When you’ve reached the point where the traditional crunch is too basic, then move on to the jackknife exercise. The main muscle targeted during this move is your rectus abdominis, which is that muscle on the front of your abdomen known as your "six pack." The jackknife can be completed in several variations, some of which can challenge certain core muscles more than other traditional abdominal exercises.
When you do a basic jackknife situp on the floor, you are using nearly a dozen muscles, yet your rectus abdominis is the targeted muscle. You also use your obliques, and muscles throughout your thighs, gluteus, chest and calves to complete this move. To recruit even more muscles, try doing this move with a power wheel or on a stability ball.
With so many opinions out there as to the "best" abdominal exercise, it can be difficult to know which one really is. A study published in the “Journal of Bodywork and Movement Therapy” in 2009 examined four abdominal exercises: the curlup, Swiss ball curlup, Swiss ball jackknife and Swiss ball rollout. Muscle activity in the upper and lower rectus abdominis was tested and activity was strongest when moves were performed on a stability ball. The jackknife proved to be the most effective move in targeting your lower rectus abdominis, or lower stomach.
To maximize the benefits of the jackknife exercise, grab a stability ball. Begin by placing your hands on the floor supporting your upper body and walk your hands forward so your shins are positioned on top of the ball. Start in a plank position so your wrist, elbows and shoulders create a straight line. Bend your hips and knees and bring your knees toward your chest. Your shins will pull the ball toward your chest simultaneously. Pull forward until your heels are nearly touching your buttocks, then return to the starting position and repeat.
Because the stability ball jackknife is considered a more advanced move, make sure you tackle other more basic variations first. When you first use the ball, avoid lifting your hips and just tuck your knees into your chest; this makes the move easier. For a more basic jackknife, begin lying on the floor face up with your hands by your sides. Simultaneously raise your torso as you bend your knees and hips and bring your thighs toward your chest. Return to the starting position and repeat.
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