The leg press exercise is a compound movement, meaning it involves more than one joint action. Compound exercises work more than one muscle group, unlike isolation exercises, which target only one muscle. Leg presses work the upper thigh, buttocks and a couple other smaller muscles in the upper and lower leg.
The Movement
Sit in a leg press machine. Place your feet in the center of the foot plate. Push through your heels, extending your legs. Straighten your legs all the way, but keep a slight bend in your knees. Pause for a count. Slowly lower the weight to the starting position. There are several versions of the leg press machine, such as a lying leg press, 45 degree leg press and a seated leg press. The different versions still target the same muscles.
Primary Mover
The quadriceps, or front thigh muscles, are the primary movers during the leg press exercise. All four heads of the quadriceps muscle are involved in this exercise: rectus femoris, vastus lateralis, vastus intermedius and vastus medialis. The quadriceps muscles are mainly responsible for knee extension, which is the movement that occurs as you push the weight on a leg press machine.
Dynamic Stabilizers
A biarticulate muscle crosses two joints. Biarticulate muscles are often involved in compound exercises. Exrx.net defines a dynamic stabilizer as a biarticulate muscle shortens at one joint while simultaneously lengthening at the other. This results in no significant change in the length of the muscle. The hamstring muscles cross the hip and the knee joints. They flex the knee joint and extend the hip joint. During the pushing part of the leg press exercise, the quadriceps muscles extend the knee joint, so the hamstring muscle is lengthening at the knee. Simultaneously, the hamstring is contracting, or shortening, at the hip, resulting in hip extension.
Synergists
A synergist is a muscle that assists another, usually larger muscle, in a movement. The main glute muscle, the gluteus maximus, is a synergist during the leg press movement. The gluteus maximus assists in hip extension. The adductor magnus, a small muscle in the inner thigh, also assists in hip extension during the leg press exercise.
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