Single-Leg Plank: Lie on your back with your knees bent and your feet flat on the floor. Cross your arms over your chest and raise the lower half of your right leg until it's in line with your left thigh (A). Press your left foot into the floor and contract your glutes as you lift your torso so it's in line with your thighs (B). Hold. Rest for 1 minute, then repeat, raising the opposite leg.