MUSCLES ENGAGED Back (lower) The lower back is often not trained because it is not visible in the mirror. If you have weak lower back muscles, there is nothing to help pull back against the spine, putting you at risk of developing poor posture (hunched over). Weak lower back muscles can also lead to lower back pain. 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 in a prone position with your arms and legs stretched out while holding the medicine ball in your hands. Step 2 Lift your body up off of the floor and raise the medicine ball and both legs as high as you can. Step 3 Lower your legs and the medicine ball back to the floor and repeat.