In this post we dive into the reasons behind why the dreaded seated knee extensions are not only safe, but extremely effective during your ACL rehab process. We do this primarily by breaking down three of the most common arguments supporting the dangers of seated knee extensions.
There are honestly so many good blog posts out there already debunking this topic (see references) that I feel a little annoyed that I still have to write about this at all in 2024… but the myth continues to circulate so here we go.
Hopefully, this gets through to you and you can help not only smash your own ACL rehab, but minimise misinformation by spreading the good word too!
1. "Knee extensions will damage your new ACL"
One common belief is that knee extensions put too much strain on your new ACL, causing harm. But here's the thing: all exercises put some strain on the ACL—and that’s actually a good thing! Strain is just a term describing how a structure responds to force, and a healthy amount is necessary for healing.
Now, let’s put this in perspective. How much strain do knee extensions really put on the ACL compared to other common rehab movements?
Seated Isometric Leg Extensions (30Nm Torque at 15 degrees): 4.4% strain on ACL2.
Walking: 10-17% strain on ACL, depending on the stage of the gait cycle2.
Squatting: Around 4% strain on ACL1.
So, the strain from seated knee extensions is actually lower—or very similar—to exercises like walking and squatting. Yet, the myth persists that knee extensions are more dangerous for the ACL. Perhaps this is because knee extensions make your quads feel more “toasted” than squats, leading to the incorrect assumption that they’re harder on the ACL.
The truth is, knee extensions are unbeatable when it comes to isolating your quads—which brings us to our next point.
2. "Knee extensions aren’t necessary for quad strengthening"
Here’s an ACL rehab fact: your quadriceps are the kings of the recovery process.
Research shows that weak quads on the surgical side are a major predictor of reinjury when returning to high-level sports (like those involving jumping, pivoting, and cutting). In fact, for every 1% increase in quad strength symmetry, there’s a 3% reduction in reinjury risk4.
Sadly, 25% of athletes under 25 who return to sports after ACL surgery will tear their ACL again5.
So, quad strength is non-negotiable when you’re looking to run, jump, or pivot safely post-surgery6.
The seated knee extension machine is one of the only exercises that fully isolates the quads. Other exercises like squats, lunges, and leg presses are great, but they don’t ensure that your quads are doing all the work. With these exercises, there’s a high potential for compensation, meaning your body may unconsciously use other muscles to protect your knee.
Let’s take a squat for example. When you perform a compensation on your surgical side, it can look like your hips & ankles are working more than your knees. When you perform a compensation on your non-surgical side, it can look like you swaying to that side to offload your entire surgical limb7.
In contrast, the knee extension machine removes those compensations, ensuring that your quads bear the load. Squats and lunges are great, but they leave too much room for error, and compensations can often go unnoticed, fooling you into thinking you’re doing more for your quads than you actually are.
3. "Knee extensions aren’t functional exercises"
Ah, the good old “functional exercise” debate. Let’s clear this up.
If “functional” means improving your ability to run, jump, and cut, then how can any quad-strengthening exercise be considered non-functional? Improving quad strength directly translates into better performance in these activities.
If you’re arguing that exercises should mimic real-life activities to be functional, then sure, a seated knee extension doesn’t look like running or jumping. But does that make it less effective at getting you back to those activities? Not at all.
By building stronger quads through knee extensions, you’re increasing your ability to apply force during those “functional” movements on the field. In other words, you’re improving your performance in the very activities that matter most.
In Summary:
Seated knee extensions aren’t the dangerous exercise they’re made out to be. They:
Put less strain on your ACL than commonly accepted movements like walking and squatting.
Maximize quad activation, helping you build strength essential for a safe return to sport.
Remove compensations that could hinder your progress and leave your quads weaker than they should be.
Are highly functional when it comes to improving your ability to perform in sports.
Knee extensions are safe. They’re effective. And they’re one of the best ways to rebuild your quads post-ACL surgery.
References:
Recent Advances in the Rehabilitation of Anterior Cruciate Ligament Injuries Kevin E. Wilk, Leonard C. Macrina, E. Lyle Cain, Jeffrey R. Dugas, and James R. Andrews Journal of Orthopaedic & Sports Physical Therapy 2012 42:3, 153-171
Taylor KA, Cutcliffe HC, Queen RM, Utturkar GM, Spritzer CE, Garrett WE, DeFrate LE. In vivo measurement of ACL length and relative strain during walking. J Biomech. 2013 Feb 1;46(3):478-83. doi: 10.1016/j.jbiomech.2012.10.031. Epub 2012 Nov 21. PMID: 23178040; PMCID: PMC3552116.
Grindem H, Eitzen I, Engebretsen L, Snyder-Mackler L, Risberg MA. Nonsurgical or Surgical Treatment of ACL Injuries: Knee Function, Sports Participation, and Knee Reinjury: The Delaware-Oslo ACL Cohort Study. J Bone Joint Surg Am. 2014 Aug 6;96(15):1233-1241. doi: 10.2106/JBJS.M.01054. PMID: 25100769; PMCID: PMC4116562.
Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016 Jul;50(13):804-8. doi: 10.1136/bjsports-2016-096031. Epub 2016 May 9. PMID: 27162233; PMCID: PMC4912389.
Wiggins AJ, Grandhi RK, Schneider DK, Stanfield D, Webster KE, Myer GD. Risk of Secondary Injury in Younger Athletes After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis. Am J Sports Med. 2016 Jul;44(7):1861-76. doi: 10.1177/0363546515621554. Epub 2016 Jan 15. PMID: 26772611; PMCID: PMC5501245.
Sinacore JA, Evans AM, Lynch BN, Joreitz RE, Irrgang JJ, Lynch AD. Diagnostic Accuracy of Handheld Dynamometry and 1-Repetition-Maximum Tests for Identifying Meaningful Quadriceps Strength Asymmetries. J Orthop Sports Phys Ther. 2017 Feb;47(2):97-107. doi: 10.2519/jospt.2017.6651. PMID: 28142362
Sigward SM, Chan MM, Lin PE, Almansouri SY, Pratt KA. Compensatory Strategies That Reduce Knee Extensor Demand During a Bilateral Squat Change From 3 to 5 Months Following Anterior Cruciate Ligament Reconstruction. J Orthop Sports Phys Ther. 2018 Sep;48(9):713-718. doi: 10.2519/jospt.2018.7977. Epub 2018 Jun 12. PMID: 29895231.