Here’s What You Need To Know…
1. It is estimated that over 200,000 ACL injuries occur in the United States every year. Despite approximately 90% of people opting for ACL reconstruction surgery, significant risk for re-injury after repair remains.
2. Despite the current trends in the physical therapy and rehabilitation communities, squatting BELOW 90⁰ while monitoring closed chain knee flexion provides the most advantageous environment for reimplementing gross motor patterns.
3. The low-bar squat represents a safe and effective exercise to incorporate into ACL rehabilitation, even in the early stages of healing of a post-operative ACL reconstruction.
4. Traditional exercises like the supported physio-ball wall squat just don’t provide enough training stimulus in the rehab process to protect against re-injury. Providing a controlled environment for reimplementing load in addition to proprioceptive challenges is the future of ACL rehabilitation.
5. Simply put, the biomechanics of the low-bar squat encourage maximal strength development with little risk to the ACL. This evidence is more than enough to change the way we rehab ACL reconstruction as a community of rehab specialists. Compound loaded movements will always prevail!
Staggering ACL Injury Rates
It is estimated that over 200,000 ACL injuries occur in the United States every year. Despite approximately 90% of people opting for ACL reconstruction surgery, significant risk for re-injury after repair remains. In fact, research has shown that subjects who undergo ACL reconstruction are still 15 times more likely to sustain an ACL injury in the following year and six times more likely in two years than those with no history of ACL injury (1-2).
Current criteria to return-to-sport after an ACL reconstruction usually involve a battery of testing including, but not limited to, isokinetic strength testing, jump evaluation, and knee arthrometer testing. Typically, strength criteria includes achieving less than 10% deficit in strength of the quadriceps and hamstrings on isokinetic testing at 180⁰/s and 300⁰/s (3). While this may be a great method to compare isolated muscle strength to the non-affected leg, I question its transfer to safety and performance on the field where isolated muscle contractions rarely occur.
The physical therapy community as a whole has done a tremendous job at implementing plyometric training into ACL reconstruction rehabilitation. Clinicians now routinely teach proper jumping mechanics and have made great progress in reducing the risk of ACL injury, but with re-injury rates as high as they are, are we, as a profession, still missing a major piece of the puzzle?
Barbells In The Clinic
Barbell training represents one of the most effective methods to increase strength and muscle mass, two key components to athletic performance. While barbells are often utilized in the gym and athletic training settings, their use in sports physical therapy is not standard. Exercises such as the squat and deadlift often are erroneously labeled by some as dangerous, stating “Squats will hurt your knees,” or “Deadlifts will hurt your back”.
Current dogma also suggests that squats should not be performed at knee flexion angles greater than 90⁰. The combination of fear of injuring patients and lack of knowledge on the biomechanics of a proper full ROM barbell squat often leaves this vital exercise in the dust, when, in reality, it may be EXACTLY what we need.
First, Some Anatomy and Kinesiology
The primary role of the ACL is to resist anterior tibial translation at the knee, especially at low flexion angles (4). Additionally, maximal anterior shear forces during the squat occur at the first 60⁰ of knee flexion, with peak ACL forces occurring at 15-30⁰. The forces on the ACL appear to actually decrease at higher flexion angles. As one descends into the squat, increased hamstrings activation exert a posterior force on the tibia assisting the ACL in reducing anterior translation. Also, anterior tibial translation further decreases in the deep squat due to the soft tissue approximation between the distal hamstrings and proximal calf.
When squats are performed in the clinic, however, they typically are associated with the caveat to avoid going past 90⁰ knee flexion. This reasoning is based on an observed increase in patellofemoral compressive forces at deep flexion angles in some studies (5); however, it may be flawed for a few reasons.
First, in closed-chain knee flexion greater than 90⁰, the wrapping effect occurs (6). In this phenomenon, the additional contact between the quadriceps tendon and the intercondylar notch actually enhances load distribution at the knee leading to an overall decline of retropatellar and compressive forces. Additionally, the highest dynamic peak forces occur at the transition phase of the squat. Therefore, it would be ill-advised to perform a squat to only 90⁰, as this reversal of eccentric to concentric movement would occur where the highest patellofemoral compressive forces are observed (6).
Another aspect to consider is the amount of external resistance needed during the partial vs. deep squat. Compared to deep squats, quarter and half squats require higher weights to stimulate a training effect. In fact, gym students are capable of moving about four times the weight of a deep back squat on a quarter squat (6). This phenomenon is something that, unfortunately, happens far too often.
High school and even college strength and conditioning programs are plagued with partial ROM barbell squats. Sixteen year old kids are performing quarter squats for sets of 10 with 315 lbs. when, in essence, they may not even be able to complete a single rep with 135 to proper depth. Perform a squat with a load that is too heavy within a ROM where maximal shear forces occur at the knee, and you are creating a recipe for disaster. In fact, a load of 1-2x bodyweight in a deep squat only produces a force equal to 28% the overall tensile strength of the ACL, whereas that same load in a half squat produces between 33-42%! (6). Overall, squatting deeper than 90⁰ is safer on the knee and less stress on the ACL.
A Note on Plyometrics
Plyometrics represent a major component in ACL reconstruction rehabilitation, and rightfully so. Provided most ACL injuries are non-contact in nature, it is imperative that the athlete knows how to correctly jump. In the later stages, plyometrics are performed as intense as the single-leg triple hop. Research tells us that a bodyweight double-leg jump to 70⁰ results in similar forces on the knee to a half squat with 175 lbs (6).
It amazes me that patients are repeatedly jumping with maximal effort on one leg, but there is still hesitancy to implement heavy strength training. What’s ACTUALLY more likely to cause injury to the reconstructed ACL? Instead of constantly focusing on creating exercises that mimic the exact motions on the field, we need to put more emphasis on exercises that are the best at creating strength, and barbell training will always be at the top of the list.
The Low-Bar Squat
Now that we know squatting below 90⁰ is preferred in post ACL reconstruction rehabilitation, what variation would be most beneficial? Regarding the back squat, there are two common variations that come to mind. The high-bar back squat represents one variation and is typically seen in the weightlifting, CrossFit, and the average weekend warrior community. With the high-bar back squat, the bar rests firmly on a muscular shelf created by the upper trapezius. In order to keep the bar centered over the midfoot during the descent, this will require a more upright torso, and increased knee flexion and ankle dorsiflexion ROM. Due to the more upright torso, this variation tends to create a higher demand on the quads and less demand on the posterior chain. Additionally, because of the increased knee flexion and forward migration of the knee, the high-bar squat has been shown to increase both patellofemoral and ACL stresses (4).
The low-bar squat represents another variation, typically seen in powerlifters. In the low-bar squat, the bar rests lower on the back, on the rear deltoids. This requires more forward trunk lean in order to keep the bar centered over the midfoot during the descent. The low-bar squat requires less knee flexion and ankle dorsiflexion but more hip flexion than the high bar squat. Due to these biomechanical properties, the low-bar position effectively reduces patellofemoral compressive forces and ACL stress due to greater hamstrings activity and less forward migration of the knees (4).
Notice the increased forward migration of the knees in the high-bar squat (left) in the image above. The low-bar squat (right) is more posterior chain dominant and reduces anterior migration of the knees.
Although quad strengthening is vital for ACL reconstruction rehabilitation, so is protecting the knee joint. While the high-bar squat may preferentially target the quads, due to the increased hamstrings activity and reduced forward migration of the knees, the low-bar squat represents a safe and effective exercise to incorporate into ACL rehabilitation. In fact, one source even recommends that exercises done in the standing position with the knees flexed and trunk anteriorly flexed can be performed safely even in the early stages after ACL reconstruction (7)! Due to only minimum anterior shear forces generated during the squat, it can represent a safe and effective rehab exercise to perform for those who wish to minimize tensile loading of the ACL (8).
Typical Strength Exercises in ACL Rehabilitation
Two staple exercises commonly utilized in ACL reconstruction rehabilitation that come to mind are the leg press and stability ball wall squat. The leg press may represent a great tool in the early stages of rehabilitation when the patient is physically too weak to perform a bodyweight squat. It represents a good stepping stone to squatting, but once an athlete can properly perform a bodyweight squat, the benefits of squatting far outweigh those of leg pressing. In fact, it has been shown that squatting increases both the squat jump and countermovement jump whereas the leg press does not improve these parameters at all (9). It would instead be optimal, in a field where you have limited time with your patients, to dedicate more focus working on the squat instead of the leg press. If performance is the goal, (usually the case in athletic development) the squat should be preferred because it yields better transfer effects.
Another typical exercise used in the clinic is the stability ball wall squat. During this exercise, the patient places his/her back against an exercise ball that is placed against the wall. This mimics a hack squat or a squat done on the Smith Machine with the feet placed significantly forward. The patient is instructed to squat down to 90⁰ and complete 3 sets of 10. Many aspects of this variation are not ideal. First, due to the vertical trunk and lack of anterior pelvic tilt, very little hamstrings are involved, making this a primarily quad dominant exercise and increasing the load on the ACL. Second, this variation of the squat is usually performed to 90⁰ of knee flexion. Despite little to no forward migration of the knee in this variation, allowing the turning point to occur where retropatellar forces are the greatest simply cannot be advised.
Additionally, it is very difficult to sufficiently load this exercise. Dumbbells can be held in the hands, but eventually it can become an awkward movement if the dumbbells become too heavy. Finally, with this variation, the patient is reliant upon leaning on a ball for stability. As a result, he/she does not learn how to effectively move his/her body through space, a vital skill to any sport.
Back To The Basics
Let’s, for a moment, revisit the isokinetic testing I mentioned in the beginning of the article. Again, we usually aim for recovery within a 10% deficit in strength compared to the un-involved side to return to sport, but I’d like propose a scenario of two patients.
Both Patient A and Patient B have a 5% deficit in isokinetic strength compared to their uninvolved side, and they each pass the jumping and knee stability tests with flying colors. On paper, the are both EQUALLY prepared to return to sport right? However, there is one key difference. Patient A can squat 315 lbs. for a set of 5 with pristine form; Patient B can barely squat 185 lbs. for a set of 5 without some form of maladaptive compensation. Both of these patients score the SAME on isokinetic strength testing under the criteria of percentage of strength of the uninvolved side. The problem with using a side-to-side comparison is that it tells you NOTHING about how the patient moves or his/her absolute strength of the entire lower body as a whole. How is it that isokinetic testing is currently the only strength requirement to return to sport? I have a hunch that Patient A will have an easier time returning to his/her sport than Patient B.
Barbell training is a very simple concept. Progressively loading basic human movement patterns is THE single most effective way to build strength and power that will transfer over to all athletic endeavors. The biomechanics of the low-bar squat encourage maximal strength development with little risk to the ACL. The physical therapy profession has made major progress in incorporating plyometric training and instructing proper jumping mechanics in ACL reconstruction rehabilitation, but are we still missing something? Incorporating a multi-joint strength exercise as simple, yet as effective as the low-bar back squat may be exactly what we need.
About The Author
Michael Mash, CSCS, FMSC is a physical therapy student and strength coach located in Pittsburgh, PA. He started his company, Barbell Rehab and Performance, with the mission to bridge the gap between physical therapy and strength & conditioning. His vision includes implementing the barbell lifts into clinical practice to both increase athletic performance and quality of life for all ages. Michael currently operates and can be contacted through his website, BarbellRehabandPerformance.com
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