Make the Most of the Weight Machines at Your Gym

Targets: Abs and obliques

Why it's key: Bypass ab machines, like the trunk twist and weighted crunch, because the ball does it all. Studies have shown that doing your crunches or sit-ups on a stability ball works more muscle fibers than doing those same reps on the floor. "Your ab muscles are constantly contracting as you curl forward on an unsteady surface, as you use more muscles to help stabilize the body," Porcari says.

How to use it: Sit on the center of the ball and lie back so that only your lower back is touching it. "Beginners may need to start with their upper back on the ball, but that limits the ab muscle activity," Schoenfeld says. Raise your torso no more than 30 degrees. "Above 30 degrees, you'll start to engage hip flexors instead of your abs," Schoenfeld notes. To make your rectus abdominus, aka six-pack muscle, feel the burn with fewer reps, hold a five-pound weight plate. To target love handles, move your feet closer together so your obliques work harder.