Abs for All Seasons:  

Cardio machines might seem like convenient options for avoiding the cold, but they aren't always the best Plan B. "The human body wasn't designed for conveyor-belt training or repetitive, one-dimensional movement," says Dan John, a fitness coach in Burlingame, California, and the author of Never Let Go. Try one of these cardio drills instead.

The "55" workout

Start by doing one body-weight squat and 10 pushups. Rest for 30 seconds, and then do 2 squats and 9 pushups. Gradually work your way up to 10 squats and down to 1 pushup. You'll complete 55 reps of each exercise by the time you're done--and reap both the cardiovascular benefit of aerobic training and the muscular pump of a strength session.

10-meter sprints

Find an area in your gym where you can sprint for 10 meters. Once you've covered the distance, pause just long enough to inhale and exhale once through your nose. Sprint back and pause, this time inhaling and exhaling twice through your nose. Continue the drill--breathing normally as you sprint, and adding an additional nose inhalation and exhalation when you pause--until you can no longer breathe through your nose. "It takes more effort than breathing through your mouth--even during rest--which increases the intensity of the exercise," John says. The result: more gain in less time (and distance) than on a treadmill.

Jumping-jack pyramid

Do as many jumping jacks as you can in 10 seconds. Rest for an equal amount of time. Next, do as many jumping jacks as you can in 20 seconds, and rest 20 seconds. Then do 30 seconds of jumping jacks followed by 30 seconds of rest. Now work your way back down the pyramid (30, 20, 10). Repeat three times.

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