Two Workouts in One

You know the routine: Do a set, rest, repeat. You've been doing it for years, and, not surprisingly, your fitness gains have started to slow. It's time to kick things up a notch. "By filling your rest periods with exercises, you can improve conditioning and fire up your metabolism," says Jim Smith, C.S.C.S., co-owner of Diesel Crew. 

Sound a little intense? Good. But the trick is to replace your rest periods with fillers that work different muscle groups than your primary exercise—jump squats between sets of dumbbell chest presses, for example, or mountain climbers between sets of chinups. In so doing, you'll allow one muscle group to recover while you kick another into gear. The result: More muscle and less fat in half the time. Ready to give it a try? Click here for all 5 instant workout boosters.