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 the body part being exercised is anchored in the squat (feet on ground), and free to move with the leg extension (padded bar moves) -- thus the chain of movement is "open" in the leg extension.
At this point of agreement, a relatively passionate debate has arisen in the 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, that is, that leg extensions are dangerous.
From reading quite a few opinions on this, including quite a few scientific and biomedical opinions, my position is somewhere in between. Even so, this is not sitting on the fence, this is a considered opinion that leg extensions can be used safely with a few precautions.
Doing Leg Extensions Safely - Points to Note
- If you have a knee/thigh to be rehabilitated, be guided by 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 . . . but 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 -- for variety if you wish -- that also includes squats for lower-body conditioning.
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.