Get down on all fours, with your knees under your hips and wrists under your shoulders. Pull in your abs, draw your shoulder blades back and down, and lengthen your spine. Keeping your right knee bent, lift that leg behind you, keeping your thigh and knee in line with your shoulder. Flex your foot and squeeze your right buttock. Press your foot up to the ceiling, giving your buttock an extra tight squeeze at the top of the movement [shown]. Do 15-20 reps on one side, then switch legs.