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. Equipment Used Bench A weight training bench is a piece of equipment that has a resemblance to a normal (e.g. park) bench, but is cushioned and designed for use in the weight room. Exercise Instructions Step 1 Start in a standing position in front of the bench. Step 2 Perform a bench push-up on the bench and then stand back up. Step 3 After the push-up, perform an air squat.