CURRENT REGIMEN: Cardio, 2-4 times/week

Regularly making your heart pound helps pump oxygenated blood to your brain and muscles. “Minute for minute, you enjoy the biggest health benefits from cardiovascular exercise,” says Mohr. “And high-intensity cardio work is the best way to burn calories.”

Plus, if you're running or cycling, you're probably outside enjoying the fresh air and reconnecting with the natural world, says Tom Holland, a certified athletic trainer and endurance-sports coach based in Darien, Connecticut. Even in a city, says Holland, going outside keeps the routine fresh and attractive.

What's missing: Flexibility and balanced strength. “When you do just one sport,” says Holland, “you overdevelop the muscles needed to do that sport, while other muscles grow weak by comparison. It's a classic setup for injury. Runners have overdeveloped hamstrings [the muscles on the back of their thighs]. Cyclists overdevelop their quads [the muscles on the front of their thighs].”

Performing only one type of exercise causes your range of motion to shrink, as well. Because the body is smart about minimizing the energy and muscle it needs to do a certain activity, any movement outside of the one it's used to doing can strain both those big, developed muscles and the ones that get little use. The result can be a painful muscle pull or nagging joint pain.

The fix: For runners, bikers, and elliptical fans, Holland recommends twice-weekly trips to the yoga studio for Ashtanga yoga, or “power yoga,” which focuses on lunges, push-up-type positions, and balance to build strength and flexibility.

Lindsay Hyman, a coach to triathletes, runners, and cyclists at Carmichael Training Systems in Colorado Springs, Colorado, also suggests that cardio addicts take one of their scheduled exercise days to go for an easy bike ride. “So many people burn out on their sport because they go all out all the time in their quest to burn maximum calories,” says Hyman. “But the body — and mind — need to recover in order to bounce back feeling fresh.”

Next page: Strength Training Rut-Busters