Basic body-weight exercises like squats and push-ups are a simple way to build more metabolism-revving muscle in minutes, and research shows they're just as effective as hitting the gym. "Your muscles don't know the difference between working against your body's own resistance and on a fancy piece of equipment," says Wayne Westcott, PhD, fitness research director at Quincy College and Prevention advisor. "The one rule to follow is that each exercise should fatigue your muscles within 60 to 90 seconds."