MUSCLES ENGAGED Abdominals The abdominal muscles (abs) provide movement and support to your core area. The deeper and closer to the spine the particular abdominal muscle is, the more effect over body posture it will have, and this often contributes significantly to a healthy back. 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. Exercise Instructions Step 1 Start by lying on your back with your arms straight and fully extended in line with your shoulder. Your legs should be bent at a 90 degree angle. Step 2 Return to the starting position, alternate sides and repeat the action. Step 3 Return to the starting position, alternate sides and repeat the action.