Bosu
Credit: Courtesy Bosu

Bosus are used to create an unstable platform for everything from squats to push-ups, forcing you to recruit the core along with the big muscles of the arms and legs. But, as Boyle points out, using Bosus for lower-body work adds unnecessary risk of injury (except for skiers or surfers); in life, we rarely engage our legs on an unstable surface. When we push with our upper bodies, however, it's rarely against a stable surface. That makes Bosus perfect for engaging the small muscles that stabilize the shoulder.

Try: Bosu Mountain Climbers
With the rounded side down, hands on the outer edges of the platform, start in a push-up position and jump your feet up toward your hips, alternating sides.