Your lower body is the easiest and quickest place to build calorie-torching lean muscle. You have over 200 muscles below the belt, including the largest muscle in your body — your gluteus maximus. So get off your glutes and get your lower body going in everything you do. Stand on one leg for shoulder presses or squat during biceps curls. The more you work your lower half, the faster you’ll see results. Need more ideas? Try this total body routine.