I recently had the opportunity to speak to an audience of strength coaches and athletic therapists regarding optimizing nutritional and exercise strategies to enhance connective tissue adaptations. One component of this talk was to highlight a relatively complex analysis of the Achilles tendon structure and how this work produced an ultimately simplistic exercise prescription (i.e. do more seated calf raises). So, while the focus of my talk may have been maximizing connective tissue adaptions, my secret agenda was to highlight a fundamental principle of my research and clinical practices, that:
“While the analysis may be complex, the application must always be simple”
Over the last year I have been working regularly to improve the assessment and treatment of patients following ACL reconstruction, with an emphasis on late phase care, or specifically, return to sport assessments. It’s not uncommon to discover rather large deficits in quadricep strength in these populations, even though they are many months post operative and often have rather well-developed strength, conditioning, speed and agility programs.
It’s also not uncommon to find resistance from patients to my somewhat simple recommendation, to add isolated knee extension to their programs. Many have been scared off loaded knee extensions for fear of graft rupture, while others in the S&C community prefer to rely on multi-joint exercises with the belief they more closely relate to functional or athletic movements. And yet this simplistic exercise prescription is an excellent example of reducing complex science to simple, effective interventions to improve performance.
Exercise selection following ACL Reconstruction
Considerable time and attention have been given to whether open kinetic chain (OKC) knee extension is an appropriate exercise for patients following ACLr 1. Originally, the argument against them centred on the idea that “isolated” quadriceps activity within shallow degrees of knee flexion (0-30o) would excessively load the ACL graft, risking rupture of the immature graft and ultimately increased knee laxity. This was largely based on knowledge of ACL strain rates throughout knee range of motion, observations of comparable anterior tibial displacement between knee extensions with standard clinical tests of the ACL (Lachman) 2 and concerns over isolated quadriceps activity creating excessive load on the ACL graft. These fears are not entirely baseless but are also not consistent with the available evidence.
Results from trials comparing OKC against closed-kinetic chain (CKC) are mixed with some demonstrating no difference in knee laxity with improved quadriceps strength 3–5, no difference in laxity or strength 6, and others finding increased knee laxity 7. The bulk of the literature suggests no difference in laxity, which isn’t entirely surprising since ACL strain is comparable between squats (CKC) and knee extensions (OKC) 8, although we must consider of increased loading will increase ACL loading 9 through increased quadriceps activation 10. This increase occurs disproportionately in OKC exercises as compared with CKC exercises 11. While it’s likely that knee extensions are safe, we still must be specific and intentional in our approach to programming them.
While knee extensions may be safe following ACL reconstruction, questions remain as to whether they are necessary for these patients? Surely, we can remedy these persistent deficits in quadriceps function with heavy compound exercises like barbell squats, the leg press and loaded step ups? And these CKC exercises are often thought to be more “functional” and therefore more likely to transfer to “real life”. Some even argue that simply returning to modified sports activity and gradually progressing intensity is sufficient.
I’m not convinced.
Knee-shielding with compensatory movements
Asymmetric movement patterns are common in those following lower extremity orthopedic surgery. Considering patients following ACL reconstruction specifically, numerous authors have demonstrated compensatory movement patterns in common rehabilitation exercises like the squat 12–17. This phenomenon, termed “knee shielding” can occur through two mechanisms used in isolation or combination: 1) between-limb compensatory patterns – whereby increased loading is shifted to the uninjured, contralateral limb, and 2) within-limb compensatory patterns, where loading is shifted to other joints within the affected limb.
In the early post-operative period, these compensatory movements require no specialized training to spot. If you’ve seen someone limp, then you’ve seen them in action. But these compensations can persist, albeit with greater subtly. Asymmetric loading of the operative limb during squats has been observed from three months post operative up to seven years following ACL reconstruction 13, with minimal association with passing time 14,15. Even in single leg exercises, like the lateral step up, where between-limb compensations cannot occur, emphasis shifts from the knee to the hip (within-limb compensatory patterns) 18. Some have observed partial resolution of compensatory patterns with increasing time, and with significant variation in rehabilitation practice patterns 19, it would be interesting to define what role rehabilitation had in this effect.
Ultimately between and within-leg compensatory movements can persist following ACL reconstruction, and they may limit the ability of CKC exercises to improve quadriceps strength. Simple exercises like knee extensions may be the missing link in athletes with otherwise sound return-to-sport programming with persistent quad deficits.
Program design is never a zero-sum game
Finally, I think we should acknowledge that exercise selection is not a zero-sum game. Squats do not need to be programmed at the exclusion of knee extensions and vice versa (both may be superior 4). We do not have to adhere to a single methodology of completing knee extensions – we can manipulate the position of the shin pad or restrict range of motion, or even do multi-position isometrics initially. We may not need to do them right away 3,20, although in some cases, there may be a slight edge to starting them with reduced ROM and earlier in the program 5.
Complex analysis, simple solutions
None of the cited experiments here were easy. Whether it’s arthroscopic implantation of strain gauges into knees to full human randomized trials – all of these require substantial money, time, and brain power. Humans, by default, are complex organisms, and it’s not surprising that the ways in which we study ourselves can be complex as well. But we are not bound by the complexity of scientific methodology. Rather, I’d argue the opposite, that we owe it to ourselves, our clients and our patients to generate solutions that are as simple as possible. In this case, do knee extensions.
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2. Yack, J. H., Collins, C. E. & Whieldon, T. J. Comparison of closed and open kinetic chain exercise in the anterior cruciate ligament-deficient knee. Am J Sports Medicine 21, 49–54 (1993).
3. Morrissey, M. C. et al. Effects of open versus closed kinetic chain training on knee laxity in the early period after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatology Arthrosc 8, 343–348 (2000).
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