Rehabilitation after ACL Reconstruction: Leg Extensions, Knee Laxity, and Training Load (Not What You Would Expect)


Is knee laxity change after ACL injury and surgery related to open kinetic chain knee extensor training load? Am J Phys Med Rehabil. 2009 May;88(5):369-75. Morrissey MC, Perry MC, King JB.

From the study:

OBJECTIVE:
The purpose of this study was to evaluate whether knee anterior laxity changes after anterior cruciate ligament injury and surgery are related to aspects of thigh muscle resistance training during rehabilitation.
DESIGN:
Forty-nine subjects (13 females) diagnosed with an anterior cruciate ligament-deficient knee or who had undergone anterior cruciate ligament reconstructive surgery participated in this study. The subjects trained their knee extensors in the open kinetic chain during a 6-wk program, and the relationship of aspects of training (for example, absolute resistance load) and other factors to anterior laxity change during this period were analyzed using linear regression analysis.
RESULTS:
The only factor found to be significantly related (r = -0.347) to anterior knee laxity change was average absolute load used in training the knee extensors.
CONCLUSIONS:
These results offer some early clinical support for increasing the strain on the anterior cruciate ligament graft (in patients treated with reconstruction) or other passive restraints to anterior tibial displacement, during rehabilitation after anterior cruciate ligament injury and reconstruction surgery to promote decreased knee anterior laxity.

Chad’s comments:

This one is very interesting in that the researchers found the opposite of what they would have expected and the opposite of what I would have expected. Resistive leg extensions have been considered taboo after ACL reconstruction for decades because they were thought to stretch out the graft resulting in a loose knee joint, prone to arthritis and reinjury. However, subsequent research found that adding leg extensions to a rehabilitative program does improve functional outcomes. Also timing is important with regards to when you should add them into a program, with earlier than 12 weeks post-op perhaps being too soon, particularly with hamstring graft reconstructions.

Researchers of this study expected that increased weights used in rehabilitation of both ACL deficient and ACL reconstructed knees would be the important factor in whether the graft stretched or not. They found that it was, but in the opposite direction of what they thought. Those that trained at a higher intensity (>10 kg seeming to be particularly protective) had less knee joint laxity than those who trained at a lower intensity. Subjects worked to relative high intensity 6RM loads.

These results still leave a number of questions. Is it the weight itself used that resists changes in laxity?  Or are those with stronger muscles better able to protect their ACL, which then leads one to believe higher resistance levels are good because they build stronger muscles? The patients did prone leg curls as well. I would assume use of heavier weights in extensions would be strongly associated with use of heavier weights in flexion. Hamstrings (worked by knee flexion) do act to resist strain on the ACL, so perhaps the difference is related to hamstring strengthening. However, this study found correlation of knee laxity for leg curl load was less so than for quadriceps load. Or perhaps the heavier weights used are the important thing, and using them causes greater compressive stress on the knee during exercise and this fights strain on the ACL.

Also worth noting is 6 of the 24 reconstructions were hamstring graft reconstructions and the rest were patellar grafts, and differences in laxity between these types of reconstructions was not reported. ACL deficient knees however also had lesser laxity after heavier training loads. Leg extension exercises training 90 to 0 degrees were not added until 8 weeks post-op. Other researchers have found that leg extensions added as early as 6 weeks post op did cause increased knee laxity in hamstring graft repairs, but they did not address training load. Perhaps starting the exercise later allows one to train at heavier resistance levels, which are then protective. And of course it could be all those things and more, which keeps physical therapy interesting.

As always, if you have any further questions or need for clarifications, please don’t hesitate to ask. Being aware that some of my blog ideas are contentious and occasionally a bit out of the field of my expertise, I encourage my readers to come forth with any questions/comments that are of interest or concern. Your comments are valued and welcomed.

Chad Reilly is a licensed physical therapist, located in North Phoenix, practicing science based medicine with treatment protocols unique and effective enough to proudly serve patients from Phoenix, Scottsdale, Mesa, Chandler, Tempe, Peoria, and Glendale.

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