Thermoregulation during exercise

Question 1

What physiological changes occur in hot conditions? (2)

Answer 1

1. changes in circulation to increase peripheral blood flow:
   - sympathetic NS signals skin arterioles to dilate and divert blood to peripheries to cool core
   - increase HR, contractility and Q to compensate in BP fall
2. increase in sweating (evaporation) through the stimulation of eccrine sweat glands to secrete sweat

Question 2

What physiological changes occur in cold conditions? (3)

Answer 2

• behavioural related changes
• reverse process of heat reponses
  1. peripheral vasoconstriction: peripheral artieries constrict to maintain blood closer to core, acts to limit heat loss via skin.
  2. non-shivering thermogenesis: sympathetic NS stimulates metabolism to generate heat = increased energy use
  3. shivering: rapid involuntary cycling of contraction/relaxation of skeletal muscles to generate warmth similar to exercising.

Question 3

What is the physiological range of core temperature? (6)

Answer 3

normal range = 31-41 deg.

> 45 = possible death without treatment
40 = profound clinical hyperthermia (heat stroke)
38.5 - 39.5 = mod. hyperthermia (heat exhaustion)
36.5-37 = normothermia
33-25 = mod. hypothermia
<24 = possible death without rewarming

Question 4

How is the thermoregulatory response controlled? (3)

Answer 4

1. temperature changes measured by thermoreceptors in the blood as it passes through the hypothalamus and peripheral temperature sensors in the skin, abdominal viscera, spinal cord and greater veins
2. Hypothalamus collates efferent signals
3. Stimulates processes to cope with heat change (sympathetic ns)

Question 5

What is conduction? How does it relate to human thermoregulation?

Answer 5

direct heat transfer from one molecule to another through a liquid, solid or gas
requires direct contact

depends on:

1. temperature gradient between the surface
2. thermal qualities of the surfaces

when exercising our muscles create heat which is conducted out to surrounding tissue and blood

Question 6

What is evaporation? How does it relate to human thermoregulation?

Answer 6

loss of heat as water heats and converts into a gaseous form
- aided by convection

sweat allows heat to evaporate away from the skin - must evaporate not drip or be wiped away
- primary source of heat loss during exercise

Affected by water vapour pressure of the surrounding air and the gradient between the skin and the surrounding air

Question 7

How does heat affect exercise performance? (endurance, short sprints, repeated sprints)

Answer 7

endurance performance decreased

• competition for blood supply may limit supply to musculature
• decreased VO2 max, LT2 and time to exhaustion

Short sprints are unaffected

Repeated sprints performance decreased

• decreased glycogen stores
• similar to endurance

Question 8

Describe cardiovascular drift

Answer 8

Occurs when exercise continues for any length of time in the heat. HR increases even when exercising at a fixed intensity due to:

• continued competition for blood supply
• additional sweating causing plasma volume losses
• plasma is shifted from blood to tissues
• loss of plasma volume -> decreases EDV (decreases preload) and compromises Q

Question 9

What is acclimation?

Answer 9

using artificial means to expose the body to various conditions

Question 10

How does cold affect exercise performance? why?

Answer 10

overall decreased performance

• muscle require warm temp. to perform optimally
• decrease in muscular force
• decreased function of small muscles in peripheries
• as fatigue sets in and work rate decreases the ability to create heat decreases
• catecholamines form part of the sympathetic response to the cold; vasoconstriction to fatty tissues to maintain warmth means that access to these fuels is more difficult -> greater use of glycogen stores
• shivering increases VO2 required for a given intensity

Question 11

Describe critical temperature theory

Answer 11

At approx 40deg most people will voluntarily cease exercise.
Possibly a protective mechanism related to:
- critical temp of brain
- circulatory maintenance
- protect proteins from destruction

The CNS may act directly to inhibit motor recruitment and force production

also consider psychological factors

Question 12

What are the adaptations to cold conditions?

Answer 12

Limited ability to adapt to the cold. any adaptations are likely to occur during resting exposures.

Behavioural changes have a greater benefit e.g. clothing

Question 13

What are the adaptations to hot conditions in 5-10 days? how does it affect performance?

Answer 13

1. lowered salt content of sweat - dilute sweat preserves extracellular electrolyte concentrations

Question 14

What are the adaptations to hot conditions in 7-10 days? how does it affect performance?

Answer 14

1. improved cutaneous blood flow - transports metabolic heat from deep tissues to bodies shell
2. increased sweat output - maximizes evaporative cooling
3. lowered sweat threshold - evaporative cooling begins sooner
4. more effective distribution of sweat over skins surface - optimum use of surface area for cooling

Question 15

How would you design an acclimatization program for heat performance? (time frame, temp, intensity, length of session etc.)

Answer 15

• 7-14 days
• 30-35 deg. humidity to match venue
• short, mod. intensity (30-35 min @ 75% VO2 max)
• long, low intensity (60min @ >50% VO2 max)
• interval or continuous
• ideally every day but no more than 2-3 days between sessions
• dry heat better than humid

Question 16

What are the adaptations to hot conditions in 3-6 days? how does it affect performance? (1)

Answer 16

1. increased plasma volume - appropriate circulation to skin and muscles to meet demands of metabolism and thermoregulation; greater CV stability during exercise

Question 17

What are the adaptations to hot conditions in 5-10 days? how does it affect performance? (1)

Answer 17

1. lowered salt content of sweat - dilute sweat preserves extracellular electrolyte concentrations

Question 18

What are the adaptations to hot conditions in 7-14 days? how does it affect performance? (6)

Answer 18

1. improved cutaneous blood flow - transports metabolic heat from deep tissues to bodies shell
2. increased sweat output - maximizes evaporative cooling
3. lowered sweat threshold - evaporative cooling begins sooner
4. more effective distribution of sweat over skins surface - optimum use of surface area for cooling
5. decreased reliance on CHO during exercise
6. lowered Tcore, Tskin and HR for standard exercise

Question 19

What physiological changes occur with exercise in the heat? and why? (7)

Answer 19

1. working muscles and increased metabolism increased heat load
2. Tcore raised and heat balance maintained at a higher level
3. HR, contractility and Q increase to account for demands: blood needs to be diverted to peripheries to enhance cooling, decreased supply of O2 blood to working muscles
4. blood supply decreased to non-essiential tissues (digestive system, liver, kidneys)
5. HLa- clearance decreases since hepatic blood supply is reduced
6. greater contribution from anaerobic energy sources due to competition for blood supply to muscles

Question 20

What is pre-cooling and what are some methods?

Answer 20

Lowering resting Tcore prior to exercise to delay time to critical temp. 
Improves the sensation of thermal stress
Increases exercise performance
Methods:
- cooling jackets
- ice slushies
- cold towels
- cold water immersion (CWI)

Question 21

What effect does heat acclimatisation have on exercise performance? (6)

Answer 21

1. PV better maintained
2. HR remains more stable
3. time to reach critical Tcore improved/delayed
4. ability to withstand higher Tcore
5. improved endurance and team sport performance
6. improved time to exhaustion