Your top 5 questions about riding in the heat: answered

With the Olympics scheduled to take place in Tokyo, sports institutes around the world have turned their focus towards performance in hot and humid conditions, in order to understand the effect of heat on performance.

I was fortunate enough to be a study participant in one such study, looking into the effect of the menstrual cycle on time-trial performance in hot and humid conditions. We got to do a 22km/500m vert time trial in 35-degree heat with relative humidity of a gazillion (70%). It SUCKED. The things we do for science hey?

However, though these gruelling time-trials I learned more about my own physiology and thought about how to best manage my–physiologically confirmed–SIZZLING HOT self (I exceeded maximal allowable temperature of 40 degrees, though all but one of the time trials…).

The thing is, what we know about female performance throughout the menstrual cycle is, at this point, pretty contentious. For every study that says 'X' happens, there is another that refutes it; studies don't have large enough cohorts to make validated and confident claims about the effects of hormones on performance (and treat with suspicion anyone who states they have 'the answer').

But we do have some information, more generally, about riding and racing in the heat.

1. Why is my heart rate so high?

Exercise increases your core body temperature, and hence exercising in the heat adds an extra level of stress to your body. In order to combat this, your body aims to reach homeostasis (that equilibrium and balance which is tightly controlled at all times, unless there is severe illness or aberration) by diverting extra blood to the skin and eliciting a sweat response. When you're working hard (like in a bike race) you also require bulk perfusion and blood flow to skeletal muscle to allow you to push hard on the pedals, and due to these demands heart rate is increased. This can be heightened in humid temperatures, as the evaporative cooling that occurs in drier heat is diminished.

1. Why do some people fare better in the heat than others?

Heat tolerance is highly individual and varies with many factors, including body size and fat mass (fat is insulating), preconditioning, metabolic workload, sweat rate, hydration status and hormones. That list isn't likely exhaustive, but a good starting point. Workload is key here, and related to rider size. A larger rider generally generates more power (watts) than a smaller rider, and while the surface area of the larger rider is...larger, the rule of efficiency being a key determinant of endurance exercise performance comes in. It's accepted that cycling efficiency is around 18-23%, higher in trained athletes. What does this mean? It means that for an athlete that is 20% efficient, riding at 200w, the metabolic cost will be at the rate of 1000w. Imagine if we were 100% efficient beings!

Why is this important? Well that metabolic cost results in heat generation. Therefor a larger rider, even one that has the same w/kg as a smaller rider (say an 80kg rider averaging 300w compared to a 60kg rider at 225w: they both pan out at 3.75w/kg) is more likely to get hotter and be affected by heat, as their gross energy cost is (theoretically, at 20%) 1500w compared to 1125: 25% more.

Sweat rate and heat tolerance are two other related factors, that can be addressed with training, as can hydration.

1. I'm a female cyclist and some days I feel rubbish and have poor heat tolerance, and other days its fine WHYYYYY?

There is irrefutable evidence that in the later part of the menstrual cycle, there is a rise in body temperature that correlates with ovulation and lasts until the start of the next cycle. At this point, the effect of the cycle on absolute performance in temperate conditions draws a blank: there are studies that say there is a deleterious effect and ones that find nil effect. A systematic review and meta-analysis (gold standard in evidence!) indicates there is no strong evidence that performance is changed throughout the cycle. There is some evidence that in strength and endurance sports there is a mild reduction in the first phase of the cycle (immediately post period to ovulation), however the strength of this evidence is 'low' and not compelling.

There is evidence that athletes can feel hotter and have higher RPE in the later stage of the menstrual cycle, and it makes sense with the underlying physiology occurring that heat would be tolerated more poorly in this phase. If you are in this boat, it's a good idea to track your cycle and how you're feeling in training, and put in place some helpful interventions in the case of needing to perform in a phase that you feel is less optimal for performance!

4. Can you train your heat tolerance?

Absolutely! Heat tolerance can be trained, and in Australia this is a must for performing throughout 6 months of the year! You probably know the feeling; that first hot day of the year and that first hot ride of the year, where everything is harder than it should be, you're seeking the water and your heart rate is sky high for pace or output you're doing.

The good thing is, changes occur with low exertion in the heat, and it's advised to start with easy and endurance paced rides in order to elicit adaptations before completing more intense work. These adaptations can increase performance even in temperate environments!

Remember that efficiency that we talked about? We want to mitigate both the extra stress of heat in an athlete that already has an external stress (training) until they have adapted to the heat. Changes begin to occur in as little as three days, and physiological changes occurring in the heat include: increased plasma volume (more blood! That's like cheating!) decrease in heart rate, increased peripheral blood flow (ie to skin), increased sweat rate and onset, and decreased electrolyte loss through sweat.

1. Holy dooley I live in Australia and I have to race in the hot, hot heat WHAT DO I DO????

PANIC! No...actually don't do that. Panicking will increase your sympathetic nervous system and increase your heart rate: you don't need extra physiological stress leading into a demanding event in the heat.

There is evidence that pre-cooling could be a good option for competition in hot and humid events: there is evidence that a multimodal approach yields the best results in better performance outcomes in the heat. This approach includes both internal and external cooling: an ice slushie (thanks 7/11) and an ice vest worn prior to competition.

There is evidence that a reduction in RPE can increase performance in the heat, so for the individual constrained by logistical and financial factors, looking at implementing makeshift cooling prior to racing could be useful, but it's worth trialling prior to a big event. The key principles of training can be implemented here: specificity. If you know you're racing in not and humid conditions, seek to ride in them prior. If you're in a cooler climate but heading to race in a warmer one, this is where weird training adjuncts like training in a hot, steamy bathroom or some permissive dehydration may be effective in eliciting those changes we have discussed.

Key things that anyone can implement, even if they don't have a fancy jacket full of ice:

-Reduce warm up time

-Stay out of the sun. For god's sake don't hang out in the heat if there is shade nearby

-If an ice slushy isn't available, try a crisply chilled hydration bottle prior to start

-If you don't have a fancy ice jacket, try ice in a stocking around your neck or in your jersey, or wet and freeze a towel to use prior to the race

-Avoid excessive stimulants, sleep deprivation and external stress (increases potential for heat illness)

-Don't write cheques you can't afford: race smarter not harder remembering the metabolic cost of big numbers

McNulty KL, Elliott-Sale KJ, Dolan E, Swinton PA, Ansdell P, Goodall S, Thomas K, Hicks KM. The Effects of Menstrual Cycle Phase on Exercise Performance in Eumenorrheic Women: A Systematic Review and Meta-Analysis. Sports Med. 2020 Oct;50(10):1813-1827. doi: 10.1007/s40279-020-01319-3. PMID: 32661839; PMCID: PMC7497427.

Burdon, Catriona & Hoon, Matthew & Johnson, Nathan & Chapman, Phillip & O'Connor, Helen. (2013). The Effect of Ice Slushy Ingestion and Mouthwash on Thermoregulation and Endurance Performance in the Heat. International journal of sport nutrition and exercise metabolism. 23. 458-69. 10.1123/ijsnem.23.5.458.

Carly Brade, Brian Dawson & Karen Wallman (2014) Effects of different precooling techniques on repeat sprint ability in team sport athletes, European Journal of Sport Science, 14:sup1, S84-S91, DOI: 10.1080/17461391.2011.651491