Surely you've run nonstop for 15 minutes before. But have you ever performed 15 minutes of resistance exercise without rest? Try it with these challenging body-weight exercises from trainer Craig Ballantyne, C.S.C.S. "You'll be surprised how hard you can work without a gym," he says. (Before you get started, check out the five rules of body weight training.) Alternate between the Y squat and the Spider-Man pushup for three sets of each. Then perform the remaining three exercises consecutively (again, without rest), doing three sets of each.
Stand with your shoulder blades pulled back and your arms extended up and out so your body forms a Y. With your feet slightly more than shoulder-width apart, sit back at your hips to lower your body. Go as low as possible without allowing your back to round. Squeeze your glutes and push yourself back up to the starting position. Do 12 repetitions.
Assume the classic pushup position with your legs straight and your abs tight. As you lower your body, bend your right leg and rotate your right knee outward until it's outside your right elbow. Don't drag your foot, and try not to allow your torso to rotate. Return to the starting position and repeat, pulling your left knee to your left elbow. Do eight reps per side.
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