Leg Extensions: Why This Exercise Isn't the Best Choice

29 January 20252 min read By Dushyanta Dushyanta

The leg extension machine is a gym staple. It feels easy and isolates the quadriceps. The mechanics carry a downside that better alternatives avoid. Resistance applied at the ankle creates an anterior shear force on the tibia against the femur. The same muscle group can be loaded with less knee strain through squats, leg presses, lunges and step-ups. For most adults the swap is worth making.

The problem in plain terms

In a squat or a leg press, the load passes vertically through the knee joint as compression. In a leg extension the lever arm pulls the tibia forward, generating shear stress on the anterior cruciate ligament and the patellar tendon. Wilk and colleagues at the American Sports Medicine Institute documented in 1996 (Am J Sports Med) that open kinetic chain knee extension produces substantially higher anterior tibial shear forces than closed-chain alternatives. For a healthy knee with moderate load, the exercise is rarely the worst thing in a programme. For a knee with prior ACL injury, patellar tendon irritation, or pre-existing patellofemoral pain, it adds load exactly where the joint is least equipped to take it.

Four better quad builders

Back squat
Compression load through the joint. Builds quads, glutes, hamstrings and core in one movement.
Walking lunge
Single-leg work. Quads with balance and stability. Natural range of motion.
Leg press
Heavy quad loading without the shear of leg extensions. Good for adults who struggle with squat form.
Step-up
Functional pattern. Load progressively with dumbbells or a barbell.

When leg extensions make sense

Controlled rehabilitation under a physiotherapist. End-range quad activation work for bodybuilders with healthy knees. Light, slow, partial-range sets as a finisher when the squat rack is taken. Skip heavy loads, fast cadences, and full hyperextension.

Bottom line

Leg extensions are not the worst exercise. They are rarely the best choice when squats, lunges, leg presses and step-ups build the same muscle with less knee stress and better carry-over to real life. Use them sparingly, if at all.

Work with DT Fitness London

For a strength plan that protects knees and progresses safely, book a consultation at www.dushyantatomar.com.

Dushyanta Tomar, MSc Applied Sports and Exercise Physiology, CIMSPA Accredited Personal Trainer.

Sources

  1. Wilk KE, Escamilla RF, Fleisig GS, Barrentine SW, Andrews JR, Boyd ML. A comparison of tibiofemoral joint forces and electromyographic activity during open and closed kinetic chain exercises. Am J Sports Med. 1996, vol 24, issue 4, pages 518 to 527.
  2. Escamilla RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc. 2001, vol 33, issue 1, pages 127 to 141.
Back to blog