Start in a high plank position, with your lower legs resting on a gym ball. Keep your body as straight as possible, with your core and buttocks engaged. Alternately pull one knee towards your chest and then return it to the starting position. Gradually increase the pace in a controlled manner, without allowing the gym ball to roll back and forth too much.
Attention:
Maintain body tension; do not let your lower back sag; keep your pelvis as stable as possible; actively position your shoulders above your hands; pull your knees towards your chest in a controlled manner, not just by momentum; do not let the ball roll too far forwards or backwards.
Lighten:
Slow down the pace; pull your knees only slightly towards your chest, or simply support yourself in place (without lifting a leg); rest your lower leg more firmly on the ball or bring the ball closer to your body.
Harden:
Increase the pace; pull your knees further towards your chest (or move them diagonally towards your elbow); add extra weight (on your back); use an unstable surface for your arms (e.g. a balance board).
Variation:
Pull both legs in at the same time (the ball rolls towards your body).
