Leg Extensions: Safe after ACLR

The purpose of this blog is to discuss why leg extensions are safe, functional, and absolutely vital after an Anterior Cruciate Ligament Reconstruction (ACLR). It is a guest blog from the website of Dr. Nicole Surdyka, Physical Therapist.

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I didn’t actually think that this was such a controversial topic anymore. I thought that the myths had successfully been busted and that everyone was including both open and closed kinetic chain (OKC and CKC, respectively) exercises in their rehab programs for patients who are recovering from an anterior cruciate ligament reconstruction (ACLR). It turns out that I was wrong.

I think that it is vital to include both open and closed kinetic chain exercises into a holistic rehabilitation program. Before I get into what I think we should be doing, I should first address the arguments I have heard against utilizing OKC exercises.

Myth #1: Implementing OKC Exercises is Dangerous in ACLR Rehab

This is one of the main arguments against utilizing OKC exercises in ACLR rehab. The belief is that performing OKC exercises, such as a leg extension, will place too much strain on the ACL and cause increased ligamentous laxity. First, let’s take a look at the amount of strain on the ACL during OKC exercises, CKC exercises, and some normal functional tasks.

The primary function of the ACL is to restrain anterior tibial translation on the femur. During walking, the peak anterior shear force was found to be 355N at 16.8 degrees of knee flexion.(1) The peak anterior shear force during a seated leg extension from 0-90 degrees of knee flexion was found to be 248N at 14 degrees of knee flexion.(2) Typically, a physical therapist will begin gait retraining with an athlete within the first week or two after ACLR, yet many will not begin any type of OKC exercise until after 6-8 weeks. If there is more anterior shear force (and thus, loading of the ACL) during normal walking than there is during a leg extension, then why the hesitation to prescribe it earlier in the rehab program?

Another commonly prescribed exercise early in the rehab program is the squat. There is a similar amount of strain on the anteromedial portion of the ACL during a squat as there is during an OKC active knee flexion-extension exercise.(3)

Similarly, a Lachman Test, which is usually performed by the surgeon right on the operating table immediately following surgery, performed at 30 degrees of knee flexion with 150N (or about 34 lbs.) of force causes about a 3.5% strain on the ACL.(4) A dynamic OKC seated knee extension exercise from 0-90 degrees of knee flexion causes a maximal amount of strain on the ACL at 10 degrees of knee flexion, and this is only 2.8% strain with no added resistance, and only 3.8% strain with 10 lbs. of added resistance.(5) If you are fine with performing a Lachman Test on an athlete during the first few weeks after ACLR, then you should also be fine with prescribing OKC seated knee extensions as there is a similar amount of ACL strain.

It has been shown that the ultimate strength of a native ACL is 2000N.(6) The amount of ACL loading during an isokinetic knee flexion-extension exercise at 60 degrees per second was 349N at 35-40 degrees of knee flexion.(7) Granted, I don’t think anyone would have an athlete perform an isokinetic knee flexion-extension exercise at 60 degrees per second from 0-90 degrees of knee flexion range of motion immediately following surgery; this is something that is more likely to be progressed to a bit later in the rehab program. This shows us, though, that the overall amount of ACL loading during OKC exercises is really pretty low given its potential capacity.

Overall, neither OKC nor CKC exercises commonly prescribed in ACLR rehab cause very much strain on the ACL. More strain is produced when the knee is between about 10 and 30 degrees of knee flexion, and there is almost no strain on the ACL at knee flexion angles greater than 60 degrees.(8) There is an increase in ACL loading with increasing resistance for OKC exercises, and that tensile force is almost twice as great when the resistance is applied to the ankle as opposed to being applied at the mid-shin.(9) This essentially means that we should be gradually progressing the intensity of the exercises we prescribe, which is a basic principle we should be following for all exercises anyway.

Finally, recent systematic reviews have shown that there are no significant differences in knee laxity between patients who perform OKC exercises and those who do not.(10,11) Since there is similar loading of the ACL in both CKC and OKC exercises, and there is no difference in knee laxity between patients who perform OKC versus CKC exercises, then can we finally put to bed the myth that OKC exercises are somehow harmful or dangerous to the ACL?

Myth #2: Open Chain Exercises Aren’t Functional

I must admit that I’m not entirely sure what is meant by the term “functional” in rehab and performance training anymore. We’ve somehow gotten to a point in which standing on a ball is functional, but getting really strong quads isn’t. The function of an OKC exercise such as the seated knee extension (commonly referred to as a leg extension) is to strengthen the quadriceps muscles. Having really strong quads is crucial for athletes to be able to decelerate, cut, and change directions. We can train these movements all day long, but if an athlete does not have the requisite quad strength to handle those loads, then they are not going to be very efficient in performing those movements. Further, they will likely develop inter- and intra-limb compensations as a result of training a movement that they do not have the capacity to perform.

Quadriceps strength is one of the best predictors of second ACL injury, and for every 1% increase in quad strength limb symmetry index (LSI), there is a 3% reduction in reinjury risk.(12) If there was an exercise that could isolate the quads and apply a load so as to improve their capacity to generate torque, wouldn’t it seem like a really good idea to prescribe that exercise? Well guess what? There is such an exercise! This is exactly the function of a leg extension. Besides, I don’t see too many athletes performing step-ups or doing speed ladders on the soccer field and yet we don’t seem to have any issue prescribing those. The bottom line is that after a primary ACLR, we need to get the athlete’s quads strong again, and OKC knee extensions do exactly that.

Myth #3: You Can Strengthen the Quads Just as Well with CKC Exercises

No, you can’t. Not after a primary ACLR.

Several studies have shown that patients who perform OKC exercises have significantly greater quad strength at follow-up than patients who perform only CKC exercises.(13,14) Yes, front squats, goblet squats, lunges, step downs, and trap bar deadlifts all load the quads. However, these more complex, multi-joint exercises also grant the opportunity for compensations to occur. As early as 3-5 months post-surgery, doing something as simple as a bodyweight squat can be performed with significant intra-limb compensations. The athlete will unload the surgical knee and shift that load to the ipsilateral hip and ankle.(15) The real kicker is that we cannot always see these compensations occur without the use of kinetic and kinematic analysis. In other words, relying solely on CKC, multi-joint, “functional” exercises may not actually be loading the quads as much as you think they are, and even if you think that your athlete is performing these movements “perfectly”, you really can’t tell just through observation.

Further, the main mechanism of second ACL injury is likely to be employing a more hip-dominant strategy for movements such as landing, cutting, and changing directions on a more extended knee. If we never strengthen the quads enough to allow for proper loading of the knee during these tasks, then we are potentially perpetuating this more hip dominant strategy once they step back onto the field.

Of course, there is certainly an important role for CKC exercises in ACLR rehab, and of course, we should be training movements such as squats, deadlifts, lunges, etc. We just also need to be programming OKC exercises as well in order to ensure a complete and holistic program.

Usually, on an athlete’s first visit with me post-ACLR, after I have taken their history and relevant measurements, I have them squat, hinge, ride a bike, do some gait training (depending on concomitant injuries and WB status), and then I have them sit at the edge of the table for some OKC knee extensions. My entrypoint to this exercise varies based on the individual, their surgical history, graft type, and any concomitant injuries. Sometimes this ends up being a simple manually resisted knee extension isometric at 60 degrees of knee flexion. Other times it’s an active knee extension-flexion exercise throughout their entire available range. I rarely add load other than some light manual resistance in the first 2-4 weeks, not because I’m afraid of damaging the graft, but because I understand the importance of gradually progressing exercises. It’s the same reason I wouldn’t just throw a barbell on someone’s back the first time they squat after surgery – we have to gradually increase the load.

Here is a basic progression of OKC knee extension that I use as a guideline. As I said, I always see where that individual’s entrypoint is and then progress them from that point. I also don’t always follow exactly in order and usually have some overlap. For example, I might have the athlete perform active flexion-extension from 90-45 degrees of knee flexion and also add load isometrically at 60 degrees of knee flexion. Every individual is different, and this is simply a guideline.

  1. Isometrics at 60 degrees knee flexion or greater
  2. Active flexion-extension 90-60 degrees
  3. Isometrics at 45 degrees or greater
  4. Active flexion-extension 90-45 degrees
  5. Isometrics at 30 degrees or greater
  6. Active flexion-extension 90-30 degrees
  7. Active flexion-extension through full available range
  8. Isometrics at varying angles throughout available range
  9. Resisted knee extension with load at mid-shin
  10. Resisted knee extension with load at ankle

The reasons behind this progression are that there is almost no strain on the ACL at knee flexion angles above 60 degrees, and I want to get to work right away on getting a quadriceps contraction in those ranges. As we get closer to knee extension, there is gradually more strain through the ACL, so I want to gradually get there. Loading at the mid-shin produces significantly less strain on the ACL than does loading at the ankle, so I usually will add resistance to the mid-shin first before adding it to the ankle. Again, this is so that we provide gradual loading instead of making larger jumps in the load. Once they are able to perform a full OKC knee extension with load at the ankle, we continue to gradually increase intensity. This can be done by manipulating the load, speed, or tempo.

Another thing I like to do during these leg extensions from very early on is to have the athlete perform the movement to the sound of a metronome. Doing this forces the athlete to have to perform a motor task driven by something outside of themselves. They must move in reaction to an external stimulus. This will be important down the road when they must perform motor tasks in reaction to what is happening on the field around them. For more on the importance of neuroplasticity in ACLR rehab, you can watch my interview with Dr. Dustin Grooms here.

A quick word about graft types:

If the athlete I am working with has a hamstring autograft, then I may be a bit more conversative initially than I would be with an athlete who has a bone – patella tendon – bone autograft. The reason for this is that it takes a little longer for soft tissue healing of the hamstring tendon as the new graft. This doesn’t mean that I do not do OKC exercises until 8-12 weeks post-ACLR when that soft tissue graft has had a chance to heal, it simply means that I am more conservative in the progression until that time point. If the athlete has had an allograft, I am also a bit more on the conservative side in the progressions of OKC exercises. Overall, 9 times out of 10, I am prescribing some form of OKC knee extension exercise the very first time I see an athlete post-ACLR.

In conclusion, there really is no reason not to include OKC exercises in ACLR rehabilitation. Doing so is not dangerous to the graft, does not lead to increased knee laxity, and is vital for improving quadriceps strength. As with any other exercise, we should gradually progress OKC knee extensions based on the amount of load being applied to the newly reconstructed ACL.

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If you want to learn more about Nicole Surdyka, DPT, CSCS, make sure to head over to her website – nicolesurdykaphysio.com.

Want to learn more? Check out our some of our other similar blogs:

Are Leg Extensions Bad for Your Knees?, Banded Leg Extensions, Reverse Nordics

Thanks for reading. Please leave any questions or comments below.


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  2. Wilk KE, Andrews JR. The Effects of Pad Placement and Angular Velocity on Tibial Displacement during Isokinetic Exercise. Journal of Orthopaedic & Sports Physical Therapy. 1993;17(1):24-30. doi:10.2519/jospt.1993.17.1.24
  3. Beynnon BD, Johnson RJ, Fleming BC, Stankewich CJ, Renström PA, Nichols CE. The Strain Behavior of the Anterior Cruciate Ligament During Squatting and Active Flexion-Extension. The American Journal of Sports Medicine. 1997;25(6):823-829. doi:10.1177/036354659702500616
  4. Fleming BC, Beynnon BD, Renstrom PA, Peura GD, Nichols CE, Johnson RJ. The Strain Behavior of the Anterior Cruciate Ligament During Bicycling. The American Journal of Sports Medicine. 1998;26(1):109-118. doi:10.1177/03635465980260010301
  5. Beynnon BD, Fleming BC, Johnson RJ, Nichols CE, Renström PA, Pope MH. Anterior Cruciate Ligament Strain Behavior During Rehabilitation Exercises In Vivo. The American Journal of Sports Medicine. 1995;23(1):24-34. doi:10.1177/036354659502300105
  6. Woo SL-Y, -Y. Woo SL, Marcus Hollis J, Adams DJ, Lyon RM, Takai S. Tensile properties of the human femur-anterior cruciate ligament-tibia complex. The American Journal of Sports Medicine. 1991;19(3):217-225. doi:10.1177/036354659101900303
  7. Toutoungi DE, Lu TW, Leardini A, Catani F, O’Connor JJ. Cruciate ligament forces in the human knee during rehabilitation exercises. Clinical Biomechanics. 2000;15(3):176-187. doi:10.1016/s0268-0033(99)00063-7
  8. Escamilla RF, Macleod TD, Wilk KE, Paulos L, Andrews JR. Anterior cruciate ligament strain and tensile forces for weight-bearing and non-weight-bearing exercises: a guide to exercise selection. J Orthop Sports Phys Ther. 2012;42(3):208-220.
  9. Pandy MG, Shelburne KB. Dependence of cruciate-ligament loading on muscle forces and external load. J Biomech. 1997;30(10):1015-1024.
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  12. 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. British Journal of Sports Medicine. 2016;50(13):804-808. doi:10.1136/bjsports-2016-096031
  13. Mikkelsen C, Werner S, Eriksson E. Closed kinetic chain alone compared to combined open and closed kinetic chain exercises for quadriceps strengthening after anterior cruciate ligament reconstruction with respect to return to sports: a prospective matched follow-up study. Knee Surgery, Sports Traumatology, Arthroscopy. 2000;8(6):337-342. doi:10.1007/s001670000143
  14. Tagesson S, Oberg B, Good L, Kvist J. 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 Sports Med. 2008;36(2):298-307.
  15. Sigward SM, Chan M-SM, 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;48(9):713-718.

1 Comment. Leave new

  • I had a partial acl tear now i am on my 45th day of rehab should i start doing knee extensions now?


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