The human body evolved expressly to move. We run, jump, and climb as soon as our tiny limbs allow it. But a select few thrill-seekers go much further, ­hungry to push beyond our species’ more-mundane corporeal capabilities. The most extreme among us fling, squeeze, and stretch their bodies in the name of guts, glory, and even entertainment. Here’s what all that abuse does to their agile-but-fragile frames.


Ultra running

An elite endurance runner can log hundreds of miles on their feet. While they pound the pavement, their brain diverts blood away from the intestines to give heart, lungs, and muscles a boost. That means the risk of ­gastrointestinal ­distress increases. Sleep deprivation—especially combined with lack of oxygen and glucose—triggers hallucinations in most racers. And all the outdoor training means greater exposure to pollen, leading to an above-​average incidence of allergies and asthma.


Air pressure decreases during the ascent but shoots up during the fall, a shift that can cause vertigo and ruptured eardrums. A 10,000-foot drop lasts a minute or less, but fright can throw off the brain’s stopwatch: Research suggests that fearful flyers will think they’ve fallen for longer. All divers see a spike in hormones such as adrenaline, which tenses muscles and speeds up breathing and heartbeat. The rush is meant for fight-or-flight, but ­skydivers don’t have much of a choice.

Racecar driving

Professional drivers go beyond 200 mph, and some races last 24 hours. Car interiors can top 130 degrees, which equals a lot of sweat; racers typically lose around 5 pounds during a three- to four-hour event. Stress and G-force keep heart rates high—­comparable to a mara­thon run—so even though driving is sedentary, pros stay in shape with cardio workouts. They also strengthen their neck muscles, which support the equivalent of an extra 60 to 90 pounds at peak acceleration.


Many land-dwelling species share a trick called the diving ­response: When we stop inhaling and get our nostrils wet (surefire signs of aquatic submersion), our heart rates drop by up to 50 percent, and peripheral blood vessels constrict, conserving oxygen for tickers and brains. The Bajau—​an indigenous population near Indonesia who freedive for fish—have spleens twice the typical size. The organ regulates red-blood-cell circulation; a larger spleen delivers more oxygen-​carrying cells.

This article was originally published in the Spring 2019 Transportation issue of Popular Science.