Start out by working the lower body with some overhead squats in order to get the heart pumping without a lot of impact. When you squat, hold your arms above your head with your hands in line with your ears and your palms facing forward. Hoebel says it's like holding a bar over your head, which forces your back into alignment and is more difficult than a standard squat.
Advanced version: Front squats with a small jump at the top of the squat
Brett's Tip: Don't move your arms from their starting, overhead position.