Leg extensions are a weight-training exercise that targets the quadriceps muscles located at the front of the upper thigh. A leg-extension machine is used, where you sit on a chair and lift a weight bar, using your quadriceps muscle. The lever leg extension machine is the most common, but many varieties exist. There are both pros and cons to using a leg-extension machine to work your quads.
Sit on the chair with your back pressed against the back rest. Place the front of your lower legs under the padded lever. Make sure your knee joint is on the same level as the lever fulcrum. Hold the handles for support, and exhale as you extend your knees to lift the lever forward and upward. Continue until your knees are straight but not locked. Return slowly to the starting position to complete one rep. Do two sets of 12 to 15 repetitions. Choose a weight that offers a challenge but is not so heavy that you cannot complete the set with proper form.
The leg-extension exercise targets the quadriceps and is a good machine on which to start training the muscles at the front of the thigh. The exercise is simple and is hard to do incorrectly, aside from having the knee at the wrong height. This exercise is a good one to perform if you wish to work only your quads, because it involves no other muscles. The leg-extension exercise helps people improve the force of kicking movements.
The leg extension applies constant ACL tension, so people with ligament injuries should avoid this exercise, particularly when heavy weights are in use. There is an increased risk of lateral patellar deviation, no hamstring activity and increased joint stress in regards to shear forces. When you perform a leg extension with heavy weight, you drastically increase the chance of sustaining a knee injury, due to the forces involved. Finally, the training benefits realized in a leg extension do not carry over into real life except for kicking movements, so this exercise is not very practical.
Do not add leg-extension exercises to your fitness routine without first speaking with your doctor, physical therapist or personal trainer. If you have ligament damage, are recovering from ACL surgery, or any other knee injury or surgery, do not perform this exercise without first speaking with your doctor. If you feel a sharp pain at any time during this exercise, stop immediately. Always use a light weight when performing this exercise, to reduce the chance of injury to your knee. Make sure you use proper form. Incorporate hamstring-strengthening exercises such as hamstring curls into your training program as well. When the quadriceps are stronger than the hamstrings, particularly in women, your knee loses stability and is at a greater risk of injury.