According to About.com, the quadriceps are the primary muscles used in leg extensions. The quadriceps are located on the front of the thighs and include the rectus femoris, vastus intermedius, vastus lateralis and vastus medialis muscles.
The leg extension is a seated strength-building, resistance weight-training exercise performed on a lever machine. According to the Miami Dade College website, the leg extension is considered an isolation exercise because it focuses tension only on the quadriceps. Other leg exercises, like squats and leg presses, work other leg muscles, such as the calves, hamstrings or glutes, but leg extensions are the only leg exercises to focus solely on the quads.
According to Miami Dade, the leg extension involves three different types of muscle contractions. The concentric contraction that takes place when lifting the weight is the primarily focus, but the quadriceps also contract during the other phases of the movement.
Although the leg extension is a popular strength-training exercise, it is not without its critics. Men’s Health warns that the seated leg extension puts unnecessary stress on the knees when compared to squats, citing research performed by the Mayo Clinic and Auburn University. They recommend free-weight squats, lunges and split squats as better alternatives for working the quadriceps muscles without putting unnecessary strain on the knees.