WHY This works the butt, hamstrings and core. HOW Partner A, lie on the floor, arms by your sides, palms down. Bend the left foot to the floor, and loop the towel around the right foot. Partner B, stand at head level, knees bent, abs engaged. Grasp the ends of the towel in both hands. Partner A, squeeze your glutes and roll up through your spine, straightening your right leg. Partner B, create resistance as their leg extends. This is 1 rep. Repeat on the other side.