Compensatory Strategies That Reduce Knee Extensor Demand During a Bilateral Squat Change From 3 to 5 Months Following Anterior Cruciate Ligament Reconstruction

Journal of Orthopaedic and Sports Physical Therapy,  June 2018

Sigward Et. Al   

Human Performance Laboratory, Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, CA.

Do all your patients squat like this prior to returning to sport from ACLR?

Okay, I’m sure they’re looking better than that prior to hitting the field, though to be fair, we should give the guy in the video some kudos for his raw and heroic effort.

Compensation for Decreased Quad Strength After ACL Repair

You’ve all seen the classic weight shift in the first months out during ACL repair rehabilitation. Everyone has it at first, but is everyone able to get rid of it?  What’s driving these compensations in patients with acquired quadriceps weakness and are they able to recover their quad strength before returning to sport?

As the knee joint is healing and recovering from surgery it’s natural for the body to adopt strategies to underload the knee joint for protection-purposes. For example, a study demonstrated that those who were 1 month out from ACLR showed a 38% reduction in ground reaction forces (how much force is put down through the affected side) during a simple sit-to-stand task.

Unfortunately, these strategies appear to persist past the point of healing. One study showed a 13% deficit in knee extensor torque with bodyweight squats in the involved vs. uninvolved limb at 13 months after ACLR. This is definitely concerning to see in a sub-maximal exercise such as a bodyweight squat, and you can imagine what this could look like with a more exertional activity. If we know that asymmetrical landing and forces play a role in increased risk for ACL tears then we have to make sure we do everything we can to regain full strength and function prior to returning to sport.

In order to dive deeper into what drives these compensations, the authors of this study looked loading patterns during bodyweight squats in patients at both 3 and 5 months out from ACLR. Furthermore, they investigated how inter-limb (weight shifting side to side) and intra-limb (force generated from an adjacent joint to the knee) compensations played a role in decreased knee extensor force output.

How Did the Squats Look?

11 participants who underwent ACLR performed bodyweight squats to a maximal voluntary painless depth in a biomechanics lab at 3 months and 5 months S/P surgery.

3 months:

• Knee extensor moment was 38% decreased compared to the non-surgical limb.
• Subjects shifted, on average, 40% more torque from the knee to the hip and shifted weight, measured in vertical ground reaction force, 13% more to the non-surgical limb.
• Interlimb compensation (weight shift to the uninvolved limb) was the major contributor to decreased force output. Intralimb compensation (increased force by hip extensors) was a secondary compensation at this time.

5 months:

• Knee extensor moment was 30% decreased compared to the non-surgical limb; however, this was not considered statistically significant for a main effect of time.
• Intralimb compensation was the primary mode of compensation at this point in time with no significant amount of interlimb compensation.

What to Make of Things

Patients may have a serious loss of knee extension force during functional tasks, such as bodyweight squats, at 3 and 5 months from ACLR. These knee extension moment deficits did not get significantly better from 3 to 5 months out. However, from other studies, it does appear that the force output increases over time, but still remain in a deficit with regard to surgical vs. non-surgical limb.

It appears that earlier on, a weight shifting compensation is the primary method for making up for a loss of knee extension strength, whereas later-on in recovery, an intra-limb compensation is preferred with the hip extensors taking on more of the load from the knee extensors. From this, we could consider addressing strategies to restore symmetrical weight-bearing early on in rehab and then make sure we appropriately load the quads at a high level as rehab progresses.

Remember these compensations could conceivably persist past the 5-month mark, so make sure that they are addressed before returning to sport if we want to decrease our problematic ACLR revision incidence rate. Having your patients do straight leg raises with 1 lb on their leg 6 months out and then marking sufficient strength as 5/5 on an MMT is simply not enough. Nobody wants their patient’s squat visual presentation to end up on YouTube, so make sure they get strong!

Tyler Cope

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