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Cracking the Code on ACL Injuries

**NOTE: This is a preliminary article that will be followed by a piece coming soon to the Sportsmith platform.


What We’re Doing is Not Working

With the tremendous improvements in technology- athlete monitoring, data tracking, video analysis, we would assume this expansive accessibility to resources would improve our ability to reduce injury rates. But despite this, collective injury data suggests that we are still underserving our athletes when it comes to injury mitigation and restoration (3). This appears to be most prevalent for knee injuries, particularly ACL injuries. ACL tears are among the most common and severe injuries in sports, affecting athletes across multiple sports and virtually all levels of competition (7). Although ACL injuries have been the centerfold of sport related research for over a decade, we still can’t seem to crack the code on mitigating these injuries. In this article, I would like to present a handful of strategies along with a conceptual framework for how we can better navigate ACL injuries from both a preventative and restorative perspective.

Primary ACL Risk Factors

As with any injury, there is an extensive list of potential contributing factors for ACL injuries. As sport practitioners (i.e., strength coaches, physical therapists, athletic trainers), our job is to recognize and separate the modifiable and non-modifiable risk factors. The non-modifiable factors have been well documented- previous injury history, body structure, playing surface, environmental conditions, and so forth. But speaking to the factors sport practitioners may have influence over, we should look to the following:

  • Total workload and sport demands (practice and team training volume/intensities)

  • Rapid changes in acute workload or demand

  • Training inputs (exercise selection, volume, intensity, density)

  • Improving mechanical (force) coupling

  • Body structure and mechanics (i.e., base of support relative to center of mass)

  • Non-functional asymmetries (i.e., greater than 15% differences between left/right extremities… quad atrophy, SL vertical jump height)

  • Managing compounding stressors (i.e., stress management strategies, sleep profiling, nutritional inputs)

While aiming to minimize injury risk prospectively is one challenge, there is also work to be done on the restoration side after an injury has already occurred. This is where we need to be mindful of “orthopedic cost” considerations, and how we may need to ratify our approach from chasing peak outputs to facilitating longevity and durability as athletes mature. It’s been said that previous injury the best predictor of injury, and as athletes experience a greater number of injuries throughout their career, this risk is amplified by creating a compounding effect. Whether it is an athlete being rushed back throughout the return to play timeline, or it’s simply a case of doing the right amount of the wrong things, we need to be cognizant of both the workload and the inputs. An incomplete or rushed return to play process will almost inevitably lead to re-injury or consequential injury.

Mechanics of ACL Injuries

Despite several areas of ACL research being inconsistent, one of the few universal understandings of ACL injury is the body position and joint angles upon injury. Although superficially the mechanism of injury may appear simple, there is an abundance of nuance and factors that we need to consider. The conversation must extend beyond the knee in isolation, as ACL injuries are a result of multiple joints, velocities, vectors, and neuromuscular factors (3). Predominantly, sport-related ACL injuries occur during planting, cutting, deceleration, landing, and unanticipated changes of direction (5,7). In more recent years, this has led researchers to a thorough debate of other potential contributing factors which may include visual processing, proprioceptive acuity, and vestibular function all potentially being prominent factors (2).

Where Have We Gone Wrong?

This is the (hundreds) of million dollar question, and frankly if I had the explicit answers, my career would be in a much different state. But truth of the matter is it’s difficult, if not impossible to specifically say what has caused this rise in ACL injuries. Presumably, it has been consequential of multiple factors and depending on the lens you look through, you could point to a few different things. One most people are quick to point to, is the rise in year round sports and training, namely for younger athletes. Additionally, the earlier specialization in sport is probably not doing anyone any favors from an injury standpoint. I’m a strong advocate of playing multiple/many sports until at least 15-16 years old, no matter the apparent talent or affinity for one sport. There are other extraneous factors that we can speculate on- changes in modern footwear, field surfaces, and even just the faster rate of play in most team sports are all items to consider. But to focus this in on the training side, there may be a few things we can pick out.


As Dan Pfaff and Stu McMillan (Head Coach & CEO of ALTIS) have adamantly suggested over the years, improper training input, incomplete programming, and poor coaching decisions have all without question been a part of our injury crisis. I would agree. Despite our bias of wanting to look outward for explanations surrounding injury data, it is imprudent for us to not acknowledge the potential role we play in injury manifestation. Coaches and practitioners have become more inclined to attach themselves to specific “systems” and niche “methods” to amplify their marketability more so than for the sake of pragmatism. Consequentially, along with improper and outright poor training ideologies, this has also resulted in athletes being in a constant state of “working” and often without compatibility or coherence.


Please note that this is just my opinion on the matter, and there is plenty of room for debate/interpretation, but here is a short list of where I see strength coaches contributing to the ACL epidemic:

  • An overzealous pursuit of “grinding” (unnecessarily high volumes/intensities) in lieu of valuing quality movement and proper dosages of training.

  • Too much bilateral loading, not enough single leg and split stance.

  • An overcommitment to max load with neglect for dynamics of movement.

  • A reductionist view of exercise variation as it relates to sports performance. In other words, training individuals like powerlifters instead of athletes.

  • A lack of emphasis on foot and lower leg strength/function.

  • Improper or insufficient recovery modalities and soft tissue care.

  • Absence or neglect for multiplanar loading.

  • A poor understanding of injury mechanisms and how they can be accounted for in training.

So, What Can We Do?

Building off the bullet points outlined above, I believe the first place to start is auditing the programming for exercise selection and parameters. One training parameter of particular interest should be single leg to bilateral ratios. The reason I’ve continued to distance myself from bilateral loading speaks back to the kinetic relationship between the foot and hip, and consequentially how that influences the knee. By utilizing more split stance and single leg loading, we are better able to promote appropriate foot-hip mechanics. I think a fair heuristic here is to follow a 3:1 ratio for unilateral:bilateral exercises.


I don’t believe bilateral movements should be done away with entirely, however, I do think they hold less value than we’ve been led to believe for athletes. In my view, I want to replicate as much in training as we observe in sport. And with very few exceptions, athletes spend the majority of time in sport predominantly on one leg. Moreover, in most cases this is with a forefoot center of pressure and a forward leaning center of mass. These attributes are better reflected in split and single leg positions in the training setting.


Train the Feet

A topic I’ve spoken about at length and a hill I’m happy to die on. Directly and specifically training the foot and lower leg should be a staple in any athlete’s program, irrespective of sport or goals. The foot is the origin for force interactions, and for that reason alone should be seen as a priority in training. Nevertheless, as this relates to ACL injuries, the foot provides the base of support, and as outlined above, the base of support and center of pressure are critical attributes for preserving the ACL. As such, these are things that need to be reflected in training and cued with precision. Congruency between trunk and shin, directing where the athlete’s center of pressure should be, and maintaining a slightly forefoot dominant position under load are efficient ways to bolstering the knees.

The advantage here is that training the feet is very simple, rarely requires you to do anything outside of standard training and can be worked in several ways. My approach is to aim for about 10 minutes of direct foot work at least 2-3 days per week. This can be applied as warm-up or movement prep options, used as intraset options, or can be done within the accessory blocks. In addition, at-home options can be distributed to athletes for them to manage on their own. These can include isolated foot strengthening drills (i.e., jump rope, spring ankle series), or can be more restorative in nature (i.e., voodoo floss, manual therapy).


But the majority of your foot/ankle specific work should come by the way in which you are programming and coaching your exercises. Adding in a floating heel is a simple but highly effective training strategy for strengthening the arches of the feet. Similarly, performing portions of your training barefoot is another low hanging fruit for improving foot strength. Adding in more lateral and rotational movements that challenge the foot compliance is something else I find a lot of value in. And of course, being active coaching and cueing foot positions and pressures. An improved ability to stabilize and pressurize across the foot, particularly during dynamic actions, the more robust the knee will be.


A Spectrum of Planes & Ranges

The debate surrounding the influence of squat depth for knee and ACL health has been exhaustive. But definitive above or below 90° squatting is the wrong argument to be having. The ACL is maximally loaded (tensed) beyond 90° (7), however, almost all (non-contact) tears occur between 10-30° (2,5,7), so how do we navigate this? My approach is simply we should have some balance of both deep and shallow knee angle movements depending on the specific demands of the athlete or individual. For instance, a basketball or volleyball athlete I will likely do more above than below 90° squat variations, whereas a defensive lineman would be the opposite. In both cases, we need the deeper angles to fully load the ligaments of the knee, but the shallower angles to strengthen where knee injuries tend to occur. Above 90° is also where we can maximize peak concentric impulse and jump takeoff angles.


Training paths of motion can be approached similarly in that there’s a time and case for anything in isolation, but we want to emphasize challenging a wide variety of vectors. Although the knee has very limited motion outside of the sagittal plane, the relationship between hip and foot is directly challenged when working in other planes or vectors. This relationship, and ability to produce/tolerate non-sagittal forces is critical for knee and ACL health. You can envision is as simply as the better the above & below joints can rotate, the more prepared the knee is to accommodate torque. The better the hip can rotate, and the more compliant the foot is, the less direct stress we will experience at the knee (5).


Proportionality, Not Peaks

Ultimately this is what I believe it comes down to with regard to ACL injuries- do we have a force, proportion, or timing problem? Of course we can take the copout in suggesting it’s a combination of all three, but this is too generalized to provoke effective strategies. When we analyze the more conventional strategies of ACL training, albeit preventative or restorative, I believe there has been an overzealous and myopic focus on the force applications. While viewing peak isometric forces in uniplanar motions, or even isolated force plate metrics (jump height, ground contact time), we are only seeing a piece of the puzzle. There must be a wider perspective for how we evaluate these injuries and protocols.


We should be mindful of the relative proportions athletes demonstrate with force signatures and expressions, not just the values they produce. In other words, how an athlete executes something is likely as important as what they produce. With regard to injuries, the way in which movement occurs is often the best evidence we can utilize for understanding mechanisms and levels of risk. Several studies have shown good evidence that “more weight” isn’t always the answer, and in certain cases, increasing isolated or singular strength measures without compatibility could actually potentiate injury risk (2,4). For instance, it’s been a long held belief that eccentric quad strength is a primary factor for preventing ACL injuries. Although this isn’t a false statement, it does require context. As demonstrated in the study from (4), disproportionally greater eccentric strength relative to peak concentric force was actually shown to be an amplified risk for ACL injuries. This is a great example of how blindly pursuing more force output, again in a singular measure, can be leading the athlete down the wrong path.


Closing

It should go without saying that sport related injuries will never be entirely preventable. But at the same time, I feel we have grown too passive in what our potential influence on injuries could be- especially within the strength & conditioning side. This void in the S&C world is largely predicated on only seeing exercise through a very refined lens. Thinking “outside of this box” presents a challenge for coaches and can often be intimidating. But rather than rushing athletes to being thrown back into the mix and expected to do the same type of training we see on the developmental side; we need to approach athletes recovering from injuries differently. albeit with Band-aid solutions such as avoidance, using the same exercises with lighter loads or reduced ranges of motion, or other transient strategies will not cut it. There’s more that we can do, and we need to take ownership of this.


If you enjoyed this article, please check out our recently released Restorative Training Protocol eBooks. These eBooks are short, comprehensive guides that provide actionable strategies for restoring and preventing injuries.


References

1. Allahabadi, S. Gatto, AP. Pandya, NK. ACL tears in the NFL from 2013-2020. J Ortho Sports Med. 2022;10(2).

2. Boden, BP. Sheehan, FT. Mechanism of non-contact ACL injury: OREF clinical research award 2021. J Ortho Res. 2022;40:531-540.

3. Bullock, GS. Murray, E. Vaughan, J. Kluzek, S. Temporal trends in incidence of time-loss injuries in four male professional North American sports over 13 seasons. Nature.2021;11:8278.

4. Pontillo, M. Hines, SM. Sennett, BJ. Prediction of ACL injuries from vertical jump kinetics in division 1 collegiate athletes. IJSPT. 2021;16(1):156-161.

5. Sigurosson, HB. Karlsson, J. Snyder-Mackler, L. Briem, K. Kinematics observed during ACL injury are associated with large early peak knee abduction moments during a change of direction task in healthy adolescents. J Ortho Res. 2021;39:2281-2290.

6. Schick, S. Cantrell, CK. Young, B. The mechanism of ACL injuries in the NFL. A systematic video review. Cureus. 2023;15(1).

Webster, KE. Hewett, TE. Meta-analysis of meta-analyses of ACL ligament injury reduction training programs. J Ortho Res. 2018.

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