Leg Extensions: Benefit or Risk?

Work your legs, without injury.

Updated June 04, 2018
Woman doing the leg extension machine at the gym
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Leg extensions are exercises usually done with a lever machine. You sit on a padded seat and raise a padded bar with your 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, which is different from a "closed chain kinetic exercise," such as a squat.

The difference is that in the squat, the body part you're exercising is anchored (feet on the ground), while in the leg extension, you're moving the padded bar, which means your legs aren't stationary as they work, and thus the chain of movement is open in the leg extension.

Pros and Cons of the Leg Extension Machine

Fitness pros haven debated the leg extension exercise, with regard to the safety of the exercise. 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. But after examining the research, there is a safe way to execute the exercise. All you have to do is follow proper form and mix it up in terms of how you approach the move. 

Doing Leg Extensions Safely

  • If you have a knee or thigh injury, seek guidance from a qualified physical therapist, or strength and conditioning coach who specializes in weight training rehabilitation. Just don't be surprised if they say to avoid the leg extension machine.
  • Don't lift heavy. This is not the machine to try 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.

    If you follow this sensible approach, there's no need to worry about the exercise. You can still effectively use the leg extension machine for building both strength and muscle. Just make sure to still incorporate squats and lunges. 

    View Article Sources
    • 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.