What’s your favorite lower body exercise? Knee extensions are a popular exercise for strengthening the quadriceps muscles in the front of the leg. You often hear squats are harmful to your knees, but there’s little evidence to support this idea if you’re using proper form. In fact, there are more concerns about leg extensions than squats when it comes to preserving knee health. Are leg extensions harmful to your knees? Let’s look at what the evidence shows.
What is a Leg Extension?
A leg extension is a simple exercise where you extend your legs against resistance. You can do the exercise using a chair or a bench, but most people use a leg extension machine, available at most gyms. The leg extension machine works your quadriceps muscles in the front of your thighs. You sit on the seat and use your legs to raise a bar of whatever resistance you set the machine for.
Leg extensions is an exercise that works the quadriceps muscles in isolation. Since you’re in a sitting position, your quads do most of the work, and there’s minimal input from your glutes or hamstrings with this exercise. This contrasts with squats, a compound exercise where you work multiple muscle groups in your lower body, including quads, hamstrings, and glutes. Plus, your core muscles and calves also contract when you do this exercise. So, you’re getting several muscle groups involved.
Leg Extension Build Stronger Quads
When comparing leg extensions and squats, leg extensions are modestly more effective for building quadriceps strength and size because it’s an exercise targeted to the quadriceps. Squats work your quads, but also your hamstrings and glutes. If you’re looking for an exercise that works your entire lower body, squats are your best option, as you’re getting a complete lower body workout. But if you specifically have weak quads and your quadriceps are weaker than your hamstrings and glutes (not a common scenario), leg extensions are best for correcting that imbalance because you can more precisely target your quads.
How Safe is the Leg Extension?
Knee injuries are something every person who lifts or exercises should be concerned about. A painful knee injury can make it hard to train for weeks and be a source of pain and frustration. Plus, you must worry about the long-term health of your knees. Who wants achy, stiff joints in 20 years? Are your knees at risk if you do leg extensions?
As Brad Schoenfeld Ph.D. and personal trainer points out, leg extensions create shearing forces, forces applied parallel to the leg, on the knee. You often hear squats are risky for the knees, but squatting creates compressive force more than shearing forces. Compression applies forces perpendicular to the axis of the leg. Shearing forces are more damaging to the knee than compressive forces. Shearing forces also stress the ligaments that support and stabilize the knee, including the anterior cruciate ligament. (ACL). Based on this, leg extensions are more likely to harm your knee joints.
Considering leg extensions are more problematic than squats, for people with knee pain or a history of knee injury you might think one belongs in your workout and the other doesn’t. It’s not quite that simple. Both squats and leg extensions are safe exercises if you use impeccable form, and neither may be safe if you have known knee problems. As studies show, the amount of force placed on anterior cruciate ligaments doing leg extensions is far lower than the threshold amount of force needed to injure the ligaments. In fact, one study found the force was less than what walking creates.
Of the two exercises, the squat is safer, as it places less shearing force on the knees. However, the risks of leg extensions may be overstated based on the amount of force they place on the knees. With either exercise, it’s important to use proper form. The squat is a trickier movement to master and do correctly, whereas machine-based leg extensions are more straightforward and have fewer “moving parts.” So, the latter is more intuitive and easier to master.
Benefits of Squats Over Leg Extensions
If you could only do one exercise for your lower body, squats are your best option. With squats, you work your whole body, including your calves and core, whereas, with a leg extension machine, you only work the quadriceps muscles in your thighs. Since you’re working multiple muscle groups and moving more than one joint, you also burn more calories than you do with a single joint exercise like leg extensions. Plus, squats teach the muscles in your lower body and core to work together, so you gain functional strength. If you play sports, squats will improve your performance in many sports endeavors, while leg extensions alone do little to improve sports performance or enhance functionality.
The Bottom Line
If you have healthy knees, there’s a place for leg extensions and squats in your lower body training. Leg extensions allow you to focus on a single muscle group, the quadriceps, to correct a quad weakness. You can also target one quadriceps muscle at a time by extending only one leg. It’s more challenging to do a one-legged squat.
If you have a history of knee injury or knee pain, squats are a better choice. But regardless of what exercises you do, focus on using impeccable form. You can injure your knees doing either exercise. However, the risk of straining or tearing a ligament is higher with leg extensions.
If you experience pain when you do leg extensions, it’s not the exercise for you. Listen to the feedback you get from your knees when you do the exercise and remember that you don’t have to do leg extensions to strengthen or build your quads. Squats will do it nicely too.
- com. “Are Leg Extensions Good or Bad, Safe or Dangerous, Effective or a Waste of time, Functional or NonFunctional? – Exercise Expert Brad Schoenfeld has the Surprising Answers!”
- “Are the Seated Leg Extension, Leg Curl, and Adduction ….” .nsca.com/education/articles/ptq/are-the-seated-leg-extension-leg-curl-and-adduction-machine-exercises-non-functional-or-risky/.
- . Escamilla, RF, Macleod, TD, Wilk, KE, Paulos, L, and Andrews, JR. Anterior cruciate ligament strain and tensile forces for weightbearing and non-weight-bearing exercises: A guide to exercise selection. Journal of Orthopaedic and Sports Physical Therapy 42: 208-220, 2012.
- Shelburne, KB, Pandy, MG, Anderson, FC, and Torry, MR. Pattern of anterior cruciate ligament force in normal walking. Journal of Biomechanics 37: 797-805, 2004.
- Ema R, Sakaguchi M, Akagi R, Kawakami Y. Unique activation of the quadriceps femoris during single- and multi-joint exercises. Eur J Appl Physiol. 2016 May;116(5):1031-41. doi: 10.1007/s00421-016-3363-5. Epub 2016 Mar 31. PMID: 27032805.