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BOSU "Get Down, Get Up".  This is a great way to build strength, flexibility, and core strength all in one movement.  As I've said in the Daily Standard description, you want to find the ideal ratio of hip, knee, ankle flexion for your body during this movement.  The idea is to minimize stress on the knees and the back.  Practice allows you to find more efficient usage of your body. 

The medicine balls make the bottom position quite a challenge and are great for building strong abdominals.