Introduction to Environmental Exercise Physiology

Lecture #1 W1

Pollution Overview:

The larger particulates aren’t so bad because as we breathe them in their captured by the mucous and nostril hairs. But it’s the smaller particles that get through and potentially be troublesome to disease.

Do breathing masks actually help filter smaller particulates?

It can, it depends on the type of filter. But remember that air masks restrict airflow and can put more stress on the cardiorespiratory system especially if you’re exercising with them on.

Thermoregulation and fluid balance

Lecture #2 W2

Excess fluid – Hyponatraemia

Body Water Content

40-70% of body mass
- the more muscle you have the more water you store because muscle contains more water than adipose tissue. So the more muscle mass you have the more water content you have.
65-75% of muscle weight = water
10% of fat mass = water

Example of someone with 53% total body water how much fluid other compartments make up.

Intracellular: water stored within any cells. Extracellular: water stored outside any cells Interstitial: water moving between the cells itself Plasma: water in your blood

There is no true cut point of exercise dehydration because of how water loss affects us all can be different.

Water balance

Fluid Intake:

Increases in fluid intake are required when athletes undertake intense exercise in hot climates.
Water provided from foods, liquid and metabolism.

Fluid Loss:

Water lost from urine, faeces, skin and lungs.
One individual lost 13.6 kg of water in a 2 day, 55 mile run across Death Valley. With fluid and salt ingestion actual body weight loss only amounted to 1.4 kg.

Most of the water lost through the skin via evaportation.

There is no clear cut specific number of how many litres per day you should drink because water intake is dependent on…

Who you are
what you’re doing
Where you live

Heat balance

The aim of temperature regulation is to maintain a constant core-body temperature and prevent overheating or cooling.
Heat loss must match heat gain to do this.
An increase in body temperature above 45ºC may destroy the protein structure of enzymes resulting in death. A temperature below 34°C may cause slowed metabolism and abnormal cardiac function.

Variable set-point

Circadien Rhytm and Core Temperature

However, the temperature around which heat balance is achieved (set-point) is not a fixed value.
The set-point of core temperature varies with factors such as ovulation, food intake and digestion, exercise.
There are also daily variations in the set-point (~1°C) that occur in a cyclical fashion (over 24 h), known as circadian rhythms.

Heat production

Heat production during intense exercise/movement is the main form of volentary heat production in the body.
>75% of energy produced during muscle contraction is lost as heat.

Heat loss

The body has four methods for heat loss:

1.Radiation

2.Conduction

3.Convection

4.Evaporation

The first three of these mechanisms require a temperature gradient to be effective. ie. The difference between the temp of the environment and your body.

Radiation

Radiation: heat exchange via electromagnetic energy transfer between a relatively warm and cool body (occurs without contact).
Driven by temperature gradient between skin and environment.
In hot and sunny climates, thermal radiation (mostly solar) is added to the body.
Differences in thermal radiation can be as high as 15°C in some countries like Australia.
Posture and orientation of the individual to the sun determine the effective radiative area of the body and thermal radiation absorbed.

For radiation heat transfer from the sun to occur the environmental temperature has to be higher than skin temperature of 37~

You won’t gain heat via radiation from the sun by going outside if the outdoor temp is lower then your core temp. To lose the heat the body must instil mechanisms of heat loss to cool the body. e.g. sitting outside in 30 degrees you will not gain heat because the thermal temperature is cooler than your internal core temperature.

Conduction

Conduction: heat transfer from direct contact of body with other objects. E.G. Sitting on a chair.
Driven by temperature gradient and thermal qualities of the surfaces.
Heat is produced in the muscle as a by-product of movement. We get heat conduction from the muscle through through the tissues to the skin (temperature ~33 – 35°C). Heat moves through your muscles conducted through your skin and is lost at the epithelial surface via air convection if the envrionment permits (vapour pressure – absolute humidity is not too high)
External heat conduction is negligible for most exercise scenarios (e.g. running) but may be some heat loss in sports such as kayaking.

Convection

Convective heat loss is facilitated by the flow of air (or water) across the skin. The cooler air molecules are replacing the hotter air molecules on the skin which is dependent on wind speed but its very effective in water – water is a better conductor of heat loss than air.
Also dependent on temperature gradient between skin and ambient environment.
When air temperature is equal to skin temperature no convective heat exchange occurs irrespective of air velocity. At higher temperature (> 35°C) heat loss becomes convective heat gain.
Heat is transferred internally from core to the surface (skin) via the convection of blood.
Some additional convective heat exchange via respiration but contribution is small in warm environments.

The 3 above methods depend on heat of the body and heat of the environment.

Evaporation

Mostly relies on a vapour pressure graident.

Primrary method for heat loss.

For evaporation, heat is transferred to water on the surface of the skin. When this water gains sufficient heat (energy) it is converted to gas (water vapour) and evaporates from skin taking the heat away.
Evaporation Requires a vapour pressure gradient – a difference in humidity between skin surface and ambient air.
- Singapore is a very inefficent place for heat loss. Because the humidity, the water in the air and water on your skin there isn’t much of a vapour pressure gradient, therfore a lot of the water just drips off your skin instead of evaporating. Because of that you’re body is ineffective at lossing heat.
- Why is humidity a poor environment for thermoregulation? Because there’s less of a difference of gas pressure between your skin and the air. Humidity dictates there’s more relative moisture in the air making it much less efficient for sweat to evaporate into gas and pull heat away from the skin.
What’s happening at the epithelial level: Conduction and convection taking water molecules to the skin surfaces..when those wate moleceles gain sufficient heat they are evorpoarted. It is only when they are evoaported that the heat is taken away. Because if the sweat just drips off your body you do not lose heat.
It is important to remember that it is only the evaporation of sweat that liberates heat from the body (2430 J for every 1 g of sweat vaporized) and not the production of sweat.

Effective evaporation

Evaporation from the skin is dependent on:

The temperature and relative humidity (together these give absolute humidity).
The convective currents around the body.
The amount of skin surface exposed to the environment = a greater surface area potential for the evaporation to occur.

Humans have an upper limit for evaporative heat loss (Emax) determined by different characteristics depending upon the type of environment.
In hot dry environment Emax is capped by the physiological capacity to secrete sweat; but in humid environments is determined by the maximum proportion of the total body surface area that can be completely covered with sweat i.e. maximum ‘skin wettedness’ (ωmax).
ωmax is lowered in athletes with spinal cord or burn injuries that have greatly altered regional sweat function which affects ANS function to signal body to sweat.
Its not about how much you sweat, some people produce excess sweat (high sweat rate) – it doesn’t mean all the sweat lost is contributing to heat lost. Their autonomic NS may just be more inefficient/efficient at sweating.
KARRIE: Its not about how much you sweat, some people produce excess sweat (they’re people with high sweat rates like me) – just because you sweat a lot doesn’t mean all the sweat lost is actually contributing to heat loss. (Remember that’s why we sweat, so it can be evaporated and cool the body via the gas exchange between the air and the water on the skin). SO your autonomic nervous system which controls your sweat rate may just be much more efficient at regulating your body temperature. Your low sweat rate may be a positive thing because your body is producing the minimal amount of sweat it needs to maximally cool your body INSTEAD of having a higher sweat rate and losing a lot more fluid. Hope that makes sense.

Importance of evaporation & how effective our bodies are at regulating temperature

“Elite marathon runners sustain an oxygen uptake of about 4 L/min and….power output of about 1200 W…..a rate of heat production of 1200 W would cause the body temperature to rise by 1°C approximately every 3 minutes IF their body had no mechanism to lose the heat they would be at the thermal death point in about 20 minutes. Our bodies are very effective at losing heat.

Heat loss via evaporation

Example:

Exercise intensity: VO2 = 2 L min-1
Exercise duration = 20 minutes
Efficiency = 20% of the work they do is going into producing the exercise (80% into heat loss)
Evaporation of 1 L of sweat = 580 kcal (2.43 MJ) of heat loss

Total energy expenditure = 20 min × 10 kcal min-1 = 200 kcal
Total heat produced = 200 kcal × 0.80 = 160 kcal
Total evaporation to prevent heat gain = 160 kcal/580 kcal L-1 = 0.276 L

Heat storage

The extent to which an athlete’s body temperature change during exercise is determined by:

The net difference between internal heat generation and skin surface heat dissipation (body heat storage) AKA how much heat is coming out of your muscles lost through the skin into the envrionment
Body mass: how much weight you have to transport
Specific heat capacity of the body’s tissues – different tissues have different capacities for heat transfer

As (2) and (3) do not change during typical sporting events body temperature is altered primarily by heat storage: S = (M-W) ± K ± C ± R – E

S = Heat stored; M = Metabolic energy expenditure; W = External work performed; K = Conduction; C = Convection; R = Radiation; E = Evaporation
The change in body heat storage during exercise is determined by the cumulative difference in metabolic heat production (M – W) and net heat dissipation from skin to the surrounding environment (M-W) ± K ± C ± R – E).

The environment

Absolute humidity (AH) not relative humidity (RH) is the critical environmental factor influencing evaporation.
- AH takes into account the environmental temperature which is what makes a difference to the vapour pressure.
- RH just talks about humidity which misses the important factor of air temperature.
Absolute humidity at a fixed RH increases exponentially with ambient temperature.
You have twice the moisture present at 30%RH/38°C than at 30%RH/26°C. Why? The vapour pressure graident is higher in higher temperatures which is where the problems with heat loss/evaporation occur. 0 degrees at relative humidity of 100% has a much lower vapour pressure compared to 30 degreesa at 100% RH. You will do a much better job at evaporting sweat at lower temperature higher humidity because the vapour pressure is much lower compared to hotter humid environment. So both RH AND temp are key in dicating heat loss efficiency.
- Similarly, absolute humidity is higher at 30%RH/40°C than 50%RH/30°C.
Humdiity is often the greater challenge than the overall radiated heat from the sun

Wet bulb-globe temperature (WBGT) index

1WB-GT = 0.1 × DBT + 0.7 × WBT + 0.2 × GT

where: DBT = Dry bulb temperature / WBT = Wet bulb temperature / GT = Globe temperature

The environment can be evaluated for its potential thermal challenge using the WBGT index.
The WBGT index depends on ambient temperature, relative humidity and radiant heat.
Dry bulb temperature recorded by ordinary mercury thermometer.
Wet bulb temperature recorded by similar thermometer with a wet wick surrounding the bulb.
Globe temperature recorded by a thermometer with a black metal sphere surrounding bulb.
The WBGT is a climatic index and does not account for metabolic heat production or clothing and therefore cannot predict heat dissipation. E.G. NFL athlete wearing a lot of gear contributing to high metabolic heat production which is a limitation of WBGT that it doesn’t account for.

Several sporting bodies have recommendations based on the WBGT to alter (longer rest time) or end competition but these are contradicted often.

Environment and sweating

Increases in sweat rate (and core temperature) will occur in a hot and humid environment increasing risk of hyperthermia and leading to decreases in performance.
The rate of whole-body sweating required to achieve the required rate of evaporation is altered by sweating efficiency, which itself is determined at a fixed exercise intensity by a combination of ambient humidity and air flow across the skin.

Summary

Water makes up 40-70% of body mass.
Active person in a warm environment may require 5-10 L of fluid per day.
Maintaining core temperature is a critical function of the body.
Heat production can be classified as voluntary or involuntary.
Heat loss may occur by radiation, conduction, convection, or evaporation.
Evaporation is the most important method of heat loss during exercise and occurs because of the vapour pressure gradient between the skin and air.
The extent to which an athlete’s body temperature change during exercise is determined mostly by body heat storage (methods of heat loss + the metabolic heat production)
At high environmental temperatures absolute humidity is the most important factor determining evaporative heat loss.
The environment can be evaluated for its potential thermal challenge using the wet bulb-globe temperature index.

Exercise in the heat I

Lecture #3 W3

Heat storage

Body temperature during exercise is altered primarily by heat storage:

S = (M-W) ± K ± C ± R – E

S = Heat stored; M = Metabolic energy expenditure; W = External work performed; K = Conduction; C = Convection; R = Radiation; E = Evaporation

The change in body heat storage during exercise is determined by the cumulative difference in metabolic heat production (M –W) and net heat dissipation from skin to the surrounding environment (M-W) ± K ± C ± R – (any change in heat storage is dependent on these things)

Heat loss in steady state exercise

Radiative loss doesn’t change if the internal temp is stable (if we’re outside there’s going to be a change depending on duration)
The convective loss would be higher if we were outside due to airflow/resistance from wind but in this graph it’s stable because it’s inside on a bike
Metabolic energy production = total energy productio (work + heat production)
During submaximal steady-state exercise, heat production increases early and then remains constant.
Most heat loss is from evaporation.
There are small radiative and convective heat losses which will remain constant in a controlled environment.

Oxygen uptake and temperature

Core temperature during exercise is directly related to exercise intensity and is independent of ambient temperature at low humidity.
All this graph is saying is that the higher intesnity (higher O2 uptake) you work the more your core temp will increase irrespective of upper or lower body isolation work but lower seem’s to increase it more due to more / larger muscles in the lower limbs being worked.

Steady state exercise under different environmental temperatures

The method of heat loss during steady state exercise is modified according to the ambient temperature conditions.
With increasing ambient temperature convective and radiative heat loss decrease because of the lower thermal gradient being created as the room temperature increases, whilst evaporative heat loss increases to componensate for this reduced effectivness to loss heat through convection/radiation.
Note that energy output and heat production are constant.

Exercise intensity and heat production

Under controlled ambient conditions, convective and radiative heat loss do not increase with exercise intensity.
However, there is a consistent rise in evaporative heat loss with increasing exercise intensity.
About 80% of the energy output is produced via heat production
The sum of lines 4-6 = line #3 (total heat loss)

Ambient conditions just modify your method of heat loss but heat production is BECAUSE of the muscle contraction not because of the ambient conditions the amvbietn conditons simply modify or impair heat loss

Physiological effects of exercise in the heat

Sweat rate and core temperature

Sweat rate is higher and core temperature increases faster during exercise in a hot and humid environment compared with a cool environment.
It’s not the the heat production is greater in hot/humid conditions, it’s you end up storing more of that heat because you end up being less effective at lossing that heat
In a cool environment a lot of that heat loss is from convection/conduction compared to a hot humid envrionment where we compensate via evaporation heat loss

Sweat rate and ambient conditions

Low convection and increased humidity increase the whole body sweat rate at different ambient air temperatures.
Once we add humidity even even if its windy the humidity impairs the evaporation and we sweat more because less of that sweat is actually evaporating.

Sweat electrolyte loss

Sweat contains a wide variety of organic and inorganic solutes.
Significant losses from the body of some of these components will occur where large sweat volumes are produced
Sweat composition varies between individuals but can also vary within an individual depending on sweat rate, fitness and heat acclimatisation.
Losing electrolytes like these can affecct functions like the sodium potatsium atp pump which affects nerve conduction

Increased oxygen consumption

Prolonged submaximal exercise in a hot and humid environment leads to an upward drift in oxygen uptake.

Cardiovascular drift

Theortically during steady state cardiac output and HR should stay at a constnt but during prolonged exercise in a hot and humid environments there is an increase in heart rate and a decrease in stroke volume.
The decrease in stroke volume may partly result from a decreased plasma volume and increased cutaneous (skin) blood flow. The water we lose from sweat results in a decrease of plasma volume.
Heart rate increases to maintain cardiac output.
Another cause is increased levels of catecolmines (adrenalin/noradrenalin) = more SNS activation > IN HR

Slower recovery

In recovery from exercise in the heat, the post-exercise fall in heart rate back to resting level is prolonged.
This may result from the decrease in plasma volume and/or from the increased catecholamine response experienced during exercise in the heat.
AKA it takes longer to get your HR back down from exercising in the hot/humid conditions (due to above reasons) so take more time to do breathing post workout.

Central fatigue – reduced motor drive

Hyperthermia (heat gain) induced central fatigue may reduce motor drive to skeletal muscles.
Experiments showing that hyperthermia (core temperature 40°C) after exercising in the heat affects maximal voluntary contraction (MVC) of either exercised or non-exercised muscle compared with exercise in cooler conditions (control) when core temperature was not substantially elevated (38°C).
However, electrically stimulated (EL) MVC were unaffected by hyperthermia which suggests that the CNS is the component that is being adversely affected.

Ventilation in the heat

Ventilation increases during exercise in the heat without any change in the partial pressure of arterial carbon dioxide.
Ventilation is increased because of increases in breathing frequency.
There’s no difference in the arteial pressure of CO2 – you’re not losing more CO2 in hot/humid conditions but you do breath (ventiliate) more despite the fact there’s not change in PCO2. We breathe more because we lose heat through ventilation.

PCO2 = partial pressure of CO2

Increased carbohydrate metabolism

Carbohydrate oxidation is greater during exercise in the heat or when individuals are dehydrated before beginning an exercise bout.

Other effects of the physiological effects of exercise in the heat

Vascular constriction and dilation – constriction of splanchnic (internal organs/gut) and renal (kidneys) blood flows. May increase liver and renal complications during heat stress.
Maintenance of blood pressure – vasoconstriction in the viscera and vasodilation to the skin maintains blood pressure. As exercise intensity increases less blood is diverted to the skin and more to muscle preventing heat dissipation and impairing heat loss as the body priorities supporting working muscles (exercise) from a blood flow perspective
Blood lactate accumulation – earlier accumulation of lactate, encroachment on glycogen reserves, premature fatigue. Decreased lactate uptake by liver from reduced blood flow.

Performance in the heat

Exercise capacity

In laboratory conditions, ambient temperature (constant relative humidity of 70%; air velocity of 0.7 m/s) appears to have a clear effect on exercise capacity (cycling at 70% of maximum oxygen uptake) which follows an inverted U relationship.
As temp increases time to exhaustion increases somewhere between 11-21 degrees

Soccer performance

Indices of soccer performance are reduced in experimental conditions with exercise in the heat (43°C) compared with cooler (21°C) conditions from a total distance and total high intesnity running measure.

Tennis play

The duration of each point doesn’t really change much
Players try and componsate by taking more time between each point which is interesting
Therefore the effective playing time is reduced
But the measures here aren’t really that interesting – we see a beahvuoural change in heat vs cool but not any other performance markers

Summary

Core temperature during exercise is linearly related to exercise intensity.
The method of heat loss during exercise is modified according to ambient conditions.
Evaporation is the primary method of losing heat during exercise.
Exercise in the heat increases submaximal oxygen consumption, heart rate and ventilation and decreases stroke volume and plasma volume.
Exercise in the heat increases carbohydrate metabolism and lactate formation.
Central fatigue during exercise in the heat may lead to decreased motor drive.
Indices of endurance capacity and performance are affected with exercise in the heat in a range of sports and players may modify playing strategies based on the environmental conditions.

Exercise in the heat II

Lecture #4 W4

Minimising risk for exertional heat illness

Heat ilLecture-4-Exercise-in-the-Heat-IIlness during exercise may take various forms:

Exercise associated muscle cramps (heat cramps)
Heat exhaustion
Heat stroke

What strategies can be implemented to sustain/enhance performance during training and competition in the heat, and minimize the risk of exertional heat illness?
- Behavioural: training at different times/conditions on purpose
- Aerobic fitness: higher aerobic base = better prepared to exercise in the heat
- Heat acclimatization
- Cooling strategies
- Hydration

Definitions

Acclimatisation: refers to physiological adaptations that occur in naturally hot ambient conditions.
Acclimation: refers to physiological adaptations that occur in artificially hot indoor environments.
Heat stress: refers to environmental (including clothing) and metabolic conditions that tend to increase body temperatures.
Heat strain: refers to physiological (e.g. body temperature) consequences of heat stress.
Compensable heat stress: exists when heat loss occurs at a rate in balance with heat production so that a steady-state core temperature can be achieved at a sustainable level for a requisite activity.
Uncompensable heat stress: occurs when the individual’s evaporative cooling requirements exceed the environment’s evaporative cooling capacity. Heat gain excees heat loss,

Behavioural strategies

1. Cooling breaks and rest periods

Rest breaks provide an opportunity for body temperature to reduce via reduced heat production because your skeltal muscle is no longer contracting.
Several sporting bodies (ITF, ITF-WTA, FIFA – see lecture 2 for list) provide additional breaks for sportspersons when the heat stress is high (wet bulb-globe temperature).
This strategy is also useful for individuals in the military or in professions such as firefighting, ships stokers, steel workers.
May not be a realistic strategy in many exercise events.

Showing how giving breaks can be effective at cooling the body. Note: Data show a 12-km forced march in 10 male SAF recruits carrying an ~28-kg load under an ambient temperature of 30°C and relative humidity of 65%. Marching was completed in three 45-min stages interspersed with a 15-min and 30-min rest after stage 1 and 2, respectively.

2. Aerobic fitness

In a recent meta-analysis, aerobic fitness stood out as the most important mitigation strategy to favourably alter core temperature for endurance exercise in the heat.
Aerobic exercise training is best combined with heat acclimation/acclimatisation for full benefits for the individual.

From: Alhadad et al, Frontiers in Physiology, 10:71, 2019

3. Heat Acclimatisation

Habituation to hot environments

“In 1768, James Lind published the first report on the ability of humans to adapt to environmental heat. He reported that when relocating to East and West Indian climates, Europeans were at first adversely affected by the environment, but over a period of time, habituated and eventually lived comfortably.” We’ve known about the effects of habititatuon for a long time.

Thermal comfort in resting individuals changes when acclimatized to a tropical (hot and humid) climate.
Comfort level is altered in relation to specific humidity (i.e. grams of water per kilogram of air), dry bulb temperature, and relative humidity.

Physiological adaptations to exercise in hot environments

Several important physiological adaptations occur with heat acclimatisation that improve thermal comfort and endurance performance. Improved sweating, reduced core temperature, reduced skin temperature and changes to the cardiovascular system are among the most important changes.

Changes to sweat rate and composition

What you can see is the body sweats earlier and quicker post heat adaptation (Post-HA). If you can sweat more you can lose more heat so it’s the body becoming more efficient at cooling.

Adaptations to sweat rate and composition occur with heat acclimatization.
Body core temperature threshold for the onset of sweating is reduced.
The sweat rate (slope) is increased.
Concomitantly, the body core temperature threshold for onset of cutaneous (skin) vasodilation is reduced, whereas skin blood flow sensitivity is increased.
- Greater increase in skin blood flow.
Adaptations occur to your sweat rate and the composition of the sweat during heat acclimitisation.
The concentration of electroyltes such as sodium and chloride lost are reduced.

Temperature and cardiovascular adjustments

Note: Participants completed an initial exercise heat test (4 h at 35% of maximum aerobic power). Then 11-wks training of 1-h for 4-d/wk in temperate (normal) conditions. Finally, 8-d of exercise in the heat training for 4-h/d at 35% of maximum aerobic power.

Aerobic training in temperate environments can reduce physiological strain and improve exercise capabilities in the heat.
Heat acclimation is the preferred method of adaptation.
Core and skin temperature are lowered with acclimation.
Heart rate reduces whilst stroke volume increases. Cardiac output improves and plasma volume increases with heat acclimation.

Cardiorespiratory and performance changes

Note: Heat acclimation group received 10-d at 40°C for 100-min at 50% of maximum oxygen uptake. Control group completed same exercise for 10-d in a temperate climate. Both groups tested pre and post in hot and temperate environments.

The control group had little change in hot or cold conditions. The experimental group who become heat acclimated improved their performane in a cool AND hot envrionment.

Acclimation to the heat leads to improvements in maximum oxygen uptake, increases in lactate threshold, increases in maximal cardiac output and improvements in time trial performance.
The magnitude of any heat adaptations depend on the intensity, duration, frequency, and number of heat exposures.

Environment

Heat acclimatisation in dry heat improves exercise in humid heat and vice versa.
However, acclimatization in humid heat evokes higher skin temperatures and circulatory adaptations, potentially increasing maximum skin wettedness and therefore the maximum rate of evaporative heat loss.
- Training in hot AND humid conditions is optimal for heat acclimitisation but training in hot dry heat still makes great improvements.
Athletes unable to travel to naturally hot ambient conditions (acclimatization) can train in artificially hot indoor environments (acclimation).
Training outdoors is more specific and allows athletes to experience the exact nature of heat stress.

Time course of adaptations to heat stress

(Périard et al., Scandinavian Journal of Medicine & Science in Sports, 25(S1):20-38, 2015)

Heat acclimation occurs rapidly: 75-80% of adaptations in first 4-7 days.

Considered:

-short-term: < 7 days
-medium-term: 8-14 days
-long-term: > 15 days

Exercise in the heat is the most effective method for developing heat acclimation but passive heat exposure can also result in some adaptation.
Remember: To achieve optimal adaptation, work rate and environmental conditions should closely replicate those of the competition setting.

How Quickly Do Our Heat Acclimation Adaptations Decay

Note: Data show 14-d of heat acclimation followed by 14-d of no exercise in the heat.

Heat acclimation is transient and gradually disappears if not maintained by repeated heat exposures.
Heart rate improvements are lost more rapidly than thermoregulatory responses.
There is no agreement on the rate of decay. Aerobic fitness and regular exercise appear to contribute to retaining the benefits of heat acclimation for longer.
However, the rate of decay is generally slower than its induction allowing maintenance of most benefits for 2-4 weeks. Moreover, (re) acclimatization during this period is faster than the first acclimatization.

Heat acclimatization strategies

Heat acclimatisation strategies will vary depending on the stage of training and in relation to competition performance.

Heat as a training stimulus to improve Performance

Note: Data showed 12 trained cyclists after 10-d heat acclimation vs 8 matched controls after 10-d in a cool environment. From: Lorenzo et al, Journal of Applied Physiology, 25(S1):6-19, 2015

There has been recent movement toward examining exercise training in the heat as a stimulus to improve performance.
Some studies have shown improvements in maximum oxygen uptake, time trial performance, lactate threshold, cardiac output and stroke volume with heat acclimation.
Athletes may consider using training camps in hot ambient conditions to improve physical performance both in-season and pre-season.
However, training quality should not be compromised.
Experienced athletes requiring a novel stimulus may benefit most.
There is no consesus on the amount of heat training stress to maintain heat acclimitisation adaptations

4. Cooling strategies

Cold water immersion (CWI)

Heat loss to water is four times greater than that to air at the same temperature so we know it will cool people effectively.
CWI may take different forms e.g. prolonged duration in tepid water (22-30°C; shorter duration in cooler water (17- 18°C); cold shower or water spray/mist; recovery cooling between exercise bouts.
Most are effective at reducing skin temperature and often core temperature you still create a greater thermal gradient for heat loss. Concomitant increases in performance have been noted.
Once criticism is that blood flow to the active musculature may be reduced causing the athlete the need to re-warm up before competition to optimise CNS / nerve conduction.
An alternative is selected cooling or part-body immersion of non-active body parts.

Cooling garments

(Arngrímsson et al, Journal of Applied Physiology, 96:1867-1874, 2004) Note: Data show 5-km time trial core temperature after pre-cooling with ice vest or wearing t-shirt (control).

Ice-cooling jackets are a development from the earlier practice of using iced towels for cooling athletes before or during exercise.
Cooling garments are practical in reducing skin temperature and can be worn during warm-up and recovery.
Data shows they are effective in reducing thermal strain during early stages of exercise but they can improve exercise performance.

Cold fluids and ice slurry

You can also loss heat internal if a fluid is very cold. This is an additional way we can have a heat transfer from the ice to our body therefore heat will be lost.
AKA Cold fluid or ice slurry ingestion leads to internal heat transfer which may represent an additional method to the four avenues of heat transfer at the skin surface.
However, ingestion of cold fluids appears to lead to transient reductions in skin blood flow (because blood flow is being prioritised internally to the ice) and sweating which reduce evaporative and dry heat loss and negate the additional internal heat loss. That may be true in hot and dry conditions but in humid conditions evaportive heat loss is imparied anyway so it may be a effective way to cool the body in humid conditions.
AKA These strategies may be of most use in hot and humid climates where evaporative cooling is impaired or for athletes with physiological disruptions to sweating such as spinal cord injury or burn injuries as their capacity for skin surface evaporative heat loss is lower.

Combining strategies

Combining techniques (i.e. both external and internal cooling strategies) has a higher cooling capacity than the same techniques used in isolation, allowing for a greater benefit on exercise performance.
For athletes this can be achieved by combining simple methods such as ingestion of ice slurry, wearing cooling vests, and providing fanning.

Clothing

Dry clothing retards heat exchange. Evaporative heat loss occurs when clothing is wet!
Cottens and linens readily absorb moisture.
Sweatshirts, rubber or plastic produce high relative humidity close to the skin.
Moisture-wicking garments optimally transfer heat and moisture.
Dark colours absorb light and add to radiant heat; light colours reflect heat rays away from the body.

Summary

Strategies to sustain/enhance performance during training and competition in the heat, and minimize the risk of heat illnesses include changing behaviour, improving aerobic fitness, acclimatization, body cooling, and fluid intake.
Changes in behaviour can mitigate heat strain to the greatest extent. Whilst rest breaks are a useful strategy to reduce body temperature used in certain sports, the military and selected vocations they may not be a realistic option in many sports.
Aerobic fitness is suggested to be one of the most important measures to favourably alter core temperature for endurance exercise in the heat but improving fitness whilst training in hot and humid environments is more helpful.
Heat acclimatization/acclimation can help reduce physiological strain and optimize performance in the heat.
Heat acclimatization should comprise repeated exercise-heat exposures over 1-2 weeks.
Heat acclimatization results in increased sweating, reduced core temperature, reduced skin temperature, changes to the cardiovascular system and improved endurance performance.
Cold water immersion reduces skin and core temperature but may reduce blood flow to active musculature – part-body immersion may be a better strategy.
Cooling garments reduce skin temperature and can be worn during warm-up and recovery to aid performance.
Cold fluid or ice slurry ingestion leads to internal heat transfer but also transient reductions in skin blood flow and sweating which negate any additional internal heat loss. Optimal use may be in environment where evaporative cooling is impaired or for athletes with physiological disruptions to sweating such as humid envrionments.

Fluid provision during exercise in the heat

Lecture 5 W5

Sports Drinks

Most sports drinks are designed to support two main aims – what are they?

Fluid replacement
Energy supplementation through sugar
- Glucose is the main sugar contained in because its easier/quicker to absorb into circulation.
- We can digest about 60g glucose per hour.

However a large fluid volume impairs CHO uptake
A too concentrated sugar solution impairs fluid replenishment
The rate of gastric emptying from the stomach into the SI and SI into circulation is slowed when a liquid is more concentrated with sugar

Why are the two aims of sports drinks contradictory?

What are the main categories of sports drinks?

Isotonic: Similar concentrations of dissolved particles (osmolality: salt and sugar) in the body *generally the best to take in while exercising due to matched concentration.
Isotonic: Similar concentrations of dissolved parHypertonic: Higher concentrations of dissolved particles in the human bodyicles (osmaility: salt and sugar) in the body *generally the best to take in while exercising due to matched concentration.
Hypotonic: Lower concentrations of dissolved particles in the body *generally the best to take in while exercising due to lower concentration.

What physiological considerations should be taken into account when designing sports drinks?

Gastric emptying, which describes the rate at which substances leave the stomach into the SI.

How do you make a sports drink – what ingredients can you include and in what amount/ratio?

Why intake 150% of fluids lost post-exercise?

The reason for this is to account for continued fluid loss through sweating post-exercise as your body continues to try and cool itself post-exercise. Your body will continue sweating to lose heat post-exercise thus you will continue to lose fluids.

Hydration

Most exercise recommendations suggest that athletes should not lose >2% of body mass during exercise. That’s when we start to notice performance decrements. 2% = 1.5kg of a 75kg person.

Fluctuating between these 3 points almost every day depending on activity levels.

Hydration and performance in the heat

Hypohydration degrades work performance in men wearing protective clothing in the heat.
Hypohydration increases rectal temperature, heart rate and decreases tolerance time regardless of acclimation state in both moderately-fit and highly-fit men.
1. People who were heat acclimated could extend their time walking in the heat in a euhydrated state.
2. Although if they were hypohydrated, even after the process of heat acclimation it made no difference to how long they could exercise, whereas it did for the euhydrated group.
Result: This is a really imporant consideration: that even if you put effort into heat acclimation if you’re not adequetly hydrated your performance would be similar to if you didn’t do any acclimation at all.

Note: Data show moderately fit men (maximum oxygen uptake (VO2max) <50 ml/kg/min) and highly fit men (VO2max >55 ml/kg/min) undertaking light exercise in the heat (40°C) whilst wearing nuclear, biological and chemical protective clothing in either a euhydrated or hypohydrated (~2.5% of body mass) condition. Tests were conducted 2-weeks before and after daily heat acclimation exercise (‘Pre’ & ‘Post’). The graph show’s that even after 2 weeks of the benefits of heat acclimatisation are negligible…

Hydration and aerobic exercise performance

Hypohydration degrades aerobic exercise performance regardless if it’s 10 degrees all the way to 40 degrees.
Hypohydration increases heat storage and reduces ability to tolerate exercise-induced heat strain.
High skin blood flow and plasma volume reductions appear to mediate performance degradations in dry conditions.
One meta-analysis concluded that pre-exercise hypohydration reduces mean power output by 3.2% in comparison with control trials.
There was little difference in core temp likely due to low humdity helping their ability to lose heat via evaporation
Though skin temp did go up from 25-36 degrees because of greater skin blood flow.

Note: Four groups of men completed constant intensity (50% VO2max) cycle ergometer exercise followed by a time trial in conditions of 10, 20, 30 or 40°C either euhydrated or hypohydrated. There was little effect on core temperature but a stepwise effect on skin temperature.Data show percentage decrement in time trial performance from euhydration.

Hydration and strength performance

There is evidence to demonstrate that performance in some strength/power activities is also compromised when poorly hydrated (2.5 – 5.0%).
The amount of work done was less when they were hypohydrated compared to when they were euhydrated.

Note: Data from seven healthy men who completed resistance exercise euhydrated, 2.5% dehydrated or 5% dehydrated. ##Different between euhydrated and both hypohydrated trials. *Different between euhydrated and HY50. Judelson et al, Medicine and Science in Sports and Exercise, 39:1817-1824, 2007.

Pre-exercise hydration

Euhydration vs Hyperhydration

Pre-exercise hyperhydration (saline – water + NA+) does not appear to reduce physiological stress, increase exercise duration, or influence perceptual strain compared with euhydration in moderately fit individuals wearing personal protective equipment (firefighters)
AKA hydrating above normal hydrated levels doesn’t seem to give added benefit compared to having a normal state of hydration and performing exercise in the heat for occupation.

Note: Data from 10 individuals (8 M, 2F) who completed treadmill exercise whilst wearing personal protective equipment in a euhydrated state or after pre-hydrating with 15 mL/kg of body weight of 0.9% saline given intravenously.

(Hostler et al, European Journal of Applied Physiology, 105:607-613, 2009)

Glycerol and plasma expanders

Athletes may attempt to hyperhydrate before exercise in hot conditions where the rates of sweat loss or restrictions on fluid intake inevitably lead to significant fluid deficit.
Glycerol is a plasma expandor molecule which when ingested is absorbed and increases the concentration (osmolarity) of the fluid in the blood and tissues.
The concentration of these fluids is held constant by the body, so water consumed with the glycerol is not excreted until the extra glycerol is either removed by the kidneys or broken down by the body.

25-135% in fluid retention by taking in glycerol

(Goulet et al, International Journal of Sport Nutrition and Exercise Metabolism, 17:391-410, 2007)

Glycerol and performance

Natural performance enhancer: Glycerol does seem to make a difference to performance with its ability to increase plasma volume, decrease urine volume and improve endurance capacity, time trial performance and total power and work output. Increased blood plasma helps with cardiac output and stroke volume.
Concerns that the haemodilution (dilates body fluids) associated with the fluid retention in the vascular space may be sufficient to mask illegal doping practices by athletes meant that glycerol and other plasma expanders were prohibited by the World Anti-Doping Agency from 1st January, 2010.

Hyperhydration using water

Pre-exercise hyperhydration with water does not increase fluid volume substantially as there is a greater urine output. As it has no solutes and it just dilutes your body causing you to urinate more.
Subsequently, there appears to be no effect on performance with pre-exercise water hyperhydration.
Attempts to hyperhydrate with water may be dangerous and lead to hyponatraemia.

Note: Data from 11 individuals who hyperhydrated with water or glycerol over 150 mins pre-exercise, dehydrated during exercise over 120 mins, and then rehydrated with water or glycerol over 90mins.

Recommendations for pre-exercise fluid intake

PRACTICAL: “Athletes may achieve euhydration before exercise by consuming a fluid volume equivalent to 5 to 10 mL/kg BW in the 2 to 4 hours before exercise to achieve urine that is pale yellow in color while allowing for sufficient time for excess fluid to be voided.”#Post
- 375ml – 750ml (75kg) 2-4h before exercise
- (Position Paper: Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine. Journal of the Academy of Nutrition and Dietetics. 501-528, 2016)

“Endurance athletes should strive to start exercise well hydrated, which can be achieved by keeping thirst sensation low and urine color pale and drinking approximately 5–10 mL/kg body weight of water 2 h before exercise.”
- (Goulet, Nutrition Reviews, 70(S2):S132-S136, 2012)

Post-exercise urine colour is a bad indiactor of hydration because urine increases concentration post exercise due to vasopressin (ADH) release increasing concentration of urine.

Fluid intake during exercise

Fluid replacement and performance

Sports drinks can improve exercise capacity in the heat.
Note that this imprpovement in performance may be as much from carbohydrate intake as fluid replacement.

Recommendations for drinking during exercise from organisation

“Sufficient fluid should be consumed during exercise to limit dehydration to less than about 2% of body mass…sodium should be included when sweat losses are high, especially if exercise lasts more than about 2 h. Athletes should not drink so much that they gain weight during exercise. During recovery from exercise, rehydration should include replacement of both water and salts lost in sweat.”
- (Shirreffs & Sawka, Journal of Sports Sciences, 29(S1):S39-S46, 2011 – Statements taken from IOC Consensus Conference 2010 and citing 2004 Consensus Statement)

“Sodium should be included in fluids consumed during exercise if the exercise lasts more than 2 h. It should also be included in fluids consumed by individuals in any event who lose more than 3–4 g of sodium in their sweat.”
- (Coyle, Journal of Sports Sciences, 22:39-55, 2004)

“Ideally, athletes should drink sufficient fluids during exercise to replace sweat losses such that the total body fluid deficit is limited to <2% BW…..Sodium should be ingested during exercise when large sweat sodium losses occur. Scenarios include athletes with high sweat rates (>1.2 L/h), salty sweat, or prolonged exercise exceeding 2 hours in duration.”
- (Position Paper: Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine. Journal of the Academy of Nutrition and Dietetics. 501-528, 2016)

Commonalities for Drinking During Exercise:

Attempt to limit fluid loss to 2% of BW
Sodium replenishment if you have high sweat rate or exercise goes over 2h
Don’t drink so much you gain weight after exercise

Compared to what we thought 24 years ago…ACSM – 1996 Position Stand Controversy

“During exercise, athletes should start drinking early and at regular intervals in an attempt to consume fluids at a rate sufficient to replace all the water lost through sweating (i.e., body weight loss), or consume the maximal amount that can be tolerated.” …this leads to hyperhydration/hypernatremia

(American College of Sports Medicine Position Stand: Exercise and Fluid Replacement, 28(1):i-viii, 1996)

Post-exercise rehydration

Post exercise you don’t have the cirulatory conflict blood flow competition therefore there is more blood flow to your gut
There is a direct relationship between the sodium concentration of a drink and the fluid retained post-exercise.

The study observed people who rehydrated with different concentations of NA+

100 mmol/l is the concentration that minimises urine output and maximises sodium balance – the more conentrated the solution the more likely you are to retain your fluid
With the low sodium concentration you urinate much more out, therefore the total amount of water balance is lower

Food versus fluid

In most instances there is sufficient recovery time between exercise bouts for athletes to rehydrate slowly through the intakes of fluids (water) and foods.
A meal with water is more effective than a sports drink with the same volume of fluid for post-exercise rehydration probably because of the greater electrolyte (cations Na+ and K+) content.

Milk for post-exercise rehydration

Note: Individuals dehydrated to 1.8% of body mass in a warm environment and then rehydrated with a fluid volume equivalent to 150% of mass lost.

Closed circles:water
Open circles: carbohydrate- electrolyte sports drink
Closed triangles: milk
Open triangles: milk with added sodium

It is difficult to infuse sports drinks with a high sodium concentration because of palatability issues.
Milk is a potential candidate for an effective post-exercise solution, as it has a naturally high electrolyte content (~sodium 40 mmol/L), contains carbohydrate similar to many commercially available sports drinks and is a source of protein and calcium.
Whilst milk is a palatable drink, there may be issues with milk in lactose intolerant groups.

Alcohol and rehydration

Note: Individuals exercised and then rehydrated with a fluid volume equivalent to 150% of mass lost.

Squares: 0% alcohol
Diamonds: 1% alcohol
Circles: 2% alcohol
Triangles: 4% alcohol

Alcohol has a negligible diuretic effect when consumed in dilute solution after a moderate level of hypohydration (loss of 2% body mass) induced by exercise in the heat.
There is no difference in alcohol free beverage with 2% alcohol drinks for rehydration, but drinks containing 4% alcohol tend to delay recovery.
However, alcohol increases calorie load, suppresses fat oxidation, increases unplanned food consumption and may compromise achievement of body composition goals.
From a purley rehydration perspective, alcohol is as an a affective as a rehydration compared to plain water. Compared to what some other people say is that 4% alcohol will dehyrate you.

Menstrual cycle and rehydration

Note: Women exercised in the heat and dehydrated by 1.8% body mass before rehydrating fluid volumes equivalent to 150% of mass lost.

There is concern over whether the cyclical changes in steroid hormones that occur with the menstrual cycle may influence fluid balance.
Published data suggest that in regularly menstruating women there is little influence of these hormones on fluid restoration/urine volume after exercise-induced sweat loss in the heat.

Recommendations for post-exercise fluid intake

“After exercise that has resulted in body mass loss due to sweat loss, water and sodium should be consumed in a quantity greater than those lost to optimize recovery of water and electrolyte balance.”
- (Shirreffs et al, Journal of Sports Sciences, 22:57-63, 2004)

“Most athletes finish exercising….may need to restore euhydration during the recovery period. Rehydration strategies should primarily involve the consumption of water and sodium at a modest rate…..The presence of dietary sodium/sodium chloride (from foods or fluids) helps to retain ingested fluids, especially extracellular fluids, including plasma volume. Therefore, athletes should not be advised to restrict sodium in their postexercise nutrition…Because sweat losses and obligatory urine losses continue during the postexercise phase, effective rehydration requires the intake of a greater volume of fluid (eg, 125% to 150%) than the final fluid deficit (eg, 1.25 to 1.5 L fluid for every 1 kg BW lost). Excessive intake of alcohol in the recovery period is discouraged due to its diuretic effects. However, the previous warnings about caffeine as a diuretic appear to be overstated when it is habitually consumed in moderate (e.g., <180 mg) amounts.”
- (Position Paper: Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine. Journal of the Academy of Nutrition and Dietetics. 501-528, 2016)

Summary

Hypohydration degrades work tolerance in protective clothing, and aerobic and strength exercise performance in many laboratory-based studies.
Hyperhydrating before exercise using glycerol or other plasma expanders is effective and can improve performance but concerns over their misuse in relation to illegal doping practices has led to their prohibition by WADA.
Athletes should consume 5 to 10 mL/kg BW of fluid in the 2 to 4 hours before exercise training to ensure adequate hydration.
Recommendations suggest athletes should try to limit fluid losses during exercise in the heat to <2% of body mass.
Food with water is ideal for rehydration purposes but a sports drink equivalent to 150% of the fluid lost during exercise can also be used if food is not available.
Mild alcohol intake does not impair post-exercise rehydration but athletes should avoid alcohol for the other negative effects associated with its consumption.
The menstrual cycle does not appear to affect post-exercise fluid restoration.

Hyponatraemia

Lecture 6 W5

What is hyponatraemia?

Excessive consumption of water or fluids low in sodium before or during exercise can lead to hyponatraemia.
Defined as a serum sodium concentration <135 mmol/L.
Severe hyponatraemia can be defined as serum sodium concentration <120 mmol/L.
Mechanism: When you have EAH you get osmotic shifts of water into your intracellular. So your blood plasma has become diluted so the concentration of substances in the plasma is less than that of the intracellular compartment so water starts to shift across from the plasma into the intracellular space. That causes these cells to swell up, leading to adverse symptoms such as impaired muscle function, altered CNS function, cardiopulmonary failure, brain swelling (serious issue because there’s no where for the cells to expand because you have your cranium housing your brain which can lead to edema and death).

Symptoms and risks?

Exercise associated hyponatraemia (EAH) can be symptomatic or asymptomatic.
Athletes with symptomatic EAH can present with mild, non-specific symptoms (e.g. light- headedness, nausea) but typically present with headache, vomiting, and/or altered mental status (confusion, seizure) resulting from cerebral edema.

Symptoms associated with EAH are due to osmotic-induced shifts of water into the intracellular compartment.
In the confined space of the cranium these water shifts into the central nervous system lead to cellular edema and increases in intracranial pressure which in the extreme can lead to death.

Treating Hyponatramia:

These symptoms are similar to hyerthermia (heat stroke)/hypohydration which can result in improper unsafe treatment to individuals with hyponatremia. Unsafe treatment comes in the form of hyponatremia patients being given lots of fluids which further worsens the problem of fluid retention and low sodium contributing to more swealling (edema) of tissues (the brain in worse and where death can occur).

Prevalence of hyponatraemia

From a sample of 488 individuals in the Boston Marathon, the percentage with hyponatraemia has been recorded at 13% (serum sodium concentration <135 mmol/L).
Those experiencing critical hyponatraemia has been recorded as 0.6% (serum sodium <120 mmol/L).

This shows those with the biggest positive weight change were at highest risks due to higher fluid retention. Often victims of this are the slower and/or smaller races at the back who have more time to drink and less body mass to distrbuate fluids.

Factors associated with hyponatraemia

Hyponatraemia was associated with weight gain, consumption of >3 L of fluid during the race, drinking fluids every mile, and a racing time of >4 hours.
It is unclear why low and high BMI are associated with hyponatraemia – low BMI because smaller runners may drink more in proportion to larger runner, high BMI because larger runners may lose less water through evaporation (sweat) as a result of a lower ratio of surface area to volume.

Exercise-Associated Hyponatraemia (EAH) Statement

“Given that excessive fluid consumption is a primary etiologic factor in EAH, using the innate thirst mechanism to guide fluid consumption is a strategy that should limit drinking in excess and developing hyponatremia while providing sufficient fluid to prevent excessive dehydration.”

Dehydration and impairment of performance?

Most laboratory studies indicate an impairment in endurance performance with exercise performed in a dehydrated state.

Note: Data are from a review of 34 endurance exercise/dehydration studies. Fractions above bars represent the number of significant observations out of total observations for each level of dehydration. 41 out of 60 studies showed impaired performance with dehydration ≥2%.

Body weight change and marathon performance?

However, in real-life many runners and other athletes complete endurance races successfully with body mass losses well in excess of 2%. But would they have performed better had they hydrated more?

Note: Level of post-race dehydration vs. average running speed and finishing time for 42 km when drinking ad libitum.

Laboratory studies lack external validity?

Ecological validity refers to the extent to which the findings of a research study are able to be generalized to real-life settings.

There are many conditions like air flow (convective heat loss), motivation of a real life event etc that aren’t accounted for in studies.

Ecologically valid laboratory studies

Cyclists were dehydrated -3% in body mass during 2 hours of submaximal exercise and then blinded to the extent of rehydration during a saline infusion – that dehydrated blinded group still had very similar PPO than the other 2 groups that rehydrated.
There was no difference in subsequent 25 km time trial performance in the heat with real-life wind speeds (32 km/h) when they were fully rehydrated compared with -3% or -2% dehydration even though rectal temperatures were greater in the -3% condition.
- This could demonstrate how the importance of 2-3% dehydration may not play as an important role to PPO over middle distance events when outdoor conditions like air resitance and placebo are accounted for.
Wind speed may play an important role in skin temperature cooling.

Whilst many laboratory studies indicate an impairment in endurance performance with exercise performed in a dehydrated state that is not the case in real-life racing or in ecologically valid laboratory studies. It is important to recognize that many of these studies are not always conducted in hot and humid conditions.

Adaptation to hypohydration/dehyration?

Familiarisation to hypohydration (four successive session of 2% hypohydration) may nullify (reduce) impairments in performance (treadmill running) without diminishing cardiovascular strain.

Note: 10 recreationally active males ran for 45 mins at 75% VO2max followed by a 5 km time trial. Data show difference between euhydration and hypohydration before and after habituation.

Overall performance

Meta-analysis of five articles with 13 effects found that during cycling in ‘real world’ laboratory conditions, exercise-induced dehydration did not alter cycling time trial performance and drinking to thirst was associated with an increase in time trial performance compared with drinking above or below thirst.
Goulet, British Journal of Sports Medicine, 45:1149-1156, 2011

Ecologically valid vs. non-valid protocols

Endurance performance during ecologically valid (time-trial exercise) vs non-valid (clamped intensity exercise).
Data suggests that exercise-induced dehydration ≤4% does not impair performance in ecologically valid studies.
However, during clamped intensity exercise performance is impaired with dehydration ≥1.75%.
- Clamped intensity = exercising as long you can so people become more susitible to effects of dehydration (but not realitc to real world which always has end points to strive for.)

Too much water versus too little water pros and cons

Both over consumption and inadequate consumption of fluids (water) are associated with acute and chronic health risks for athletes and may impair performance.
Individuals working with athletes should provide reasonable guidelines related to fluid intake that do not endanger health.

Air pollution and exercise

Lecture #7 W7

What are air pollutants?

Types of air pollutants

Air pollution is caused by a variety of gases and particulate that are products of fossil fuels.

Major air pollutants include:

Particulate matter
Ozone
Nitrogen dioxide
Sulphur dioxide
Carbon monoxide

Particulate matter (PM)

Refers to a complex mix of airborne particulate matter generated mostly by human activity which has a broad range of health effects, but predominantly on the respiratory and cardiovascular systems.

PM includes PM10 and PM2.5

Refers to the diameter of the particulate matter in the air

The smaller particultes the worse because they can reach further into lungs and potentially into the blood stream then cross the blood brain barrier (highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system (CNS))

PM2.5

PM2.5 originates primarily from combustion sources.
PM2.5 are more likely to penetrate further into the respiratory system (small airways and alveoli).

PM10

PM10 are primarily produced by mechanical processes such as construction activities, road dust and wind
PM10 are monitored by most air quality monitoring systems and include particle mass that enters the respiratory tract for both the coarse (particle size between 2.5 and 10 µm) and fine particles (<2.5 µm).

From: WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide – Global update 2005. World Health Organization.

Micrograms per metre cubed

Particulate Matter – Singapore

Singapore’s Haze Guidelines are based on 24-hour particulate matter.
There are also 1-hour PM2.5 guidelines available.
Ministry of the Environment and Water Resources (MEWR) and National Environment Agency (NEA) have set ‘interim’ targets for PM2.5 of 37.5 µg/m3 24-h mean and 12 µg/m3 annual mean by 2020. ‘Long-term’ (undefined) targets are in line with WHO recommendations (Sustainable Singapore Blueprint Target, NEA Website).
For PM10 the 2020 targets are 50 µg/m3 24-h mean and 20 µg/m3 annual mean in line with WHO recommendations.

PSI = Pollutent Standard Index

Ozone (O3)

Ozone is formed in the atmosphere by photochemical reactions in the presence of sunlight and precursor pollutants, such as the oxides on nitrogen.
It is destroyed by reactions with nitrogen dioxide (NO2) and deposited to the ground.
Ozone levels tend to be higher in the countryside than cities and greater in the summer than winter.
Can reduce lung function, cause lung inflammation, trigger asthma symptoms and cause breathing problems.
MEWR and NEA have set a 2020 target for O3 in line with the WHO recommendation of 100 µg/m3 8-h mean. Most of this is to be achieved with improved exhaust emissions from vehicles (NEA Website).

From: WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide – Global update 2005. World Health Organization.

Nitrogen dioxide (NO2)

Produced directly by combustion and higher levels by road traffic or indoors where there is gas cooking.
Has multiple roles which are difficult to separate from one another.
Animal and human experimental studies indicate that NO2 – at short-term concentrations exceeding 200 µg/m3 – is a toxic gas.
Long-term exposure has varying adverse effects including causing inflammation of airways, affecting lung function and respiratory symptoms.
Most atmospheric NO2 is emitted as NO, which is rapidly oxidized by ozone to NO2 in the presence of hydrocarbons and ultraviolet light.
MEWR and NEA have set a 2020 target for NO2 in line with the WHO recommendations above (NEA Website).

From: WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide – Global update 2005. World Health Organization.

Sulphur dioxide (SO2)

Emitted from industrial sources including power stations.
Mostly contributed by petrochemical refineries in Singapore – Shell, Singapore Refining Company and ExxonMobil contributed 93.1% of the 80,111 tonnes of all SO2 emissions in Singapore in 2015 (NEA website).
Causes constriction of the airways of the lung leading to changes in pulmonary function, respiratory symptoms and asthma.
MEWR and NEA have set targets for SO2 of 50 µg/m3 for 24-h mean and an annual mean of 15 µg/m3 by 2020 (Sustainable Singapore Blueprint Target). The WHO recommendation is a ‘long- term’ target (NEA Website).

Carbon monoxide (CO)

Produced when carbon-containing fuel burns without an adequate supply of oxygen.
Not much production from modern cars.
However, can also be caused by coal combustion and improperly adjusted gas-burning and oil appliances which may be important in developing countries.
Affects binding of oxygen to haemoglobin and release of oxygen at tissue level.
MEWR and NEA have set a 2020 target for CO in line with WHO recommendations of 10 mg/m3 8-h mean and 30 mg/m3 1-h mean (NEA Website).

Health risks of air pollution

Aging: Particulate matter in air pollution is a mixture of solid particles and liquid droplets. Some evidence suggests that exposure to particulate air pollutants accelerates aging.

Depression: Depression and suicide linked to air pollution in new global study

The authors of the review conducted a meta-analysis of 14 studies involving more than 680,000 participants living in North America, Europe, and Asia. They found that as concentrations of particulate matter increased, the risk of depression and suicide increased. Specifically, for every 10 microgram per cubic meter increase in particulate matter that was 2.5 microns or less in width, the risk of depression increased by 19 percent and the risk of suicide increased by 5 percent.
The mechanisms that drive these links may be related to increased oxidative stress and neuroinflammation as a consequence of exposure to air pollutants. These findings point to the need for improving air quality and monitoring at-risk groups living in areas where air quality is poor.

Lifetime exposure

Important to recognize that the exposure to air pollution is over the entire lifetime – from within the womb to old age.
Why air quality matters: During a lifetime, a person breathes about 250 million litres of air, weighing about 300,000 kg – not all is natural gas!

Lifetime harms

Can lead to decline in lung function, cause lung cancers, is linked to the development of asthma, associated with the appearance of diabetes, linked to cardiovascular disease, and may also affect cognition.
Road traffic emissions – especially of NO2 and PM – are significant in terms of total pollutant loading of outdoor air and diesel vehicles contribute more than petrol driven vehicles.

Prevalence of air pollutants

Worlwide Regional PM10 Pollution

South East Asia Region has the highest levels of PM10 air pollution after the Middle East and Africa.

HI = high income

LMI – low/middle income

EMR = Eastern Mediterranean Region

Sear = south east asian region

Wpr – Western Pacific Region

Eur = Europe

Global PM2.5 Pollution

Global levels of PM2.5 pollution are centred in Africa, the Middle East and Central/South Asia.

Singapore PM2.5 Levels

Singapore National Environment Agency (data.gov.sg, 26th Sept, 2018)

From 2002 and 2014 the mean annual PM2.5 levels in Singapore were reported as between 16 and 23 µg/m3.
According to the WHO recommendations it should be below 10 averaging in a year – yet none of these numbers in Singapore go below 16.
The highest 1-hour haze level for PM2.5 was recorded by the National Environment Agency on October 19th 2015 at 471 µg/m3.

PM: Recommended PM.2.5 & PM10 annual and mean concentrations

From: WHO air quality guidelines for particulate matter, ozone, nitrogen dioxide and sulfur dioxide – Global update 2005. World Health Organization.

Recommended annual levels of PM have been difficult to achieve in many countries.
At the moment Singapore’s target is based on Interim target 3

Exercise and air pollution

Are athletes more vulnerable to air pollution?

Individuals who exercise – particularly outdoors – may be more vulnerable to the environmental exposure from air pollution.
Air intake (minute ventilation) can increase from 6 L/min at rest to 200 L/min during maximal exercise in some top athletes. That’s a huge amount of air.
- 1h of high-intensity exercise outside can = the same as 10h sitting outside at rest.

Your vulnerability/risk to pollution are related to 2 major factors:

Your biological susceptibility (uncontrollable)
Environmental susceptibility (controllable)

Exposure

The size of a risk estimate depends on the presence or absence of a vulnerability factor.
Vulnerable populations: can include elderly, children, pregnant, asthma or populations who have multiple risk factors (obesity, smoke)

Individuals, or groups of similar individuals, who are especially vulnerable to the effects of air pollution may:

a) for any given exposure to pollution, have a higher risk of an adverse health outcome (a)
b) for any given exposure to pollution, experience a more severe adverse health outcome (b)
c) experience an adverse health outcome at a lower level (‘threshold’) of exposure to pollution (c)
d) be more likely to experience an above- threshold exposure to pollution (d).
- That is what happens with athletes who exercie outdoors. Because you exercise outside the likelihood of exposure goes up dramatically.

Particulate matter, health and performance

Particulate matter in athletes has received considerable attention because it can have multiple effects on health and because the inhaled effects of pollution are magnified during aerobic exercise from increase PM deposition with high-ventilation rates.

All of these factors contribute to the deposition of particulates within the body which contribute to inflammatory reaction and possible impaired cardiac/immune/respiratory function.

Early Studies: Government Legistlation Improving Lead Levels

Cross-sectional data on competitors in the 1984 and 1990 Comrades marathon in South Africa showed that government legislation to halve the lead content of petrol led to significant differences in mean blood lead level.

Carlisle & Sharp, British Journal of Sports Medicine, 35:214-222, 2001

Blood lead levels in urban cyclists

Similarly blood lead levels in British competitive cyclists were correlated with the amount of training and racing in difference environments.

Rural training is not SS but so much lower correlation with blood lead levels.

Cross-sectional data children in India living in Pollution (low/high) & O2 Uptake

Boys exercising in areas of India with similar climatic conditions (altitude, temperature and humidity) but with differing levels of pollution (low/high) found that those in high pollution had lower maximum oxygen uptake and lower post-exercise red, higher white blood cell counts, and lower haemoglobin concentration, and volume of red blood cells.

Diesel exposure and vascular dysfunction

30 healthy men exposed to diluted diesel exhaust air (particulate concentration 300 µg/m3) for 1 hour during intermittent exercise (minute ventilation 25 L∙min-1∙m-2) showed impairments in vascular tone (a marker of resistance to blood flow) and endogenous fibrinolysis (the breakdown of blood clots) – a prothrombotic state.
T-PA is a protein involved in blood clot breakdown

Diesel exposure in men with myocardial infarction

Exposure to dilute diesel exhaust (particulate concentration 300 µg/m3; median particle diameter, 54nm; range 20-120 nm) compared with filtered air over 1 hour caused ST-segment depression (an indicator of myocardial Ischaemia (cardiac arrest) – lack of oxygen to the heart) in 20 men with prior myocardial infarction (heart attack) completing two 15 minute sessions of moderate exercise (5-7 METs).
Suggests that PM air pollution can trigger or augment existing myocardial ischaemia extremely rapidly.
Ischemic burden = lack of O2

Walking in polluted areas versus green space on lung function

A 2-hour walk in Oxford Street compared with Hyde Park in London in men and women aged ≥ 60 years found that in all participants, walking in Hyde Park (botanical garden) led to better lung function (forced expiratory volume in the first second [FEV1] (how much air can be forced out of your lungs in 1 sec ) and forced vital capacity [FVC]) (how much air can be forced out of your lungs in total) and improvements in markers of blood pressure.
Participants with chronic obstructive pulmonary disease (COPD) reported more cough, sputum, shortness of breath and wheeze after walking down Oxford Street.

Walking and asthma

Similarly, a 2 hour walk in Oxford Street compared with Hyde Park in young men and women with mild and moderate asthma showed greater reductions in FVC and FEV1 in those with moderate than mild asthma in Oxford Street.
If you’re already suffering from some sort of x respiratory diease your exposure to pollutants is greater and you’re at a greater risk of respitory certain ailments.

Endurance training in urban environments on White Blood Cell Count & immune Count

Bos et al, Medicine and Science in Sports and Exercise, 45:439-447, 2013

A 12-week aerobic training programme in healthy men and women found no differences in fitness improvement in those training in an urban vs rural environment.
However, white blood cell counts (inflammatory markers) increased in the urban training group compared with those in the rural environment and increases correlated with the extent of ultra-fine PM exposure.

To exercise or not?

When does harm outweigh benefit?

In comparing the health risks of air pollution with physical activity from active travel (cummuting to work) two thresholds can be used:

Tipping point: where an incremental increase in active travel will no longer lead to an increase in health benefits (i.e. maximum benefits have been reached);
Break-even point: where the risk from air pollution starts outweighing the benefits of physical activity (i.e there are no longer net benefits compared to not engaging in active travel).

Relative risk in relation to daily cycling at PM2.5 50 micrograms per metre cubed.

Up to 69-90m of exercise the tipping point
It would take exercising about 5h per day to reach the breaking point where additional PA will cause adverse health affets at PM2.5

Physical activity trumps air pollution

For half an hour of cycling (6.8 METs) per day the background PM2.5 would need to be 95 µg/m3 to reach the tipping point (<1% of cities in the WHO Ambient Air Pollution Database) and the break-even point 160 µg/m3.
For half an hour of walking (4 METs) the tipping and break-even point appear at a background level above 200 µg/m3 (very high)
If the counterfactual was driving, rather than staying at home, the benefits of physical activity would exceed harms from air pollution up to 3.5 h of cycling per day.
For the global average urban background PM2.5 concentration (22 µg/m3) the benefits of physical activity far outweigh the risks from air pollution with the tipping point only reached after 7 h of cycling and 16 h of walking per day.

Summary

Exposure to air pollution is over an entire lifetime and is linked with a number of diseases.
South-East Asia has high levels of air pollution. Singapore experiences modest air pollution annually but levels can rise dramatically during periods of haze.
Data show that bouts of exercise performed in polluted areas can cause unfavourable changes in lung function, blood pressure and other health parameters.
However, in most urban environments, the benefits of exercise outweigh the harms caused by air pollution.

Diving

Lecture 7 W8

Shallow Water Blackout

Hyperventilating lowers the PCO2 not giving the signal to respire, but it is a technique to prolong respiration.

Diving depth, pressure and gas volume

Water is non-compressible.
2 forces produce increased external pressure (hyperbaria) in diving:
- i.Weight of the column of water directly above the diver (hydrostatic pressure);
- ii.Weight of the atmosphere (ata or bar) at the water’s surface.
A column of seawater exerts a force of 1 atm (760 mm Hg, or 14.7 psi) for each 10-m (33-ft) descent below the surface.
Freshwater is less dense so a depth of 34-ft corresponds.
The human body is also made of water so is mostly non-compressible. However, volume and pressure in areas with air cavities (lungs, respiratory passages, sinuses, and middle ear) changes considerably during descent and ascent in diving. That air is compressible which is where you feel pressure as you descend.
The biggest relative pressure increase is from the sea level to the first 10m.

Diving depth and gas volume

What happens to volume as pressure increases? It decreases.

“At a constant temperature, the volume of given mass of gas varies inversely with its pressure.” (Boyle’s Law, 1662)

When pressure doubles volume halves; conversely, reducing pressure by one half expands any gas volume to twice its previous size.
A diver taking with 6 L air in their lungs at the surface would have their lung volume compressed to 0.6 L from the compressive force of water against the thoracic cavity at 90 m.
Conversely, taking in 6 L pressurized air at 10 m depth would expand to 12 L of air at the surface and counteract that external pressure.
The human free diving depth limit is mostly dictated by a person’s lung volume and ability to regulate it.
If an individual rises too quickly they can get lung expansion injury. The problem with rising too quickly in scubadiving is because you’re intaking compressed air compared to intaking air at the surface.

Snorkelling and free-diving

Limits to snorkel size: Why Can’t You Have a 10m Long Snorkel

Increased hydrostatic pressure on the chest cavity as one descends beneath the water. The respiratory muscles are not strong enough to breath aginst that pressure (that’s why you take in compressed air).
Increased pulmonary dead space by enlarging the snorkel’s volume you have to get the air all the way down the tube into the lungs.

Free diving – Diffusion of gases

Gas diffusion always occurs from an area of higher to lower partial pressure.

Free-diving (breath-holding)

Duration and depth of a breath hold dive depends on:

Breath-hold duration until arterial carbon dioxide pressure reaches the breath-hold breakpoint (arterial PCO2 of 50 mmHg) – that breakpoint is when you’re forced to breath.
Relationship between a diver’s total lung capacity and residual lung volume.

Hyperventilation and blackout

Hyperventilation considerably extends the breath-hold period by lowering arterial PCO2 which considerably extends the breath-hold time.
Combining hyperventilation with breath-holding poses serious risks including blackout.
During diving descent increased intrathoracic pressure maintains a relatively high alveolar PO2. Upon ascent, intrathoracic pressure reduces, lung volume expands and alveolar PO2 decreases to a level where no gradient exists for oxygen diffusion into arterial blood.

Glossopharyngeal breathing

Competitive breath-hold dives exceed the depth predicted from the physiological ratio of total lung capacity to residual volume by using this breathing technique to store extra O2.
In order to prevent lung collapse during diving it is desirable to achieve lung volumes greater than total lung capacity.
Glossopharyngeal insufflation (GI) is a pumplike action of the cheeks, tongue, pharynx and larynx that enables free divers to fill their lungs beyond TLC by up to 2-3 L.
The diver can increase oxygen stored in the lungs and provide additional intrapulmonary gas to prevent compression.

Glossopharyngeal exsufflation (GE)

TLCGI = their lung volume after GE

Glossopharyngeal exsufflation (GE) can help equalise pressure in the middle ear at low lung volumes.
Divers draw air from the lungs into the pharynx to equalise the pressure in the middle ear at a time when lung volumes are low.

Diving reflex

A number of physiological responses occur during immersion termed the ‘diving reflex’.

i.Bradycardia
ii.Decreased cardiac output
iii.Increased peripheral vasoconstriction
iv.Lactate accumulation

Diving in colder water is more beneficial to sustaining a lower HR and slowing down the whole metabolic rate.

Scuba Diving: Self Contained Underwater Breathing Apparatus

At depths below 1 m inspiratory muscle power cannot overcome the hydrosatic-compressive force of water against the thoracic cavity.
A Self Contained Underwater Breathing Apparatus (SCUBA) provides compressed air to promote inspiratory action and counteract the external hydrostatic pressure.

Most diving tanks contain about 2000 L of air compressed to 3000 psi. This compressed air is = to the pressure underwater.
As you dive down further the volume of the air in the tank compresses further so the deeper you go the less air you have in the tank = less time to dive.
One tank provides enough air for 30 min to 1 hour.
The start of inspiration creates a negative pressure and releases air to the diver at a pressure nearly equal to the waters external pressure. Positive pressure with exhalation closes inspiratory valves and discharges exhaled air into water.

Diving time

75% of the air in an open-circuit SCUBA tank is wasted as exhaled air is ~17% oxygen.
- Is there a way to create a system that diffuses exhaled air into another tank to then reuse? Closesd-circuit SCUBA unit below is a version of it.
Diving depth is the biggest determinant of air use.
Inhaling 5 L of air from a tank at 90 m is equivalent to 50 L at sea level.
Diving depth is the biggest determinant of air use.
Inhaling 5 L of air from a tank at 90 m is equivalent to 50 L at sea level.
They tend to mix O2 and helium to keep the body temperature regulated at a higher temperature because breathing tank water tends to give off a lot of heat and lowers body temp.

Underwater swimming

A number of factors influence the energy cost of swimming underwater – and therefore ventilation – including:

i.Sex – women use less air than men.
ii.Gear and number of tanks – the energy cost of swimming with two tanks is ~25% higher.
iii.Fin type – flexible fins have a lower energy cost than a rigid fin.
iv.Diver’s experience – lower energy cost in advanced divers.

Relationship between swimming speed and O2 Consumption

Drag forces increase energy cost of swimming.
There is a curvilinear relationship between oxygen consumption and underwater swimming speed.
The location and density of gear can alter a diver’s positioning in water and increase the energy cost of swimming by as much as 30% at slow speeds.

Closed-circuit SCUBA unit

A closed-circuit SCUBA unit prevents oxygen wastage during diving beacuse the CO2 is being rescyled into O2 again so you can stay under water for much longer up to 3h.
Also no bubbles so it’s useful for covert military operations.

Two main problems:

i.If CO2 output exceeds rate of absorption or is absorption fails diver breathes in CO2 and may become anesthetized by CO2 build-up without warning;
ii.High concentrations of inspired O2 particularly if under high pressure underwater produce a variety of adverse physiological effects, particularly on the central nervous system. So you can only go 10-20m in depth.

Dangers of scuba diving

Lung expansion injuries can occur if compressed air is not exhaled during rapid ascent to the surface and lungs can overinflate and burst.
If you run out of air during a dive they tell you to slowly exhale until you reach the surfaace as you ascend.
If lung tissue ruptures air bubbles (emboli) enter the pulmonary venous circuit which then flow to the heart and enter systemic circulation. A diver normally maintains a heads-up position on ascent so bubbles move up in the body lodging in arterioles and restricting blood supply to vital tissues.

Scuba diving hazards II

Face mask squeeze:

Air in face mask equals that of ambient air at surface at start of dive. As a diver goes deeper a considerable difference in air pressure inside to outside of mask which creates relative vacuum in mask. Wearing googles can cause eyes to bulge from their sockets leading to capillary rupture and hemorrhage of eyes and soft tissue. A facemask covering eyes and nose equalises pressure as inspired air from the tank is equal to the external water pressure and exhaling through nose into mask balances pressure on both sides of mask.

Nitrogen Narcosis: Rapture of the Deep

With each additional 10 m depth nitrogen pressure increases 600 mm Hg. Inspired nitrogen equals 4200 mm Hg at 60 m. At 20 m all tissues contain 3 times as much nitrogen as they did at surface. Increased nitrogen in tissues produces a narcotic effect characterised by a general state of euphoria similar to alcohol intoxication, termed rapture of the deep. At 30 m it is similar to consuming alcohol on an empty stomach. High nitrogen levels have a numbing effect on the central nervous system termed nitrogen narcosis. Mental processes deteriorate so that a diver may feel that the SCUBA serves little purpose and remove it or swim deeper instead of toward surface.

Decompression sickness

With rapid ascent, external pressure against the diver’s body decreases dramatically. Excess dissolved nitrogen in tissues begins to separate from a dissolved state to form bubbles in tissues (blood stream) like opening a can of coke.
Decompression sickness – ‘the bends’ – is caused by dissolved nitrogen moving out of solution and forming bubbles in body tissues and fluids.
Nitrogen reaches equilibrium slowly, particularly in fatty tissues, and leaves more slowly from the same fatty tissues. Thus, women (greater body fat) and obese men face greater risk for decompression sickness.
The only solution to this is to put people back under pressure and the nitrogen will move back into solution. This can be down in hospitals. If you have the bends and don’t put yourself back under pressure the likelihood of death is high.

Why you don’t fly within 24h of diving?

Because when you fly the pressure in an airplane is less than sealevel so that could potentially lead to nitrogen bubbling in your blood stream and lead to the bends.

Repeat dives:

You need to have a certain number of hours between your first and second dive dictated by the depth of dive.

Decompression times

The smartest thing you can do when recreationally diving is dive to the deepest point when starting and slowly come up naturally and observe the sea.

If diver exceeds depth-duration recommendations the ascent to the surface must progress in an established manner and may require stops.
A decompression chamber with hyperoxic breathing mixture can be used if diver ascends too quickly.
When diving always a good idea to go to the deepest point at the start and then ascend slowly throughout the dive.

Summary

With each doubling of pressure during diving the volume of the lungs is reduced by half.
Snorkels can only be used at the surface of the water because of limited ability of the inspiratory muscles and the increased dead space which occurs with longer tubes.
The increased partial pressure of carbon dioxide during free-diving limits the ability to hold the breath.
A scuba tank contains compressed air which is released to a diver at a pressure nearly equal to that of the external pressure of the water.
Diving time is limited by the depth of the dive, a person’s sex, equipment, experience and swimming intensity.
Scuba diving has a number of risks associated with it including air embolism, mask squeeze, nitrogen narcosis and decompression sickness.

Cold & Altitude

Lecture #8 W9

Cold

Hypothermia is defined as a core temperature <35°C.
Hypothermia is facilitated by cold air in two ways:
- By providing a gradient for heat loss + convective heat loss if there’s wind
- low water vapour pressure (more evaporation occurs) in cold air encourages evaporation of moisture
- from the skin, cooling the body.
A 2°C drop in core temperature is associated with maximal shivering rate.
Shivering can increase your metabolic rate by 4x compared to than at rest
A 4°C drop causes ataxia (nervous system damage) and apathy.
A 6°C drop causes unconsciousness followed by death.
Wet clothing is very bad because water molecules will evaporate quicker in cold air due to the lower water vapour pressure (low humidity) therefore heat will be taken away from your body quicker and further contribute to hypothermia

Wind-chill temperature index

Rate of heat loss at any given temperature is influenced by wind speed which increases the number of cold air molecules coming into contact with the skin so heat loss is accelerate because warm air molecules are being replaced by cooler air molecules
US National Weather Service provides calculated wind chill temperature for a variety of wind speeds and temperatures along with estimates of time for frostbite to occur.
°C = (°F-32)/1.8

Water

Thermal conductivity of water 25 times greater than air.
Unlike air, water offers little or no insulation at the skin-water interface, so heat is lost rapidly.
32F = 0 degrees C
Sea water doesn’t freeze because it has salt in it so it can drop lower than freezing point

Insulation – subcutaneous fat

Subcutaneous fat thickness provides an indicator of total body insulation per unit of surface area.
Higher body fatness can help maintain body temperature.
Body fat is a fuel to support shivering.
In distance swimming body fatness can aid buoyancy.

Insulation – clothing

This diagram provides practical implications to how many layers of cloths to wear during exercise in the cold

Clothing extends natural insulation by insulating warm air molecules against your skin.
Insulation of clothing is given in ‘clo’ units. Expect 1 clo to = 1 layer of clothing. Sleeping bags and outdoor clothing equipment provide clo numbers.
1 clo is the insulation needed at rest (1 MET) to maintain core temperature when the environment is 21°C, relative humidity 50% and air movement 6m per minute.
Clothing is best worn in layers so it can be removed to account for incremental increases in heat production during exercise without getting too cold.
Wearing a hat can reduce heat loss.

Wet clothing and relative humidity

Wet clothing will exacerbate heat loss in cold environments as it loses its insulating quality.
Low water vapour (dryer air) pressure in cold environments exaggerates heat loss when wet from sweating.
A cold, wet and windy environment carries an extra risk of hypothermia.
Even when relative humidity is high if temp is cold the # of water molecules in air is low, that means if you sweat that sweat evaporates easily. Its not just the relative humidity you have to take into account it’s the temp in relation to the humidity. In cold environments evaporation of swear occurs more quickly therefore more heat is taken away from your body.
Why is the south poll colder? Because the south poll is at altitude more than 9000 ft elevation versus the North Pole that is much lower and sitting on top of a thick ice sheet.

Acclimatisation to cold

Some habituation to cold is possible with regular exposure. Ama (sea woman) women divers in Korea and southern Japan tolerate daily prolonged exposure to diving for food in cold water that in winter averages 10°C.
Regular exposure to cold water can increase tolerance.
Repeated cold exposure of local tissues, such as hands or feet in fisherman, can increase blood flow through those tissues.
Yellow horizontal line = the point at which 50% of the people started shivering

Summary – cold

Cold air facilitates heat loss from the body from the temperature gradient and reduced vapour pressure causing greater evaporation.
Both wind and water exacerbate heat loss from the body.
Increased subcutaneous fat and clothing, ideally worn in layers, provide insulation from cold environments.

Introduction – Altitude

Air density is lower at altitude
The proportion of O2 stays the same at altitude, its just that there’s less of it
Nitrogen and O2 make up 99% of atmosphere

Into Thin Air – Mount Everest

Everest is ~8848 m at its summit.
By the end of 2017 there had been 8,306 summits by 4,833 individuals – most of these ascents were since 2000.
There have been 288 fatalities – 3.5 for every 100 summits.
Many fatalities were in people who had already reached the summit and were descending.
Over 8,000 m has been termed the ‘Death Zone’. The air is too thin for a helicopter to rescue you. The stratosphere begins at 8000m – you’re literally entering another atmosphere.

Olympic performances

Reduced air density at altitude offers less resistance to movements at high speeds and so may benefit short distance events.
Long distance events are challenging at altitude because of reduced oxygen level which the effects can begin being noticed at 1000-1500m.

Physiological stress at altitude

Oxyhaemoglobin dissociation curve

The partial pressure of oxygen determines the amount of oxygen that combines with haemoglobin (protein iron molecule that transports O2)
The normal partial pressure of oxygen at sea level is 159 mm Hg (760 mm Hg × 0.2093).
In the alveoli at sea level the normal partial pressure of oxygen is 104 mm Hg meaning that haemoglobin is 98% saturated with oxygen.

Reduced ambient PO2 at altitude

At 5486 m pressure of a column of air equals about one-half of its sea-level pressure.
The higher you go in altitude the more the column of air above you reduces in pressure.
At Everest summit ambient air pressure drops to 251-253 mm Hg.
However, alveolar PO2 is ~25 mm Hg. This means that haemoglobin is only ~30% saturated!!

Oxygen availability at Sea Level vs Altitude

The ‘oxygen transport cascade’ refers to the progressive change in the environment’s oxygen pressure and in various body areas.

Cardiovascular function at altitude

Oxygen uptake = Cardiac output × arterial-venous oxygen difference (L∙min-1) (L∙min-1) (ml∙100ml-1)
aVO2: Difference between O2 in your artery (away from the heart to tissues) to that in your venous blood (deoxygenated blood back to heart) – how much as been extracted from the blood stream.
There’s less O2 in the air per litre of blood so your heart has to work harder to make up for that.

Submaximal exercise: What does your body do to compensate for these altitude changes?

In order to compensate for less blood per L of O2 your heart rate increases during submaximal work at altitude as each litre of blood is carrying less oxygen. Thus, more blood must be pumped per minute in order to compensate.

Increased effort

Decreased maximal oxygen consumption at altitude compared with sea level means the same exercise at altitude represents a greater proportion of effort.

Increased ventilation

Positive of altitude: There is less pressure on your body/lungs so ventilation is easier and requires less ‘effort’, though you do have to ventilate more frequently.
To consume the same amount of oxygen at altitude as at sea level ventilation has to increase.
At ~5500 m (Everest base camp) the number of O2 molecules is half of sea level. Thus, a person has to breathe at least twice as much to get same amount of air.
Maximal attainable ventilation is increased in hypoxic environments and values above 200 L/min are often observed.

Adaptation at altitude

Main advantages for altitude training is for aerobic athletes as anerobic dominent activities are too short to allow the low O2 levels be a major factor to performed

Acid-base adjustment

Hyperventilation at altitude decreases arterial and alveolar carbon dioxide.
Ambient air contains essentially no CO2 so increased breathing volumes at altitude dilute normal alveolar CO2 concentrations creating a larger than normal gradient for diffusion of CO2 from the blood to the lungs = the diffusion of CO2 from the body is improved.
Exposure to 3000 m reduces alveolar PCO2 from 40 mmHg at sea level to 24 mmHg. Alveolar PCO2 decreases to 10 mm Hg during a prolonged stay at altitude.

Lactate paradox

On immediate ascent to altitude a given submaximal exercise load increases blood lactate concentration because of greater reliance on anaerobic glycolysis because there’s less air molecules in the air.
Some suggestion that long-term exposure to altitude reduces lactate concentrations at submaximal and maximal workloads but this has been challenged.

Blood adaptations

Exposure to altitude rapidly increases red blood cell production and haemoglobin concentrationn stimulated via a higher EPO release. This is the main adaptation benefit that athletes aim to acheive by training/living at atlitude.
Plasma volume is initially decreased at altitude but after several weeks expands in parallel with the increase in haemoglobin.
Blood of a typical miner in Andes contains 38% more erythrocytes than lowlander. Climbers acclimated at 6500 m during a 1973 Everest expedition showed a 40% increase in haemoglobin and 66% increase in haematocrit concentration.

Body composition and appetite

Reduced lean body mass from muscle fibre atrophy and reduced body fat.
The reduction in lean mass may be advantageous as the number of capillaries per muscle fibre is unaltered (so you get more capillaries serving a single muscle fibre) by altitude exposure but fibre size reduced. This would lead to more capillaries per square millimetre and a reduction in the diffusion area. The functional significance is a reduced diffusion distance and a longer transit time (oxygen transit time and extraction time is increased) at a given blood flow.
- Thus it can be advantageous to be lighter in bodyweight because more O2 is available per area of muscle
Depressed appetite and energy intake.
Decreased intestinal absorption.

Training at altitude

Whilst living and training at altitude increases the number of red blood cells, exercise intensity is reduced. Evidence for a performance improvement with altitude living and training is controversial.

From: Exercise Physiology: Energy, Nutrition and Human Performance. McArdle WD, Katch FI, Katch VL 7th Edition, 2010. Lippincott Williams and Wilkins.

Preparing for altitude

People train at sea level and live at altitude in order to maintain high intensity of training at sea level and simulate living at altitude.
Main adaptation they’re looking to take advantage of is improving RBC adaptation which living to atlitude

1.Hypobaric chamber – simulate barometric pressure of pre-selected altitude.

2.Altitude houses – houses which increase nitrogen content and thereby decrease O2 to ~15%.

3.Altitude tent – supplies 15% O2.

Summary – altitude

Altitude represents a challenge to endurance athletic performance mainly from the reduced partial pressure of oxygen and lower oxygen availability.
Maximal oxygen uptake is decreased with altitude, whilst submaximal heart rate, effort and pulmonary ventilation are all increased.
Longer stays at altitude lead to increases in haemoglobin and red cell volume, reduced body mass, muscle fibre atrophy, possible reductions in lactate and the long-term increased ventilation leads to respiratory alkalosis.
There is much debate about whether training at altitude can improve sea-level performance.
Use of hypobaric chambers may allow athletes to live high and train low to improve athletic performance, and also allow mountaineers and balloonists to simulate altitude.