HOW: Start by lying on your back with both knees straight up from your hips. Engage your core and bring both arms straight up in front of your chest. While keeping your core engaged, bring one arm back while keeping the other arm straight up. Alternate bringing each arm back. The goal is not to move your back all while your arms are moving.
FEEL: You should feel your core muscles working to prevent your back from moving while you are lifting your arms.
COMPENSATION: Keep a 90 degree bend in your knees. Don’t arch your back.