Although the leg extension exercise is potentially dangerous to the knees it is still of value to bodybuilders. Its key benefit is in development of the teardrop effect of the quadriceps, especially of the vastus lateralis and medialis muscles located on the anterior thigh. The keys to safety in executing leg extensions is to do the exercise sparingly and through a shortened range of motion which is needed for the teardrop effect. Keep in mind that the squat and its many variants is still the best exercise for full development of the quadriceps.
MAJOR MUSCLES INVOLVED The quadriceps femoris muscle group which is composed of four muscles that have the same action at the knee joint. More specifically they are: The rectus femoris, a long, relatively large muscle that runs the length of the middle thigh. It has an action at the knee joint (extension) as well at the hip joint (flexion). Underneath this muscle and located close to the middle of the thigh is the vastus intermedius a relatively large, strong muscle. Closer to the knee are the vastus medialis and the vastus lateralis muscles located on the inner and outer sides of the thigh, respectively. These two muscles are considered the teardrop muscles.
The tendons of these four muscle merge together into the patella (kneecap) and surrounding ligaments. The patella however, is a free-floating bone and attaches to the tibia of the lower leg via the patella ligament which for all practical purposes acts as though it were a tendon.
MUSCLE ACTION In leg extensions there is only one action, knee joint extension. In this action the shin moves away from the thigh as the leg straightens from a bent position of about 145? (180? is a straight leg).
Note that the name of this exercise, leg extension, is inaccurate since leg extension takes place in the hip joint not the knee joint. A more accurate term is knee extensions.
SPORTS USES Strengthening the quadriceps by doing the leg extension exercise is very important in many skills executed by athletes. This includes jumping, running, kicking, skipping, lifting, and pushing. Specifically knee joint extension is used in the high jump, long jump, jump shot in basketball, volleyball spike, and running as in track, football, baseball, soccer, etc. Knee joint extension is especially important in kicking as seen in football, soccer, karate, and other martial arts. For bodybuilders knee joint extension is one of the major exercises used to develop and define the anterior thigh muscles, especially the vastus medialis and lateral medialis. It is also important in rehabilitation.
- Assume a seated position on a leg extension machine with the thigh in full contact with the seat.
- The shins should be vertical at the start with the resistance roller pads against the lower shin close to the ankle joints.
- The trunk should be vertical or leaning slightly to the rear and the feet should be relaxed and extended (toes pointed).
- Grip the handles or hold the sides of the seat.
- Inhale slightly more than normal and hold your breath as you straighten the legs to full extension. (Push against the rollers to full leg straightening).
- Relax slightly and allow the knees to bend until there is approximately a 125-145? angle in the knee joint.
- Upon reaching the 125-145? angle, stop, inhale and then straighten the legs to full extension.
- Hold and maximally contract the muscles for 2-3 seconds when the leg is fully extended.
- Return to the halfway position and repeat.
- To place greater stress on the vastus lateralis, turn the toes out and hold them in the out rotated position as you execute the exercise.
- To place greater stress on the vastus medialis, turn the toes in as you straighten the leg through the end range of motion. Turn the toes back to the straight forward position on the return.
- To get a stronger contraction of the rectus femoris by placing the muscles on stretch at the hip joint, lean back so that the trunk is inclined 45 or more degrees to the rear.
- Do not bend the knees 90? (measured from the back of the thigh to the back of the shin) or less than 90? so that the shin is under the thigh on the return. In these cases excessive stress is placed on the knee joint especially at the start and early range of the knee extension. Work in this range only if you are physically prepared.
- Do not use excessively heavy weights. When you do this it becomes more difficult to get full extension of the knee joint. Also the stress created in the knee will be much greater and could cause injury.
- Do not lean forward as the exercise is executed. Such positioning does not allow the hip flexor muscles to stabilize the pelvic girdle and thigh which is needed to get full knee joint extension. Also, when you lean back slightly you create greater tension on the hip flexors which allows the rectus femoris to contract more strongly in knee extension.
- Exhaling as you straight the legs. When this happens the trunk becomes unstable which leads to poor stabilization of the pelvic girdle. As a result other muscles come into play and the chances of injury increase.
For more information on this and other exercises, see KINESIOLOGY OF EXERCISE by Dr. Michael Yessis.