How Aussie cyclists cope with the heat during RoadNats

The Federation University Road National Championship places enormous physical demands on riders. Learn just how challenging the race can be in our latest article from Fed Uni Sport Scientists.

By Dr. Brendan J O’Brien and Dr. Ryan Worn

The Federation University Road National Championship in Buninyong and Mount Helen is a gruelling race that presents an enormous physical challenge for competitors.

The 11.6 km course has an excruciating muscle-burning elevation of 205 metres and maximum grade of 8.4%. The women will cover 104.4 km (9 laps), while the men cover 185.6 km (16 laps).

The 2022 men’s winner, Lucas Plapp averaged an impressive 38 km/h and women’s winner, Nicole Frain averaged an equally impressive 35 km/h. To highlight the immense challenge of the road race, of the 94 men’s starters in 2022, only 21 completed the race.

Exercise generates heat from the metabolism of carbohydrate, fat, and protein to produce the energy that powers the muscles. As exercise intensity increases, metabolism, and heat production increases proportionally.

Heat production is measured in kilocalories (kcal). The body regulates its core-temperature around 37°C. At rest a person typically generates 1.4 kcal every minute (or 105 watts). The body easily dissipate this heat, so the body’s core-temperature does not increase and remains stable.

However, at maximal intensity exercise a person can generate 20 kcal every minute (1400 watts). Consequently, if the body did not have a means of eliminating heat, in theory the core temperature could rise by ~1°C every 5-7 min during maximal exercise.

Photo: Con Chronis

Compounding the physical challenge is the fact that the race occurs frequently in searing heat. On the day of this year’s road race, the ambient temperature is forecast to be 32°C.

Exercise in the heat reduces endurance performance. A 1997 study showed that cycling for as long as possible at a threshold pace in 10°C could be sustained for 92 minutes, 81 minutes in 21°C and for only 54 minutes in 30°C.

Maintaining race speeds nudging 35-40 km/hr for 3 to 4.5 hours in hot temperatures also poses a high risk of the cyclist developing hyperthermia, and more seriously, the medical condition of heat stroke. A 3°C rise in core temperature to 40°C from extreme exercise is potentially life-threatening.

It is expected that many of the cyclist’s core temperatures will exceed 40°C on Sunday. It appears that whether a person is an elite cyclist or unfit, once a critical threshold of 40°C is reached, brain function and the capability to activate muscles is adversely affected.

Exercise in the heat particularly poses a physiological challenge as the amount of blood in the body is finite (generally humans have 5-6 litres of blood), and the leg muscles require proportionally more blood flow to cycle at faster speeds.

However, one of the most important ways of eliminating heat from the body is to direct more blood to the skin, where the heat can be dissipated from the body. Ultimately, the body must compromise where its finite volume of blood is delivered.

Photo: Simon Wilkinson

Furthermore, the body loses a significant proportion of its fluid through sweating, which reduces blood volume. The average cyclist will lose 1-2 litres of sweat an hour. Consequently, some cyclists may lose over 8 litres of fluid (or approximately 15-20% of the total body water volume) from their body in a 4-hour race.

Cyclists will also lose a significant amount of sodium (as salt) in the sweat. Sodium is very important for the nervous system to generate electricity necessary to control the body.

The average adult contains approximately 80-90 grams of sodium. Sodium losses in sweat amount to 1 to 1.5 grams per litre. The combination of a loss of body water and sodium compromises the cardio-vascular system’s capability to distribute blood to cells and absorb heat, and can create life-threatening disturbances in the nervous system’s ability maintain to muscle, brain, and heart function.

Sweating is the principal method of eliminating heat from the body. It is important to point out that sweat should not be wiped away. It should be allowed to evaporate from the skin into the air, as heat from the body is transferred to the perspiration beading on the body. The evaporation of sweat is how heat escapes the body.

The maximal rate of heat loss via sweat production is approximately 18 kcal/minute. As the body of an elite cyclist can produce more than 20 kcal/minute, ultimately the heat gain potential is greater than heat loss potential. Consequently, the body’s core temperature will inevitably climb during intense prolonged exercise.

To optimise exercise tolerance in the hot weather and mitigate the risk of heat stroke, cyclists need to minimize the rise in core-temperature during exercise to the critical threshold of 40°C. There are four main strategies the cyclist can employ: 1) optimise their cardio-respiratory fitness, 2) acclimatise to the heat, 3) cool themselves prior to exercise, and 4) ingest fluid during exercise.

Photo: Con Chronis

Individuals with a higher cardio-respiratory fitness have a lower core temperature at rest. Superior cardio-respiratory fitness increases skin blood flow and lowers the threshold core temperature at which sweat appears and skin blood vessel dilation occurs. These adaptations enhance heat dissipation in the heat to reduce the rise in core-temperature during exercise.

Heat acclimation refers to the physiological adaptations that occur from repeated exposure to heat stress. Generally, within 7 to 14 days of regular heat exposure exercise training, exercise heart rate is lower, and the fluid component of blood (plasma) expands. The production of sweat and dilation of skin blood vessels to accommodate greater skin blood flow occurs at a lower core-temperature. These adaptations ensure the heat produced by the body is eliminated earlier to prolong high intensity cycling.

Pre-exercise cooling involves lowering the core-temperature before exercise to extend the body’s heat storage capacity to delay the onset of fatigue. The various pre-exercise cooling methods include cold water immersion, cold air exposure, wearing a cold vest and drinking cold fluid and ice slurries. These methods are effective in lowering core-temperature before exercise and generally reduce the thermal strain during exercise to enhance endurance cycling performance.

Drinking fluid during exercise is designed to replace water lost from sweat. Studies have compared drinking ad libitum (to individual discretion upon feeling thirsty) versus drinking specific amounts to limit body water loss to 2-3%. Drinking ad libitum while cycling replaces approximately 55% of fluid losses. Compared to drinking a prescribed fluid volume, drinking ad libitum is equally effective at maintaining performance.

Inclusion of salt (sodium) may be beneficial for events longer than 3-4 hours, particularly for salty sweaters. Therefore, rather than rigidly drinking to set fluid volume regimen, it is recommended to drink simply when thirsty, and importantly to never ignore thirst. That distinctive dry feeling in your mouth is nature’s finely tuned way of informing you when you need fluid. The current recommendation for salt inclusion in a drink is “season to taste”.

Interestingly, it is important to not over-hydrate during prolonged endurance competition. Drinking too much fluid may overdilute the blood (most sports-drinks are not as salty as blood) to cause hyponatremia, a low blood sodium condition that is potentially fatal. Generally, up to a 2% overall loss in total body water content throughout a race will not affect the finishing time.

A 2019 systematic review of the research investigating the impact of aerobic fitness, heat acclimation, pre-cooling, and fluid ingestion on endurance performance and the rise in core-temperature during exercise found aerobic fitness is the most effective heat mitigation strategy, followed by heat acclimation, pre-exercise cooling and lastly, fluid ingestion.

Brendan O'Brien (PhD) is an Associate Professor in Exercise Physiology at Federation University Australia and an advocate of physical activity for health. Dr Ryan Worn is a lecturer in the Exercise Science Discipline at Federation University and a cycling tragic.

References

1. Alhadad, S. B., Tan, P. M. S., & Lee, J. K. W. (2019). Efficacy of Heat Mitigation Strategies on Core Temperature and Endurance Exercise: A Meta-Analysis. Frontiers in physiology, 10, 71. https://doi.org/10.3389/fphys.2019.00071
2. Galloway, S. D., & Maughan, R. J. (1997). Effects of ambient temperature on the capacity to perform prolonged cycle exercise in man. Medicine and science in sports and exercise, 29(9), 1240–1249. https://doi.org/10.1097/00005768-199709000-00018
3. McCubbin A. J. (2022). Modelling sodium requirements of athletes across a variety of exercise scenarios - Identifying when to test and target, or season to taste. European journal of sport science, 1–9. Advance online publication. https://doi.org/10.1080/17461391.2022.2083526

Main Photo: Con Chronis