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. 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. Leg extensions are a key exercise in strengthening the patellar ligament and the quadriceps attachment for the knee. This exercise focuses on strengthening the quad alone and, therefore, strengthens key attachments for the knee joint at the same time.