No Space, No Equipment, No Problem: Our 20-Minute Apartment Workout

Reps: Two sets of 10.

Core muscles get a killer workout with the toe-touch crunch. Here's how to do it:

• Lie on your back and reach your arms behind you, pointing your legs toward the ceiling.
• Swing your arms toward the ceiling; as you do, lift your head and shoulder off the ground and touch your toes, while lifting your pelvis slightly off ground so your toes move toward your fingers. If your neck tires, place one hand behind your head.
• Do two sets of 10.