1 Dumbbell, 650 Muscles
It's the simplest total-body workout ever.
Editors of Men's Health

Here's a new approach for the new year: Use one dumbbell instead of two. By not tracking reps in this fast routine, you'll focus on form and trigger more muscle fibers, says creator C. J. Murphy, co-owner of Total Performance Sports in Everett, Mass. Complete the circuit by doing each exercise for 45 seconds. Rest one minute. Repeat two or three times.

Start with a 15-pound dumbbell. Add weight as you progress, but don't go so heavy that you need to rest between exercises.

1. Woodchopper

With arms nearly straight and feet just beyond shoulder-width apart, hold a dumbbell over your right shoulder. Bend your knees and forcefully rotate your torso left as you draw your arms down and across your body. When your hands go past your left ankle, reverse the motion. Then switch sides. Keep your abs tight to prevent injury.

2. Arms-Out Squat

Standing with your feet slightly wider than shoulder-width apart, grasp a dumbbell by the ends and hold it straight out from your eyes. Now try to press the ends together as you simultaneously push your hips back, bend your knees, and lower your body until your thighs are parallel to the floor. Pause, and push back up.

3. Standing Pressout

With your feet shoulder-width apart, hold a dumbbell by its ends and next to your chest. Try to press the ends together as you simultaneously push the dumbbell away from your body and slightly up (to eye level) until your arms are straight. Pause, and pull the dumbbell back as you squeeze your shoulder blades together.

4. Towel Row

Secure a towel around a dumbbell's handle. Grab an end of the towel with each hand and stand with your feet shoulder-width apart, knees slightly bent. Bend at your hips, keep your lower back flat, and lower your torso until it's almost parallel to the floor. Pull the towel ends to either side of your abdomen. Pause and lower the towel.