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 one such study, 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 year-round guide to getting your heart in shape -- and having fun while you're at it. --Richard Conniff

SUMMER

Swimming
CARDIO ADVANTAGES 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.

MUSCLES WORKED Your lats, triceps, and deltoids, as well as your glutes and hip flexors, propel you, but your core stabilizes you.

AMP IT UP 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.

Kayaking
CARDIO ADVANTAGES Kayaking is that rare upper-body-centric cardio exercise. One of the benefits of aerobic work is that it increases the number of capillaries in the muscles worked; paddling shares this perk with your chest, back, shoulders, and arms, and the natural resistance of the water means that you can't just coast along and expect to remain in motion.

MUSCLES WORKED Your lats and shoulders are in constant motion, and your legs, while not doing a lot of weight-bearing movement, play a role in stabilizing you.

AMP IT UP Felix Apfaltrer, president of the New York Kayak Polo Club, suggests sprinting out 25 meters on still water in a light, maneuverable kayak, turning, and then sprinting back to where you began. Rest 30 seconds, then repeat the sprint. Do five complete up-and-back laps, adding 10 seconds to each of the rest periods as you go.

FALL

Mountain Biking
CARDIO ADVANTAGES The steep inclines you'll face amount to built-in intervals of high-intensity work, and the ever-changing pace of biking over difficult terrain gives you the aerobic variety an effective cardio workout should.

MUSCLES WORKED Navigating the ups and downs and roots and rocks of mountain biking brings almost every muscle in your body into play: the lower body muscles (calves, quads, hamstrings, glutes, and hip flexors), plus the pecs, shoulders, and even the forearms.

AMP IT UP Three-time Olympian and U.S. Bicycling Hall of Fame inductee John Howard competes with his training partners. Pick a distant landmark and race to it. Alternate between nearer and farther finish lines to vary the duration of high-intensity work. As your crew gets stronger, reduce the rest periods between races to spend more time clipped in and cranking.

Trail Running
CARDIO ADVANTAGES The demands of the terrain keep your muscles hungry for oxygen, and the uneven surfaces will work your body harder than a track or a street can. You'll be developing balance, agility, and stability around the ankles.

MUSCLES WORKED The quads, glutes, hamstrings, and calves do most of the work, but because you're constantly compensating for terrain, your core and upper body will be in dynamic motion.

AMP IT UP "The ego is a big motivator," says Chase, who co-authored The Ultimate Guide to Trail Running. That's why he likes to turn his workouts into a race with a friendly -- but competitive -- game of follow the leader. First, recruit some trail-running partners, then pick a leader. His job is to shake off the pack. Everyone else's job is to stay on his heels. Change leaders every three to five minutes; for more of a challenge, chase the best hill runner on the inclines and the most agile runner on the twistiest trails.

WINTER

Snowshoeing
CARDIO ADVANTAGES Snowshoeing on unpacked snow, even at a sluggish 3 mph, can expend twice the energy as walking on a treadmill. You'll hit your cardiovascular system a whole new way if you do your shoeing at high altitude, where, over time, your body will produce more red blood cells.

MUSCLES WORKED Not only does it demand power from your quads, hams, glutes, and calves, snowshoeing also taxes the stabilizer muscles (the adductors and abductors) on the inside and outside of the thighs as you work to balance yourself on the shifting surface of the snow.

AMP IT UP Ryan Alford, publisher of Snowshoe Magazine, recommends alternating five minutes of shoeing on the more cooperative harder-packed, on-trail snow with five minutes on unpacked off-trail snow. The softer snow is where your body has to work the hardest, especially when you have to negotiate obstacles in your path, while the hard-packed snow offers a chance to recover a bit. Each week tack on an additional on-trail/off-trail interval; if it all feels too easy, challenge yourself whenever possible with off-trail inclines.

Cross-Country Skiing
CARDIO ADVANTAGES Cross-country skiing is such a lung-busting workout that an entire sport, the biathlon, was designed to test an athlete's ability to fire a gun -- and hit a target -- during a cross-country race. (No surprise, then, that it's such a calorie inferno.)

MUSCLES WORKED Because the sport involves moving your extremities in relationship to your core rather than to gravity, your shoulders, core, glutes, hamstrings, and calves work constantly.

AMP IT UP Champion skier and coach Nathan Schultz suggests counting your strokes across a four-minute course. Every time out strive to reduce your stroke count. You can also try skiing without poles to focus on your legs and core.

SPRING

Running
CARDIO ADVANTAGES Humans are built to jog; our ancestors are thought to have hunted this way across dozens of miles at a time. Cool spring temps also mean more oxygenated blood for your muscles.

MUSCLES WORKED Quads, hamstrings, glutes, and calves all work to keep you moving, but as your distances increase, running demands more of your core to keep you upright and stable.

AMP IT UP Scott Berlinger's clients run at a comfortable pace for 10 minutes and then at a faster, but sustainable, pace for two and a half minutes. Repeat this drill four times. As you get stronger, try cutting some of the slow periods to five minutes. When you're ready to truly test your mettle, begin decreasing the length of the slow periods as your workout progresses; you'll be pushing yourself hard when you could use the longer rest.

Road Biking
CARDIO ADVANTAGES The low-impact, easy-on-the-joints nature of cycling lets you go for hours.

MUSCLES WORKED Quads, hamstrings, calves, glutes, and core work in concert.

AMP IT UP Determine your cadence by counting how many times one of your legs rises in 15 seconds; multiply that number by 4. Berlinger suggests you ride at a 90-100 cadence for four minutes in an easy gear. Kick it up a gear and maintain the cadence for another minute. Do another four minutes at the original gear before going up two gears for another minute at 90-100. Keep pushing until you can't do a full minute at that pace. --Exercises by Steve Steinberg