Don’t underestimate a 10-pound dumbbell. They can challenge your body in an entirely new way, especially if you’re used to lifting heavy. Lighter weights target the small muscles that stabilize your joints—muscles that get overpowered by larger ones when you use heavier weights.
THE MATRIX Grab a five- to 10-pound medicine ball (or dumbbell) and kneel on the floor with your knees hip-width apart. Keep your torso upright and hold the ball against your abs (a). Slowly lean back as far as possible, keeping your knees planted (b). Hold the reclined position for three seconds, then use your core to slowly come up to the starting position. Do 12 to 15 reps.