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. Hamstrings The hamstrings play a crucial role in many daily activities, such as, running, jumping, walking and cycling. Strengthening your hamstring muscles help to protect your knees from injury by absorbing the stress placed on the knee. Weak hamstrings can lead to knee injury. Exercise Instructions Step 1 Stand up against the wall with your arms at a 90 degree angle and level with your shoulders while maintaining contact against the wall with your tailbone and shoulder blades. Step 2 Raise your arms above your head trying to keep your elbows and wrists as close to the wall as possible. Step 3 Lower your arms to the starting position and repeat.