MUSCLES ENGAGED Shoulders The shoulder is the most movable joint in the body. However, it is an unstable joint because of the range of motion allowed. This instability increases the likelihood of joint injury, often leading to a degenerative process in which tissues break down and no longer function well. Equipment Used Bosu The BOSU balance trainer stands for "Both Sides Utilized" and is most often used for balance training, various athletic drills and aerobic activities. Exercise Instructions Step 1 Stand on the Bosu on one foot while holding a dumbbell over your shoulder on the other side. Step 2 While staying as rigid as possible, fully press the dumbbell over your head. Step 3 Lower the dumbbell back down to the starting position and repeat.