You might want to reconsider doing leg extensions. The leg extension machines safety and efficiency is one of most controversial subjects. It isolates and strengthens the quadriceps, but at what cost. The risk may outweigh the rewards.
Physiologists at the Mayo Clinic determined that leg extensions place a lot more stress on the knee joint than a squat, even under load. Why? The resistance near the ankles lead to high amount torque on the knees every time you lower the weight. So do squats or lunges, they’re actually much safer.
Another issue would be this exercise can place shearing forces on the knee, which increases the risk of injury. Researchers found when comparing closed kinetic chain exercises (squats) and open kinetic chain (leg extensions) exercises that there was tension on the ACL occurred during full extension phase on the leg extension.
A study published in the Journal of Orthopedic Sports Physical Therapy in November 2003, researchers found that subluxation (knee dislocation) was greater during leg extensions compared to a squat.