Bring on the Heat

Summer is still in full swing across most of the country, and as a runner or triathlete, you are quite familiar with the effects of summer training. Including, but not limited to: squishy shoes, chafing, extreme thirst, soaked clothes, and generally questioning “What the heck am I doing out here?”.

You may also have heard the commentary of the Tokyo Olympics, where the heat has taken center stage and affected many of the athlete's performances.

What actually happens when we exercise in the heat? How do our bodies adapt? What is the best way to acclimatize? Let’s discuss some physiology of heat training, and what the research says is the best way to train for it.

Our body’s thermostat is the hypothalamus, a structure deep in the center of the brain. It’s job is to keep our body temperature at roughly the same set point, which on average in humans is 98.6 degrees F (37 deg C). The hypothalamus receives information from temperature receptors in the skin, indicating environmental temperature, and from the core via the brainstem and spinal cord, indicating the body’s response to the environment. The hypothalamus works with the pituitary gland to release the proper hormones based on this feedback. The major mechanisms in which we dissipate heat are through vasodilation (a widening of the arteries to increase blood flow to the skin) and sweating. Sweat carries the heat from the increased blood flow out into the environment, and the eventual evaporation of said sweat removes heat from the body. This is why humidity is an extra challenge for athletes, because the moisture in the air slows down this process, leaving the heat on your skin and causing core temperature to rise.

Sweat production and heat loss is highly individual, and will vary depending on heat acclimatization. As our bodies adapt to the heat, we sweat more and earlier after the onset of exercise, and lose less sodium through sweat. Heat acclimatization results in a 10%-12% increase in plasma (the portion of the blood that carries water and nutrients) volume. This is advantageous because it “dilutes” the heat throughout the body, and allows us to store more heat without changing core temperature. Additionally, the body produces more heat shock proteins, which help protect cells from damage while under duress from the heat, therefore improving exercise tolerance.

So, the big question is, are we here in Georgia making altitude-esque adaptations by training in conditions with 98% humidity? The answer is somewhat unclear, particularly due to individual differences in adaptation to heat/humidity and altitude, respectively. Altitude adaptations are primarily targeted at increasing the volume of red blood cells, therefore increasing oxygen carrying capacity and VO2 max. The aforementioned increase in plasma volume via heat training can similarly improve VO2 max, improving performance and fitness.

Finally, how do we best acclimate to the hot temps, especially if we know the conditions of a race are going to be predictably hot? We’ll get to some of the new research in a second, but we first must understand that body temperature is relatively constant at a constant exercise intensity, and increases proportionally to exercise intensity. This would indicate that to maximize success in race conditions, we have to practice harder efforts in the heat.

One(1) study assessed the effects of self-directed heat training vs. a deliberate heat adaptation protocol in a lab setting. Researchers measured variables of heat acclimation and fitness, such as body temperature, Heart Rate, and VO2 max after self directed training and then again after a 5 day lab-based protocol (60 minutes of steady-state exercise in 95 degree heat). The results suggested that the deliberate training increased heat acclimatization, suggesting that timing your training sessions during hotter parts of the day, consistently, can result in improved performance in the heat.

A second (2) study compared training in the heat vs. post-exercise hot water submersion. One group performed 60 minutes of exercise in hot conditions, and the other 40 minutes of cool exercise plus a 40 minute hot bath. The hot bath group demonstrated improved heat adaptation vs. the hot exercise group, although this may not be as useful to us who don’t have a hot tub or heat chamber at home.

It’s important to note that studies on heat acclimation use the gold standard of maintaining a body temperature of about 101 degrees F, and adjusting exercise intensity accordingly, for roughly an hour. Goodness, could you imagine if Garmin added temperature to it’s barrage of available data?

In summary, and unsurprisingly, exercising in the heat is the best way to acclimate and improve fitness in the dead heat of summer. However, we know that passive methods, such as a hot bath or sauna, can be a good adjunct to produce adaptations without introducing more training stress. But whatever method you choose, be sure to hydrate before, during, and after training!

Keep going, you got this!

Kacy Seynders, PT, DPT


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