EXECUTION
- Assume a seated position on a leg extension machine with both thighs in full contact with the seat. There should be approximately a 90-degree or greater angle in the knee joints (measured on the back of the leg) after you place the lower shins against the resistance roller pads.
- Assume a vertical or slightly backward leaning position against the back support pad. Grip the handles alongside the seat or hold on to the sides of the seat.
- Inhale slightly more than usual and hold your breath as you straighten your legs to full extension. Hold the maximally contracted muscles for one to two seconds but only if your legs are fully straightened.
- Exhale and relax the muscles as you return to the almost vertical shin position. Then stop, inhale and repeat.
- For balanced development, keep your toes pointed straight ahead or slightly to the outsides.
- If you have difficulty completely extending the legs, lean back as much as possible, especially if the base seat is angled upward. Note that some benches may not allow you to lean backwards.
WORKOUT TIPS
- Because of the immobile positioning of the thighs, great pressure is built up in the knee joint which can be dangerous. To minimize the pressure, it is important that you do not move the shins under the thighs (90 degrees or less angle in the knee joints). Such positioning is especially stressful on the knee when beginning the leg extension movement.
- Full extension of the knee joint is important for achieving a maximal contraction of the vastus medialis and lateralis muscles. Strength of these two muscles is very important in maintaining the patella (knee cap) in place.
- Do not use excessively heavy weights as they will prevent you from achieving full leg extension and produce excessive stress in the knee joint.
- Tight hamstrings may prevent you from fully extending the legs. In such cases, it is important that you lean the trunk back as much as possible to give the hamstrings some slack. Also, select a machine which has a seat level to the floor and is not angled upward, which also makes the hamstrings taut.
- For variety and for a stronger contraction of the vastus medialis, point the toes inward as you move into full extension. For more emphasis on the vastus lateralis, point the toes outward as you go into full extension.
- If this exercise places excessive stress on the knee joints, you can do a variant of this exercise that I have developed that is equally, if not more effective. With a low cable or rubber tubing attached to the ankle, stand with your thigh raised to approximately a 45-degree angle. Hold the thigh in position and execute knee extension against the resistance. In this variant, the hip muscles are capable of taking some of the stress of the exercise which makes it safer on the knee joint.
- Inhaling slightly more than usual and holding your breath allows you to exert more power to achieve full leg extension and at the same time helps to stabilize the upper body and pelvic girdle.
MAJOR MUSCLES INVOLVED
Only one muscle is involved in leg extensions – the quadriceps, which is composed of four separate muscles all of which have the same action. The rectus femoris is a relatively long muscle that runs the length of the middle thigh. It is a two-jointed muscle that acts at the knee joint (extension) as well as at the hip joint (flexion). The vastus intermedius is a somewhat smaller but strong muscle which lies mainly in the mid-thigh under the rectus femoris. The vastus medialis and vastus lateralis muscles located on the inner and outer sides of the thigh respectively, are known as the teardrop muscles. These four muscles insert together via a common tendon onto the patella which in turn inserts onto the shin.
MUSCLE/JOINT ACTIONS
The major action in this exercise is knee joint extension in which the lower leg moves away from the thigh as the leg straightens from a knee bent position. Most of the initial force in this exercise comes from the vastus intermedius and the rectus femoris (which contributes even more as you lean back to place it on stretch). The vastus medialis and lateralis play their major role at the end of the knee extension action. Because the quadriceps muscle inserts in and over the patella (which acts as a pulley wheel) and then into the shin, there is great leverage in pulling the shin forward when the muscles contract. In the seated position the hamstrings are taut at the hip joint and thus may prevent full knee extension.
SPORTS USES
For bodybuilders, the leg extension exercise (more accurately knee joint extension) is one of the better exercises to help develop and define the anterior thigh (quadriceps) muscle. When done to complete knee extension, it is the best exercise for developing and defining the teardrop muscles to hold the patella in place – one of the keys to a safe and strong knee joint. Strengthening the quadriceps muscle by doing leg extensions is important in many sports that require jumping, running, kicking, skipping, lifting and pushing. More specifically, knee joint extension is needed in all jumps such as the high jump, long jump, jump shot in basketball, and volleyball spike and block. It plays a major role in running, as seen in track, football, baseball and soccer, in jumping, to get you airborne and in all cutting actions. In addition, knee joint extension is one of the key actions in kicking as seen in football, soccer, karate, and other martial arts. The knee joint extension exercise is also important in rehabilitation, especially with light weights and going to full extension with a hold for several seconds.