Ball knee crunches, which are also called knee tucks, are an advanced abdominal exercise that uses a stability, or exercise, ball. The exercise ball creates an element of instability, which makes the exercise more difficult than other knee-to-chest crunch movements. Proper form is important during this exercise to reduce the risk of low back discomfort.
Identification
The ball knee crunch is a type of reverse crunch, which means you pull your legs toward your torso instead of pulling your torso toward your legs. Performing the reverse crunch on top of the stability ball increases the difficulty of the exercise. You have to control the movement without any torso support. During a basic reverse crunch, you lie on the floor, which supports your torso. Place your lower legs on the stability ball and your hands flat on the floor. Pull your knees forward, lifting your hips, until your knees almost touch your chest.
Function
The ball knee crunch targets your abdominal muscles; it works the main rectus abdominis muscle and the side abs -- your obliques. Your quadriceps, the muscles on the front of your thigh, also assist in the movement. Contrary to popular belief, the ball knee crunch, or any reverse crunch, does not target your lower abs more than your upper abs. According to a report by the American Council on Exercise, the ab muscle works as one sheath and you cannot isolate the upper or lower region.
Technique
Start the ball knee crunch exercise with your knees on the exercise ball and your feet hanging off the end of the ball. With your hands flat on the floor, your body should be parallel to the ground. Don't allow your hips to lift or sag in this position as this can result in low back discomfort. Engage your abdominal muscles before you begin the exercise; pull your bellybutton toward your spine and brace your core.
The Ball
Using a properly sized ball helps ensure that you perform the ball knee crunch exercise correctly. If the ball is too large or too small, it will be difficult to maintain proper form. When you are in the starting position of the ball knee crunch, make sure your shoulders are stacked directly over your wrists and your arms are perpendicular to the floor. In this position, your ankles should be in line with your shoulders. If they are higher than your shoulders, find a slightly smaller ball, and if they are lower than your shoulders, find a slightly larger ball.
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