Research proves it: the human body is designed for strenuous, long-haul adventure. Recent studies have even suggested that endurance running actually shaped the evolution of the human body. According to Harvard paleoanthropologist Daniel Lieberman, an author of many such studies and The Story of the Human Body, human beings "are fundamentally adapted to get exercise." But the gym is not enough. "There's nothing like the real thing," says Adam Chase, president of the All American Trail Running Association. The outdoors is simply more demanding. "When you're out on the trail, you have to dance through the rocks and routes." That translates into automatic interval training, which strengthens your heart by forcing it to learn to adapt to just about anything. Here's your guide to getting your heart in shape – and having fun while you're at it.
The motions involved in swimming combined with the dynamic tension created by keeping yourself afloat make for a whole-body workout. And when you exercise in water, your body spends less energy to stay cool, which allows more blood to flow to the muscles, letting you exercise longer and harder.
Your lats, triceps, and deltoids, as well as your glutes and hip flexors, propel you, but your core stabilizes you.
Amp It Up
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Scott Berlinger, of Full Throttle Endurance in New York, has his triathlete clients swim at a comfortable pace for 80 strokes (40 strokes with each arm), then speed up for another 20. Keep it up for 30 minutes, and eventually for 45. Do it in a lake or the ocean and you'll deal with the natural resistance of currents and go without the short rests that come from turning around in a pool.