Thermoregulation during exercise

Question 1

Define mechanical efficiency and what is a typical value in humans.

Answer 1

Mechanical efficiency is the % of total chemical energy consumed that is converted in external work with the rest lost as heat. Mechanical efficiency of the human body is estimated to be about 15-30%.

Question 2

Explain the relevance of thermoregulation in the human body.

Answer 2

Mechanical efficiency is about 15-30%. This means that 15-30% of the chemical energy consumed is converted into external work, and the rest (70-85%) is lost as heat. This would raise the body temperature by a lot and would kill us if we did not have mechanisms to dissipate this heat.

Question 3

What is the effect of starting an activity at a lower body temperature?

Answer 3

When we start exercising at a lower body temp, it takes more time before max temperature is reached. At around 40°C, we stop exercising voluntarily because of the perception of effort that is increased at that temperature

Question 4

How does the body react to high body temperatures during exercise?

Answer 4

It appears to be coming from the central nervous system and trigger a progressive inhibition of the brain areas responsible for motor activation when the core temperature increases above normal. Data supports a model of decreased skeletal muscle recruitment by the brain (reduced limb movement)

• In a study performed in goats, elevated hypothalamic temperature independently of the temperature of the rest of the body core reduced the ability and willingness of the goats to continue exercise.

Question 5

Why is heat released during exercise? Where is it coming from?

Answer 5

Energy lost as heat is associated with the contractile process (heat of contraction) or by inefficiencies after energy has been converted to mechanical energy.

Question 6

How do the muscles make force?

Answer 6

• In the muscle, energy is provided by ATP hydrolysis, which produces force and releases heat.
• ATP + H2O = ADP + Pi + (energy)
• Energy = mechanical work (W) + heat
• During the process of mitochondrial respiration, some of the energy released by “burning” of food is captured by ATP, to be released subsequently
• The conversion of ATP to ADP acts as the main energy source for the biosynthetic processes in the cell that are necessary for maintaining life

Question 7

What is Gibbs energy?

Answer 7

Gibbs energy (G): the amount of mechanical work which can be obtained from a given quantity of a certain substance in each initial state.
∆G = ∆H – T∆S
o ∆H is change in enthalpy (if positive heat is absorbed, if negative, heat is released)
o T is Temperature
o ∆S is change in entropy (a thermodynamic quantity representing the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.)

Question 8

What needs to occur for a reaction to take place “spontaneously”?

Answer 8

For a reaction to take place “spontaneously”, i.e. without additional energy input, ∆G must be negative. For this to happen, either entropy must rise, or energy released as heat or both. It can also take place if entropy decreases, but then an even greater amount of energy must be lost as heat
Therefore, as a living being, we constantly release heat (from mitochondrial respiration) to keep our ordered existence. Heat is not a “waste” it is a requirement for life.

Question 9

What is cytosolic ΔGATP at rest ?

Answer 9

about −65 to −70 kJ/mol

Question 10

What is the activation energy of myosin ATPase?

Answer 10

approximately 40 kJ/mol (>22°C) –> 1 molecule of ATP is enough to activate 1 myosin molecule.

Question 11

How is heat accumulation during exercise calculated?

Answer 11

S = M − (± W) ± (R + C) ± K − E

• S = rate of body heat storage
• M = rate of metabolic energy production
• W = mechanical work, either concentric (positive) or eccentric (negative) exercise
• R + C = rate of radiant and convective energy exchanges (see below)
• K = rate of conduction (important only when in direct contact with an object, such as as water; see definition of conduction below)
• E = rate of evaporative loss.

Question 12

What is radiation?

Answer 12

Infrared-mediated transfer of heat from a warmer surface to a cooler surface with no physical contact involved

Question 13

What is convection?

Answer 13

Heat is transferred either to air or water in contact with the body.

Question 14

What is conduction?

Answer 14

Transfer of heat between surfaces in contact with each other.

Question 15

Which muscles increase core temperature during exercise?

Answer 15

Heat released by the working muscles increases core temperature. While active, skeletal muscles rapidly warm during exercise, the inactive skeletal muscles demonstrate only a modest increase in temperature. Blood that perfuses active skeletal muscles is warmed and carries the heat to other body regions, which consequently elevates core temperature.
Thus, the non-working muscles and tissues also warm up.

Question 16

How do we sense changes in body temperature?

Answer 16

The human “thermostat” is located in the hypothalamus. The anterior hypothalamus deals with high temperatures and the posterior hypothalamus deals with cold temperature regulation.

Question 17

What does homeotherm mean?

Answer 17

o Homeotherms: Maintains a stable internal body temperature regardless of external influence

Question 18

Why do we maintain our body temperature stable?

Answer 18

To protect our brain

Question 19

What is usually the difference between skin and core temperatures?

Answer 19

The difference between skin temperature and core temperature on a very cold day can reach 20°C. Normally, it is 4°C (Skin: 33°C, Core: 37°C)

Question 20

What can affect our set body temperature (Tset)?

Answer 20

o	Biological rhythms (e.g. day and night)
o	Exercise training and heat acclimation (decreases set temperature) --> Allows us to train longer and/or maintain intensity in the heat if trained (see graph of esophageal temp vs training time)
o	Pyrogens (fever), set temp is increased to fight an infection.

Question 21

How does our body control our body temperature?

Answer 21

• Hypothalamus checks the core temperature (Tc) and compares it to the set temperature
• If a different is present, the hypothalamus generates a thermal error signal
• This signal, along with the skin temperature (Tsk) generates a number of responses:
  o Behaviour signals (cerebral cortex) –> simply to change our behaviours re: thermal comfort
  o IF Tc TOO WARM: Effector signal for sweating and vasodilation (sweat glands and arterioles of the skin)
  o IF Tc TOO COLD: Effector signal for vasoconstriction (skin arterioles and superficial veins) + Effector signal for heat production (e.g. shivering) in skeletal muscles

Question 22

When the body is in a state of heat stress (for instance, exercise + fever), how can brain damage occur?

Answer 22

o Decrease in cerebral blood flow = Brain damage, cerebral edema, cerebellar atrophy

Question 23

When the body is in a state of heat stress (for instance, exercise + fever), how can multi-organ system dysfunction occur?

Answer 23

o Vascular endothelium damage = coagulation, microvascular thrombosis, consumptive coagulation = multi-organ system dysfunction
o Decreased gut blood flow = ischemia, nitrosative and oxidative stress, decreased gut epithelial membrane permeability –> endotoxin leakage (gut bacteria leave the gut, reaches tissues), Systemic inflammatory response syndrome (SIRS) (activation of immune system in body, can lead to coagulation and multi-organ system dysfunction)

Question 24

When the body is in a state of heat stress (for instance, exercise + fever), how does skin blood flow change?

Answer 24

There is an increase in skin blood flow as a protective mechanism, but in cases of heat stress this is a vicious cycle leading to further increase in Tc.

Question 25

How does the body respond to an increase in heat by muscles, in terms of blood flow?

Answer 25

Hyperemia is the increase of blood flow to different tissues in the body.

• Blood flow increases with higher work rate
• Parallel to an increase in pulmonary function

Question 26

How much blood flows through the skin per min, at rest? How much heat does this release?

Answer 26

At rest, approximately 250ml of blood flows through the skin per min, releasing around 100 Watts of heat mostly as infrared radiation. During exercise this rate can go up to 8L/min.

Question 27

How does skin blood flow increase?

Answer 27

In non-glabrous skin, a sympathetic active vasodilator system is responsible for up to 80– 90% of the increase in skin blood flow during significant whole-body heat stress. The degree of vasodilation depends on the combination of skin and internal (core) temperatures.

Question 28

Up to which point can skin blood flow increase during exercise?

Answer 28

When dynamic exercise is prolonged, skin blood flow rises with internal temperature until the latter reaches about 38 °C, after which internal temperature continues to increase, but skin blood flow reaches an apparent upper limit. This limit is well below maximal values for skin blood flow and well below levels expected to attend hyperthermia at rest. In the case of hyperthermic exercise, the upper limit is about 60% of expected maximal values and does not increase further, despite further elevations of internal temperature of around 1 °C. This is due to the competition for blood flow between different systems (including skeletal muscles)

Question 29

What determines the skin blood flow?

Answer 29

The gradient between core and skin temperatures. Lower gradient = greater blood flow to the skin
Equation for skin blood flow: Qs= 1/C*h/(Tc –Tsk), where C is the specific heat of blood (∼0.87 kcal °C−1 l−1), h the heat production (in kcal min−1) and Qs the skin blood flow.
Hot skin narrows the Tsk to Tc gradient, which increases skin blood flow requirements.

Question 30

Why is there a stress on the body when we exercise in warm conditions?

Answer 30

during intense exercise, blood flow is also required in muscles (in addition to the skin when the Tsk is high, leading to a low gradient), which increases stress on the body.

Question 31

What is favored when there is competition for blood flow between skin and muscles?

Answer 31

The competition between skin and active muscle for the available blood flow during exercise in the heat favors the delivery of blood flow to muscle.

Question 32

What happens when muscles get more blood but the skin cannot shed enough heat due to the limited blood flow to the skin?

Answer 32

The increase in skin blood flow is often not sufficient to maintain heat balance and internal temperature continues to increase. This increase is self-limiting, however, as the increase in central temperature to near 40 °C causes a fatigue that limits the exercise duration and level (through nerve inputs).

Question 33

Name 4 ways used by the body to shed heat.

Answer 33

1. Evaporation
2. Radiation
3. Conduction
4. Convection

Question 34

What is radiation, and how does it work for body temperature regulation?

Answer 34

Radiation can help the body lose heat (if Tsk > environment) and gain heat (solar radiation)

Infrared-mediated transfer of heat from a warmer surface to a cooler surface with no physical contact involved. However, heat can be gained if the surrounding temperature is high, such as under direct sunlight. The sun can add 7°C under direct exposure to its rays.

Question 35

Ar rest and RT, what is the proportion of heat that is lost from the body through radiation? Evaporation?

Answer 35

At rest and room temperature (21°C), 60% of heat loss occurs via radiation since the skin temperature (33°C) is higher than that of surrounding objects.
25% of heat is lost by evaporation at rest. However during exercise, evaporation coupled to sweating is THE MAJOR cooling mechanism.

Question 36

What is conduction, and how does it work for body temperature regulation?

Answer 36

Transfer of heat between surfaces in contact with each other. Minimal heat loss mechanisms in humans normally. Conduction can occur via the application of an ice pack to the neck for example.

Question 37

What is convection, and how does it work for body temperature regulation?

Answer 37

Heat is transferred either to air or water in contact with the body. Warmed molecules are moved away and replaced by cooler ones. Highly dependent on airflow over the body. Water effectiveness is 25X than air at cooling, at the same temperature (Swimming vs running vs cycling)

Question 38

What is evaporation, and how does it work for body temperature regulation?

Answer 38

Evaporation of water from body surface or breathing passages. Sweating per se does not cool the skin, it is the evaporation of water molecules heated by the skin into gas that cools it.

Question 39

What is evaporation dependant on? (4 factors)

Answer 39

o 1- Temperature (high temperatures decreases evaporation rate)
o 2- Humidity (high humidity decreases the vapour pressure gradient required for evaporation)
o 3- Convective currents (increase air movement, increases evaporation)
o 4- Skin surfaced exposed to the environment
• Might be beneficial to have some skin still covered to diminish radiation
• Some textiles can diminish radiation while still favoring evaporation

Question 40

Why is sweating important in humans?

Answer 40

• Sweating occurs over a large surface area, making it a powerful cooling system
• Sweating occurs independently of the respiratory cycle (more freedom than e.g. animals panting)
• Efficiency of sweating is increased by lack of fur, which greatly compensates for increase in solar radiation gain
• The amount of water lost by sweating and respiration is regulated in humans depending on conditions.

Question 41

Which sports have a lower sweat rate?

Answer 41

Water sports

Question 42

What usually occurs to our fluid status during sports?

Answer 42

• We usually accumulate a deficit in water during sports. % dehydration is high
• We have a tendency to voluntarily dehydrate

Question 43

What were the Nevada research group findings on dehydration?

Answer 43

• People drank less than what they lose in sweat or urine, even in presence of available fluids. This is termed voluntary dehydration. Usually corrected after exercise or when food is eaten. –> Liquid losses are not entirely corrected during exercise
• Day-long marches in the desert were more frequently terminated in the non-drinking group compared to the freely drinking group.
• Subjects usually stopped during the marches when they experienced 7 – 10 % loss of starting body weight. They usually experienced postural hypotension. No major risk was found associated with these levels of dehydration.
• Serious risk for organ failure is believed to occur at 15- 20% dehydration.
• Rectal temperature and heart rate rose as a linear function of the level of dehydration.

Question 44

Explain the relationship between finishing times in marathon/triathlon, and fluid losses.

Answer 44

• People who finished faster in a triathlon/marathon tended to lose more weight during the race, thus more dehydration
• Dehydration per se causes a decrease in performance; However someone who has a higher decrease in wt did not run the whole time with that deficit, it accumulated over time

Question 45

Explain the relationship between sweat rate and finishing time

Answer 45

There is a close relationship (correlation) between the mean sweating rate and the time taken to complete a race. More sweating = faster time

Question 46

Explain the relationship between post-race rectal temperature and finishing time

Answer 46

• Athletes who run the fastest have the highest metabolic rate and as a result the highest sweat rates and highest post-race rectal temperature.

Question 47

What is the major determinant of core body temperature during exercise?

Answer 47

The major determinant of core body temperature during running exercise is relative exercise intensity and not the state of dehydration.

Question 48

When is heatstroke the most likely?

Answer 48

High running speeds constitute the greatest risk factor for heatstroke. For example, heatstrokes can occur in short events in ambient temperature, e.g. in near-maximal effort of 15 – 60min duration. Interestingly, in athletes running record-level performances up to 16km, there is limited blood flow to the skin, impairing heat loss.

Question 49

What is the best symptom of a fluid deficit (hypohydration)? What are other symptoms reported?

Answer 49

Thirst

- Others include feeling tired, weak, light-headed, weariness, dizziness

Question 50

What is EID?

Answer 50

exercise-induced dehydration (EID) (up to 4% BW loss)

Question 51

Does EID alter exercise performance?

Answer 51

• Compared with euhydration, exercise-induced dehydration (EID) (up to 4% BW loss) does not alter cycling performances during out-of-door exercise conditions;
• Relying on thirst sensation to gauge the need for fluid replacement maximises cycling TT performances.

Question 52

In which situations should you rely on thirst vs. forcing yourself to drink?

Answer 52

In longer runs, you sweat less because the intensity is lower, so drinking too much or too often might be detrimental (longer runs = more likely to be drinking as well)
Sometimes, it may be useful to follow thirst vs. forcing yourself to drink
It was shown that decreasing the number of hydration stations in runs can decrease the prevalence of hyponatremia cases

Question 53

Which impact does heat have on performance?

Answer 53

The wet-bulb globe temperature (WBGT) is a measure of apparent temperature that includes the effect of air temperature, humidity, wind speed and radiation (direct and indirect). -= much better reflection of the temperature we feel
- Not surprisingly, if you go from a WBGT of 10 to 25, marathon time increases due to a decrease in performance

Question 54

What occurs in heat acclimatization?

Answer 54

• Decrease in core temperature, and increase in temp with exercise is more regulated
• HR increases much more slowly in heat-acclimatized people (blood flow increases to shed heat)

Question 55

How long does it take to acclimatize to heat?

Answer 55

Heat acclimatization (active) fully develop after 7 to 14 days, however continued improvement may occur up to a month. It is retained for at least a week following return to a cooler climate and will disappear over the next 28 days.

Question 56

Name a few functional outcomes and biological adaptations associated with heat acclimation.

Answer 56

• Improved thermal comfort
• Reduced core temp
• Improved sweating
• Reduced skin temp
• Improved skin blood flow
• Spared muscle glycogen
• Increased lactate threshold
• Lowered muscle and plasma lactate
• Increased max aerobic power
• Improved submax exercise performance
• Improved thirst
• Reduced electrolyte losses
• Increased TBW
• Increased plasma volume
• Lower HR
• CO better sustained
• Increased stroke volume
• Better defended BP
• Increased myocardial compliance
  etc. ..

Question 57

Explain the challenge with exercising in the cold.

Answer 57

In a cold environment, heat loss may exceed heat production, resulting in a decreasing core temperature. Hypothermia therefore can develop.

Question 58

What occurs when core temperature drops below 35°C? 33°C?

Answer 58

• Once the core temperature falls below 35°C, mental functions are impaired, and the blood pressure falls.
• Below 33°C, mental confusion develops, and limb muscles become rigid and immobile. Loss of consciousness will occur shortly thereafter and can lead to death.

Question 59

Explain the relationship between performance and muscle temperature

Answer 59

During short duration exercise, there is a positive relationship between performance and muscle temperature, i.e., neuromuscular function is impaired by cold temperature and improved by hot temperature. The variation in performance ranges from 2% to 5% by 1°C of change in muscle temperature and is more marked for fast than slow movement. However, if central temperature increases (i.e., hyperthermia), this positive relation stops, and performance becomes impaired.

Question 60

What is the greatest determinant of performance in a cold environment?

Answer 60

Muscle temperature.

• Decreased temperature impairs muscle performance.

Question 61

What is a normal muscle temperature at rest?

Answer 61

• Normal muscle temperature at rest (33-35°C).

Question 62

Which aspects of muscle function are harmed by cold conditions?

Answer 62

biochemical, neuromuscular, circulatory

Question 63

Which different strategies can be used to regulate body temperature when exercising in the cold?

Answer 63

1- Change in behavior (clothing, shelter)
2- Vasoconstriction, restricting loss of heat via skin circulation (but extremities (fingers, toes) can suffer!)
3- Shivering, generation of heat via the muscles
4- Exercise can generate more heat than shivering, however it also increases skin blood flow and increase convection that may increase heat losses