The leg-extension exercise starts with you sitting in the chair of a leg-extension machine with your lower legs hanging off the edge. Then you place your feet under the padded lever, which is attached to a stack of weights by a cable through a pulley system, and you repeatedly extend and flex your knees to lift and lower the lever. The exercise isolates the four quadriceps muscles within the front of your thighs, which facilitate the knee-extension movement.
How the Quadriceps Work
The quadriceps contract concentrically -- the muscle fibers shorten -- to extend your knees, facilitating the upward-movement phase of the leg-extension exercise. Then they contract eccentrically -- the fibers lengthen -- to control the speed of movement as you allow your knees to flex during the downward-movement phase. Each of the four muscles contributes similarly to execute the two phases.
Rectus Femoris
While all the quadriceps muscles act on the knee joint, the rectus femoris is the only one that also crosses the hip joint, assisting with hip flexion, which occurs when you decrease the angle between your upper leg and torso. The muscle doesn't execute this function when you perform the leg-extension exercise, however, because the hip joint is fixed at one angle. The rectus femoris attaches just above the hip joint to the anterior inferior iliac spine, or ASIS, of the pelvis on top, and runs vertically down the front of the thigh, reattaching to the patella, or knee cap, via the patellar tendon, which it shares with the other quadriceps muscles.
Vastus Intermedius
The vastus intermedius runs parallel to the shaft of the femur, or thigh bone, within the front of your upper leg, and is used during leg extensions. It attaches directly to the anterior, or front, and lateral, or outside, surfaces of the femur on top and combines with the other quadriceps muscles at the patellar tendon on the other end, inserting into the base of the patella.
Vastus Lateralis
The vastus lateralis muscle runs along the front, outside portion of your thigh. It attaches at top to the greater trochanter -- the bony protrusion on the outside of your hip -- and the lateral lip of the linea aspera -- a ridge that runs vertically along the back of the femur. The lower end of the vastus lateralis attaches to the patella via the patellar tendon, just like the other quadriceps muscles.
Vastus Medialis
The vastus medialis is a thin muscle that spans the front, inside portion of your upper leg. It attaches to the medial, or inside, lip of the linea aspera on top and to the patella on the bottom.
References
- ExRx.net: Lever Leg Extension
- "Basic Biomechanics"; Susan J. Hall; 2007
- Get Body Smart: Muscles That Act on the Leg
