Leg Extensions: Benefit or Risk?

woman doing the leg extension machine at the gym
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Leg extensions are exercises usually done with a lever machine in a gym. You sit on a padded seat and raise a padded bar with your lower legs. The exercise works mainly the quadriceps muscles of the front of the thigh—the rectus femoris and the vastus muscles.

Technically, this is an "open chain kinetic" exercise to distinguish it from "closed chain kinetic exercises" such as squats.

The difference is that in the squat the body part being exercised is anchored (feet on the ground), while in the leg extension it is free to move with the leg extension (the padded bar moves), and thus the chain of movement is open in the leg extension.

The Leg Extension Benefits Debate

A relatively passionate debate has arisen in fitness circles with regard to the safety of leg extension exercises. Critics say that open chain exercises like the leg extension can damage the knees and that even full depth squatting is safer. Many trainers seem to have gone along with the loudest voices, avoiding leg extensions.

From reading quite a few opinions on this, including scientific and biomedical opinions, my position is somewhere in between. This is not sitting on the fence, however; this is a considered opinion that leg extensions can be used safely given a few precautions.

Doing Leg Extensions Safely

  • If you have a knee/thigh to be rehabilitated, seek guidance from a qualified physical therapist, or strength and conditioning coach who specializes in weight training rehabilitation. Don't be surprised if they say to avoid the leg extension machine, though not all will.
  • Don't lift heavy. This is not the machine to be trying out for a maximum lift (1RM), or even low-rep, high-load strength conditioning.
  • Don't do more than around 3 sets of 8 to 12 reps at moderate load. You don't need to do any so-called endurance sets with high repetitions on the leg extension machine.
  • Don't use the leg extension exercise exclusively for quadriceps development. Feel free to include it in a program that also includes squats for lower body conditioning for variety if you wish.

If you follow this sensible approach, you don't need to be afraid of practical use of a machine that may help in your training or rehabilitation.


Tagesson, S., Oberg, B., Good, L., and Kvist, J. (2007). A comprehensive rehabilitation program with quadriceps strengthening in closed versus open kinetic chain exercise in patients with anterior cruciate ligament deficiency: a randomized clinical trial evaluating dynamic tibial translation and muscle function. Am J Sp. Med. 36(2): 298–307.

Cohen, Z.A., Roglic, H., Grelsamer, R.P., Henry, J.H., Levine, W.N., Mow, V.C., and Ateshian, G.A. (2001). Patellofemoral stresses during open and closed kinetic chain exercises. Am. J. Sp. Med. 29(4): 480.

Fleming, B.C., Oksendahl, H., and Beynnon, B.D. (2005). Open-or closed-kinetic chain exercises after anterior cruciate ligament reconstruction? Exerc. Sport Sci. Rev. 33(3): 134–140.

Morrissey, M.C., Drechsler, W.I., Morrissey, D., Knight, P.R., Armstrong, P.W., and McAuliffe, T.B. (2002). Effects of distally fixated versus nondistally fixated leg extensor resistance training on knee pain in the early period after anterior cruciate ligament reconstruction. Phys. Ther. 82(1): 35–43.