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. Quadriceps The quadriceps (quads) are a group of four muscles on the front of the thigh. They allow your knee to straighten and provide stability when standing. Your quads are the most important muscle group for maintaining the ability to stand and to walk independently. Exercise Instructions Step 1 Start in a standing position with your feet shoulder width and your arms to your side. Step 2 Flop down to the floor until your legs and chest are making contact with the floor. Step 3 Jump back up to a standing position and then jump in the air making sure that your hips and knees are fully extended.