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 face down on the mat with your arms stretched out to your sides. Step 2 Lift one leg off of the floor, cross it over your body and touch the floor on the other side. Step 3 Return your leg back to the starting position and repeat on the other side.