Heavier athletes run 10 percent slower in hot conditions
Until December 2013, California’s 135-mile Badwater ultramarathon, which started in Death Valley and ended at the Mount Whitney Portal, was considered the ultimate endurance test in an extreme heat environment.
That’s when a temporary moratorium was placed on all sporting events in Death Valley. Obviously, the announcement threw a wrench in this summer’s 37th edition of Badwater, scheduled to take place July 21 through 23. But you can’t just axe the world’s toughest footrace, so race organizers revised the route, which now incorporates more than 17,000 feet of elevation gain between Lone Pine and Whitney Portal. Although temperatures might not reach 125 degrees, the 97 brave souls who toe the line will likely still be treated to triple-digit temps.
And although some runners will incorporate special clothing and aid-station ice baths into ther races, other runners will have a more natural advantage: their body size.
While running in hot weather, an athlete’s primary goal—besides winning—should be to maintain a constant core temperature by balancing heat production and heat loss. Exercise itself creates internal heat. In fact, 80 percent of energy produced by exercising skeletal muscle becomes heat (the other 20 percent generates adenosine triphosphate (ATP) to power the muscle. Extremely hot environments can also cause athletes to take in heat, just as cold environments cause us to lose heat.
Runners also battle heat externally via hot weather and humidity, both of which make running more difficult. Hot temperatures cause heat to transfer from the environment to the body, while humidity makes evaporative heat loss more difficult. In comfortable environments, to get rid of excess heat, blood is shunted to the skin, where warmed blood can lose heat through evaporation (sweating) or convection (if skin temperature is greater than the environmental temperature). Both evaporation and convection depend on the skin's surface area—the larger surface area, the better the heat loss.
So, bigger runners should be better at cooling off, right?
Surface area and body mass (that is, muscle mass) are not at a one-to-one relationship—for every unit of body mass you increase, you don't get an equivalent relative increase in surface area. Smaller runners actually have more surface area relative to body mass, which gives them greater heat-loss ability for their relative mass.
According to a study in the European Journal of Physiology, this “distinct thermal advantage” corresponds with speed. Because lighter runners produce and store less heat than heavier runners at the same pace, they can run faster or farther. This difference was most striking in hot, humid conditions (95 degrees, greater than 60 percent humidity) and essentially absent in cool conditions (59 degrees).
Indeed, in 2004, exercise physiologist Tim Noakes published a related study in the Journal of Applied Physiology finding that African runners ran faster in the heat than their Caucasian peers. “Larger Caucasians reduce their running speed to ensure an optimal rate of heat storage without developing dangerous hyperthermia [heatstroke],” the study reports. “According to this model, the superior running performance in the heat of these African runners can be partly attributed to their smaller size and hence their capacity to run faster in the heat while storing heat at the same rate as heavier Caucasian runners.”
In this study, the heavier Caucasian runners (169 pounds) ran approximately 10 percent slower during 30 minutes of exercise in hot conditions (95 degrees, 60 percent humidity) compared to the lighter Africans (131 pounds). The difference is dramatic when considering both groups ran the same time in the exercise test conducted in cool conditions (59 degrees).
In other words, a slower but smaller runner has a substantially better shot at beating a faster but larger runner if the temperature is high enough.
Although many other factors can help regulate core temperature (clothing, heat adaptation, genetics, age, etc.), the bottom line is that the smaller you are, the better you should be able to handle the heat. So although the Badwater 135 might not reach 130 degrees this year, the soaring temps should be sufficient to give an advantage to the slight of frame.