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“Slowing down in the heat could be a subconscious regulation to protect us from damage, such as heat stroke,”

In other words, you don’t slow down because your body has reached some critical temperature. Instead, your brain slows you down to prevent you from ever reaching that critical temperature. It’s a subtle difference – but as the cyclists in the study discovered, it means that our physical “limits” are more negotiable than previously thought.

“To obtain a substantial heat adaptation, core temperature must be elevated and high sweat rates need to occur,” Dr. Jay explains. “The best way to do this is to combine exercise with heat exposure.”The study volunteers had averaged just 18 minutes a day of “moderate” or “intense” physical activity outdoors. In contrast, researchers have found that proper acclimatization takes 45 to 60 minutes of moderate-intensity exercise in the heat, either for seven to 10 consecutive days, or four to five times a week for two to three weeks.

The adventures of Paul, Black and Breinan offer a glimpse into a forgotten aspect of the running boom of the late 1970s. Preternaturally self-disciplined, they were among about 75 children (ages 8 to 13) who tackled the early years of the New York City Marathon in a time of novelty and naïveté. Organizers were uneasy about young runners, but it was not until 1981, records show, that age 16 became the requirement. New York’s official minimum age became 18 in 1988, after an advisory set by the International Marathon Medical Directors Association in the early 1980s, and reasserted in 2001. With no conclusive study, physicians still debate risks to children who compete in marathons, like muscular-skeletal injuries, stunted growth, burnout, parental pressures and the ability to handle heat stress.

Some marathons — Houston and Twin Cities in Minnesota — allow teenagers or admit younger runners on a case-by-case basis. Los Angeles has a program for schoolchildren ages 12 to 18.