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 Start in a standing position with your feet shoulder width and your hands to the side. Step 2 Bend forward keeping a soft bend in your knees. Once you reach the floor walkout in to a push-up position. Step 3 Complete one plank-up on each side and then walk back up to the starting position.