S2, L10: Exercise in the cold

Describe the significance of heat loss in cold water

• Water thermal conductivity is is 26 times greater than air

• Convection, radiation, conduction and evaporation are 4 times greater in cold water than in air

• Heat loss increases in moving water

• Hypothermia occurs in water well above 0°C

Describe the body’s response to cold water immersion

• Cold shock response - gasping, hyperventilating and vasoconstriction

• Cold incapacitation - hyperventilation and vasoconstriction reduce the ability to swim

• Blood flow to the periphery is reduced - arms and legs full of blood reduces the ability to swim

Why would cyclists wear newspaper under their jersey if there were large descents?

To soak up sweat because there is a massive thermic effect of cooling when descending, so the newspaper reduces heat loss from evaporation

Describe thermoregulatory control for the cold

• Regulated by the preoptic-anterior hypothalamus via a negative feedback system

• Different signals to in the heat are sent from the sympathetic nervous system to induce vasoconstriction and shivering

Describe the effectiveness of shivering

Shivering gives a 4-5 fold increase in heat production, but requires fuel

What is non-shivering thermogenesis?

Heat production by brown adipose tissue from chemical reactions

What skin temperature is the thermoneutral zone?

20-35°C

At what temperatures do cold responses begin and maximise?

Non-shivering thermogenesis and shivering begin at a skin temperature of 18°C and maximise at 7°C

What is cold habituation?

Repeated cold exposure without heat loss

What are the effects of cold habituation? (2)

• Skin vasoconstriction and shivering responses are blunted

• Core temperature is able to drop further

What is metabolic acclimation?

Repeated cold exposure with heat loss

What is the effect of metabolic acclimation?

Increased metabolic and shivering heat production

What is insulative acclimation?

When increased metabolic heat production can’t stop heat loss

What are the physiological responses to exercise in the cold? (7)

• Decreased muscle function

• Altered fibre recruitment

• Decreased shortening velocity and power

• Decreased dexterity (fine motor control)

• Increased carbohydrate utilisation compared to thermoneutral, still less than in the heat

• Decreased metabolic heat production due to fatigue

• Decreased heart rate due to vasoconstriction

How does fatigue impact cold tolerance?

5 hours of exercise or a control followed by 4 hours in the cold gave no between-group differences, however there was wide inter-individual variability in cold tolerance

How is energy expenditure impacted in the cold?

It increases greatly, e.g., to cover 10 miles in Antarctica, 11,000 kcal is expended and after 22 weeks, nearly 30 kg of body mass could be lost due to negative energy balance

Explain physiological fuel usage responses to the cold at rest (2)

• Free fatty acid mobilisation is reduced because vasoconstriction reduces blood flow to subcutaneous tissue

• Muscle glycogen utilisation increases because hypoglycaemia stops shivering

Describe the effectiveness of a high carbohydrate diet for exercise in the cold

When exercising at 70% of VO_{2 max} in the cold, a high carbohydrate diet increases time to exhaustion by 69 minutes in comparison to a low carbohydrate diet

Compare carbohydrate usage in the cold to the heat

• Carbohydrate oxidation is 36 g/2 hours more than during warm conditions

• In the heat, a high carbohydrate diet still increases time to exhaustion but not to the same extent as in the cold (53 vs. 44 minutes)

Why is water lost in cold conditions? (3)

• Blunted thirst

• Increased respiration

• Cold-induced diuresis (increased urine output)

Explain why there is cold-induced diuresis

• Vasoconstriction increases blood pressure, which decreases AVP/ADH secretion

• AVP/ADH reduces urine output, so lower secretion increases urine output, potentially leading to hypohydration

What methods of cold as recovery are there? (2)

Cryotherapy and cold water immersion

What is the effectiveness of cryotherapy for recovery?

Crytherapy led to significantly lower perceived soreness than a control group at 1 hour, 24 hours and 48 hours after exercise, but perceived soreness was always significantly higher than pre-exercise until 168 hours post-exercise

What is the disadvantage of cold water immersion? Support with evidence

• 20 minutes of cold water immersion had no significant gain in maximal muscle strength post-training in comparison to pre-training, whereas a non-cooling group had a significant strength increase

• The cooling group therefore had a decrease in adaptations whereas the non-cooling group did not

• Application of cold water immersion immediately after bouts of resistance training may attenuate hypertrophic changes