What Muscles Does the Leg Extension Work?

The leg extension exercise involves extending, or straightening, the knee against resistance in a seated, lying or standing position. Although an effective upper thigh exercise, the leg extension only targets a small area of the leg musculature through one joint movement, knee extension. To fully work the quadriceps, incorporate the leg extension in a complete lower body routine with a variety of exercises.

Position the resistance against the lower portion of the shin, just above the ankle. Do not fully extend, or lock-out, the knee at the top of the movement because this places excessive pressure on the joint. Similarly, excessively bending the knee causes increased patellar compression, which can lead to knee pain and decreased functionality of the joint. When lowering the weight, stop the movement when the knee is at a 90-degree angle -- in a seated position, the lower leg is perpendicular to the floor.

The primary mover during the seated or lying leg extension exercise is a group of four muscle heads on the front of the thigh, collectively referred to as the quadriceps. The largest muscle head is the rectus femoris, which sits in the center of the front thigh. The vastus intermedius lies under the rectus femoris. The vastus lateralis runs along the outside of the front thigh and the vastus medialis runs down the inside of the front thigh. All four muscles extend from the top of the thigh to the knee joint and all are involved in knee extension. During the standing leg extension, the transverse abdominis, which is the deepest layer of stomach muscle, and the obliques, the side abs, are also primary movers; they activate to help stabilize your body as you balance on one leg.

The leg extension exercise is typically performed on a leg extension machine. Many machines have an integrated weight stack for resistance, but some must be loaded with free weights. Resistance bands offer a portable alternative to leg extension machines and allow you exercise in a standing, seated or lying position. Standing during the leg extension exercise challenges your balance and coordination more than the lying or seated versions. Select a weight that allows you to complete eight to 12 repetitions.

The leg extension is an isolation exercise -- it involves movement at only one joint and targets one muscle group. For a complete leg routine, include compound, or multi-joint, exercises in your workout, such as squats and lunges. To ensure proper muscular balance, also include exercises that target the hamstrings, which are the muscles on the back of the upper thigh responsible for knee flexion.