The exercise ball knee roll-in is an abdominal exercise that involves supporting yourself with your hands in a plank or push-up position and drawing the knees in toward the chest. This is a fairly advanced abdominal move requiring core strength.
Hits every part of the core, including the rectus abdominus or "six-pack" muscles and the deep core
Excellent way to build up to the full exercise ball pike
Place an exercise ball nearby and lay on the floor in front of it with your hands on the floor shoulder width apart in a push-up position.
Now place your lower shins on top of an exercise ball. Tip: At this point your legs should be fully extended with the shins on top of the ball and the upper body should be in a push-up type of position being supported by your two extended arms in front of you. This will be your starting position.
While keeping your back completely straight and the upper body stationary, pull your knees in towards your chest as you exhale, allowing the ball to roll forward under your ankles. Squeeze your abs and hold that position for a second.
Now slowly straighten your legs, rolling the ball back to the starting position as you inhale.