Exercising in hot weather

Since training and mountain outings normally take place outdoors, we're inevitably subject to Mother Nature's whims when it comes to the weather. Getting out in all conditions helps you build welcome experience, but you still need to expose yourself progressively. Today, we tackle the summer question: how do you prepare for and do sport when it's hot?

Our body, a heat generator

Our body is a heat engine, producing either mechanical work or heat. So when we exercise, our body temperature rises. This warming is the result of our muscles' poor efficiency: they convert only 30 to 40% of energy into mechanical movement. The rest — more than 60% — goes off as heat, dissipated through the skin and the air we exhale. During sustained effort, the largest share of this heat is handled by thermal conduction and convection. Without going into the details, two points are worth remembering:

Our body will then first try to cool itself with air, and start sweating to transfer more heat if necessary. Since the temperature difference is smaller in summer, sweating kicks in almost automatically as soon as we start moving. Now, if the cooling isn't sufficient, the heat warms up our system and raises our body temperature dangerously. The brain then starts shutting down functions one by one, at best reducing our athletic performance, at worst exposing us to heat stroke. Luckily for us, there are methods to help the body shed heat more effectively.

Acclimatizing to heat

Acclimatizing to heat: the effects

Heat acclimatization is a method available to athletes who plan ahead. If you know that at some point you'll have no choice but to go out and do sport in the heat, it's possible to prepare for it in the preceding weeks. Recent studies conducted with the help of professional cyclists have shown that training in high temperatures has the same physiological impacts as altitude. Indeed, after long training sessions in hot conditions, the body adapts to cope in the same way as if it were exposed to a deficit in atmospheric pressure. The fastest change is the increase in blood plasma volume. This increase allows better thermal regulation, reduces the metabolism's energy consumption, and increases the cells' heat tolerance. The second change is the increase in the number of platelets, which determines the blood's hemoglobin level. More hemoglobin means it's easier to supply the cells with oxygen — synonymous with increased performance. In the end, the benefits of heat acclimatization are:

Acclimatizing to heat: the protocol

Hot sessions should be integrated little by little, until you reach one hour per day, five to six days per week, over several weeks. These sessions should be modeled on aerobic development sessions — that is, walking or running at an intensity strictly below Z2. Studies have also proven the beneficial effects of 20 to 30 minutes of sauna, hot jacuzzi, or at worst, reading in an attic at the height of summer. Nevertheless, you have to take into account that the return on "hot" sessions is low. For one hour of training per day, five days per week, over five weeks, the maximum performance gain didn't exceed 10%. These sessions should be considered a supplement to your original training plan, which itself should be done in cooler conditions. Take care to properly prepare your body for this extra effort: the increase in blood volume demands very good hydration and a sufficient iron level to ease the creation of platelets. There could be differences between men and women in this adaptation, as the studies were conducted mostly on men. And since you'll be drinking more often to stay hydrated, also make sure you avoid hyponatremia by consuming electrolytes, especially sodium.

Planning your outings

During heat waves, it's possible to adapt your training to the weather. In particular, you can move intense sessions to early morning to train in milder temperatures. It's also possible to move them to late evening, but we don't recommend it. For one thing, air pollution is higher. For another, the session will be too close to bedtime, degrading the quality of your night, and therefore the quality of your recovery. Planning your outings also means defining the content of the session. Let's not forget that the heat generated by the body is a consequence of muscular work. The smaller that work, the less we need to cool down. If you have no choice but to train in the blazing sun, favor fundamental endurance sessions, in the Z1 heart rate zone. Finally, planning means anticipating the hot outing. Lowering your body temperature before heading out increases heat tolerance, as the body sets off "recharged with coolness." Several techniques exist for this. You can place wet towels on your chest and neck, drink cold drinks, or stay in the shade or in an air-conditioned place in the hour before the session. Finally, it's very important to set off well hydrated: make sure you've sufficiently topped up on water and sodium before heading out.

Cooling off

Now, long outings and races can force us to do sport at the worst hours of the day, without any choice in the pace to follow. The strategy then consists of artificially cooling the skin temperature and securing a sufficient fluid reserve for sweating.

Staying hydrated

Hydration is key to protecting yourself from the heat. On a hot outing, you'll need to plan enough water to reduce your fluid losses. The catch is, the harder the pace, the lower the hourly volume you can take in. While it's easy for a hiker to drink 1 liter of water per hour, a runner will only drink a little more than 0.5 liter to avoid reflux. It's entirely possible to train yourself to drink to increase your tolerance, but that's something to plan for over the long term. Dehydration that makes you lose 2 to 3% of body weight during effort isn't serious. Still, don't tempt fate by pushing further: being lighter, and therefore moving more easily, will be counterbalanced by a loss of both your physical and mental capacities. However, hydrating too much is also a problem. Less well known, overhydration (hyponatremia) is dangerous too. If you drink too much water, the sodium (salt) level in your blood drops, slowing the digestive system's ability to process water — and therefore your ability to hydrate. Sodium allows cells to regulate the amount of water they can hold. When there's too little sodium, cells swell, to the point of creating pulmonary and cerebral edema in extreme cases (as with mountain sickness). So favor sodium-rich drinks during your sessions.

Cooling off artificially

To cool off, you can splash yourself with cool water (better yet, immerse yourself) or drink cold drinks. For short outings, this study compared the two techniques and the possible combinations. Interestingly, the method used (drinking, splashing, both, nothing) had no impact on the athletes' performance (over a 5-kilometer running outing). Yet drinking and dousing did lower body temperature, but that only affected how the runners felt. One obvious takeaway: not drinking anything and not splashing yourself with water is the worst of the combinations. Splashing is the most effective strategy in terms of sensation, although drinking a cold drink had the same effect on body temperature. For long outings, the mental side is very important. Even if cooling has no impact on short-term performance, preserving a level of comfort helps you last over time. Our physical capacities are heavily influenced by our mental state. We can therefore imagine that keeping your head in an OK place for longer must show in your performances. In cases where water is limited, it's better to drink than to splash, in order to maintain a good hydration level. You can also target the areas to cool. The head, hands, and feet are the areas to prioritize: cooling the skin in these spots provides better cooling of the blood.

The risks of doing sport in hot weather

The risks a healthy athlete is exposed to in hot weather are:

The warning signs are, among others, fatigue, a loss of lucidity, nausea, and elevated breathing and heart rates. In those cases, you'll have to act fast to restore a good thermal balance. Quickly look to cool the body artificially, either by finding a shaded area or by splashing yourself with water. Without forgetting to hydrate with an electrolyte-rich solution. In any case, the best defense against heat remains prevention: avoid doing sport during the hottest hours and remember to hydrate properly.