Updated: Dec 18, 2019
As a physio that has rehabilitated a large number of athletes post ACL reconstruction one of the aspects I find most frustrating is the widely held belief that quadriceps strengthening utilising a knee extension machine is “dangerous” and not indicated post surgery. This myth is perpetuated by many across the health profession and particularly influences junior physiotherapists confidence in utilising this rehabilitation tool This is as a result of the potential increase in tibial translation therefore loading of the ACL graft during this exercise and the resulting concern that this may damage, stretch, weaken or even rupture the graft.
Clearly as with any rehabilitation post reconstruction the timing of strengthening exercises must be appropriate but as long as implemented as part of a structured rehabilitation programme at an appropriate stage I strongly believe this exercise provides huge benefits to the reconstructed limb and there is a growing amount of evidence to support this. This exercise is generally not completed in a loaded manner in the first six weeks post reconstruction but there is evidence beginning to suggest that this timeframe could also shift somewhat with significantly greater results in those who started open kinetic chain strengthening at 4 weeks in comparison to those that started at 12 weeks (Fukuda et al, 2013). There are also ways in which this load can be modified such as minimising the range of motion utilised (reduces load on the graft), altering the lever length and the use of occlusion whilst completing this exercise. Occlusion allows us to achieve satisfactory strength gains from the use of minimal loads (20-30%MVC) therefore achieving the desired outcome but whilst placing minimal load on the ACL graft (Hughes et al, 2017).
The question then becomes why is this so important? Well research tells us that quadriceps strength is a good predictor of function and performance on hop tests with those with <85% quadricep strength demonstrating reduced function and performance without any link to graft type, concurrent injuries or knee pain and symptoms (Schmitt, Paterno, Hewett, 2012). Physical function at the time of return to sport following ACL reconstruction was largely influenced by the recovery of quadriceps strength (Lepley & Palmieri-Smith, 2015). Greater than 80% of quadricep strength after ACL reconstruction has been found to be associated with less severe patellar cartilage damage at short-term follow-up (Wang et al, 2015). Functional outcomes have also been shown to be significantly greater in those with greater quadricep strength post ACL reconstruction (Maly, Costigan, Olney, 2006). Finally Schmitt et al, 2015 highlighted that those post ACL reconstruction that had weaker quadriceps showed more asymmetry in their landing mechanics and with high re-rupture rates of up to 30% and the risk altered landing mechanics has on ACL rupture risk this is a modifiable factor that must be corrected.
Firstly let’s talk about the actual strain forces that go through the ACL during knee extension and how we can alter these. If you are concerned regarding the utilisation of knee extension in your ACL rehab then you can ensure there is less strain placed through the ACL by adjusting the exercise slightly. You can do this in a number of ways. Firstly move the point of contact of the machine from just above the ankle to mid shin which significantly reduces the ACL strain value by up to 50% (Escamilla, MacLeod, Wilk, Paulos, & Andews, 2012).
The other alternative is to alter the angle at which the exercise is performed. Instead of moving from 90-100 degrees of knee flexion to 0 degrees you can move between 100 degrees and 45- 60 degrees of knee flexion and this removes the anterior tibial translation - which occurs between 60 and 0 degrees with peak strain between 0-30 degrees (Escamilla, MacLeod, Wilk, Paulos, & Andews, 2012). This is no different than altering the range of motion you complete exercises in conditions such as patella or achilles tendinopathy in order to ensure the desired result without the potential negative effects.
What must be remembered is that the majority of exercises used to rehabilitate after an ACL reconstruction place a strain load on the graft as clearly does a return to exercise and sport. Beynnon et al, 1997 tested ACL strain as a relative value compared to a lachmans test and showed that active knee extension produced a 3.8% strain on the ACL in comparison to a squat 3.6% and a squat with resistance 4.0%. See below for a comparison of exercises and the strain values placed on the ACL.
What we also know is that closed kinetic chain exercises such as squats also cause anterior tibial translation. They do this at the opposing end of the movement in comparison to open kinetic chain exercises such as knee extension. This means that there is little translation when the knee is in extension and yet when the knee flexion angle reaches 50 degrees this starts to increase. Also in closed kinetic chain it must be remembered that this translation and the overall strain on the ACL are affected by trunk and knee positions (McGinty, Irrgang, & Pezzullo, 2000; Escamilla, Macleod, Wilk, Paulos & Andrews, 2012; Keays, Sayers, Mellifont, & Richardson, 2013).
Overall we know that both open and closed kinetic chain exercises place load on the ACL graft with the open chain exercises likely producing the largest load. Whilst this is the case this doesn’t mean in any way that these are detrimental exercises and that any damage is going to occur to the graft as a result and there is even research suggesting that strengthening utilising knee extension results in an overall increase in knee stiffness/reduction in ligament laxity (Barcellona, Morrissey, Milligan, Clinton & Amis, 2015).
The shift towards functional strengthening in the past 15 years has bought about many benefits but unfortunately it has largely occurred at the expense of open kinetic chain exercises which when used appropriately have an essential role in isolated muscle strengthening. A large number of gyms now have no “machines” at all and the reality is in functional exercises those with significant weaknesses can “hide” these by compensating through altered movement patterns. This allows them to continue to increase their overall strength and show solid progression and yet it is likely the weakness is simply becoming more significant as it is “left behind”. It has been shown that after ACL reconstructions athletes will compensate away from their operated knee for at least 5 months post-op even when doing body weight squats with no added resistance (Sigward et al, 2018).
As physios we are all aware that quadricep wasting and weakness post ACL rupture (whether the athlete has surgery or not) is a significant issue that must be addressed if the athlete is to have a satisfactory outcome and a return to sport at their desired level. This is also essential to ensure that the detrimental long term effects of this “life altering injury” are minimised as much as possible. Statistics from the States show us that at least 50% of those who undergo an ACL reconstruction will have arthritic change that causes pain and impairments 5 – 20 years after surgery (. “Strength is KING” when it comes to ACL rehabilitation, prevention of future injury and preservation where possible of an appropriately functioning knee joint. And the “kicker” of all of this is you can squat and lunge and deadlift at appropriately high levels (achieving benchmarks) and yet you may continue to have a weak quadricep that doesn’t function as it should to assist in knee joint protection as well as power and speed development.
What we also need to remember is a return to sport doesn’t equal a return to performance and it is our role as physiotherapists to ensure that our athletes don’t return to sport until appropriate benchmarks are achieved. This requires testing with objective data not just functional tests so whether its hamstring or quadricep strength or repeated speed these are just as important as whether someone can jump, hop and complete agility and sport specific training.
It has been shown in research by Mikkelsen, Werner and Eriksson, 2000 that patients that performed both open and closed kinetic chain rehabilitation post ACL reconstruction had significantly greater outcomes than those who solely completed closed kinetic chain rehabilitation including no sign of instability, symmetrical quadricep strength and a two month earlier return to sport. These positive results of open kinetic chain exercises have also been shown in a number of other studies (Fleming, Oksendahl & Beynnon, 2005; Fukada et al, 2013; Barcellona, Morrissey, Milligan, Clinton & Amis, 2015; Melick et al, 2016). The importance of symmetrical quadricep strength prior to return to sport has been further highlighted by Grindem et al, 2016. With the significant re-rupture rates on return to sport following ACL reconstruction (30% re-rupture rate in 2 years in level 1 sports) we must ensure we are preparing the knee for the challenging demands it will have placed on it and I don’t believe there is any place for “fear” avoidance of isolated strength training.
So at the very least ensure at 12 weeks post reconstruction your athlete is completing isolated unilateral knee extension through full range of motion and for those of you who are comfortable utilising this earlier in your rehabilitation consider the option of restricting the range of motion to 45-90 degrees until 10-12 weeks.
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