Get into a push-up position with your shins resting on top of a stability ball. You can do this by kneeling in front of a stability ball, lying with your waist on top of the ball, and then crawling forward on your hands over the ball until your shins are in position.
Exhale as you flex your knees, hips, and waist and pull your knees all the way up toward your chest. Allow your shins to roll over the ball.
Inhale as you extend your knees, hips, and waist and return your legs and the ball to the starting position.
Comments and tips
Keep your neck neutral.
The lower the ball is on your shins, the smaller the range of motion will be. Therefore, in order to be able to get a full range of motion and pull your knees all the way up to your chest, start with the ball close to your knees.
Your tensor fasciae latae, sartorius, pectineus, adductor longus, and adductor brevis (most of which are out of view in the illustration) help to flex your hips (that is, they act as synergists during hip flexion).
Your rectus abdominis and obliques will contract dynamically and thus act as synergists only if you also flex your waist; otherwise, they will only contract isometrically to stabilize your waist and pelvis.
Your adductor magnus and gluteus maximus get involved only when extending your hips back to the starting position. Since there isn’t much resistance with the stability ball jackknife during hip extension, these muscles don’t get much of a workout.
The stability ball jackknife is also known as the Swiss ball and exercise ball jackknife.