Anterior cruciate ligament (ACL) knee injuries are devastating injuries that can significantly impact an athlete’s career including ending their ability to return to high level competitive sport. Individuals who suffer an ACL injury have lower self-reported knee function, quality of life and a greater risk of long-term joint morbidity including early osteoarthritis (1,2). Whilst those who undergo reconstructive surgery have lower return to sport rates with relatively high reinjury rates up to 24%, with the greatest risk being in the first 7 months (1,3).

The mean time to return to sport following ACL reconstructive surgery (ACLR) is approximately 7 months with accelerated rehabilitation program as short as 6 months (4,5). However, it has been established that when athletes return to sport following an ACLR they continue to exhibit neuromuscular and biomechanical alterations including quadriceps strength deficits resulting in altered landing patterns (1,6). These deficits may potentially result in higher risks of reinjury to the grafted and/or contralateral knee, all indicating that potentially we are returning our athletes too quickly to play following ACLR.

To investigate further we critically review the Read et.al article “Lower Limb Kinematic Asymmetries in Professional Soccer Players With and Without Anterior Cruciate Ligament Reconstruction: Nine Months is Not Enough Time to Restore “Functional” Symmetry or Return to Performance” in the April 2020 issue of the American Journal of Sports Medicine.

This study examines professional soccer players specifically looking at kinetics, focusing on asymmetries during a counter movement jump (CMJ). The framework for this article is an analytical observational cross-sectional design and although it is not the most robust design format, it enables a cohort to be observed and compared at certain time points across the study. It executes this by dividing athletes into 4 groups, those who are 3-6 months, 6-9 months, >9 months into their ACLR rehabilitation and a control group. Strengths to this observational study are the use of the CMJ as an assessment measurement tool provided the risk of errors is minimized, as well as the use of a control group. Some questions are raised over the participation inclusion criteria as there is no mention of; any current or prior ankle injuries which are prevalent in soccer and can effect landing mechanics (7,8,9,10), which knee was injured dominant or non-dominant leg, if strength asymmetries were present prior to the injury, whether all ACLR participants followed a standardised rehabilitation program, were any players involved with ACL injury prevention programs and the cohort is specific to professional soccer players.

Counter Movement Jump

The study found that jump height (which is linked to an ability to complete tasks at a high level in soccer such as sprinting and change of direction) increased in the first 3-6 months however, it plateaued at 6-9 months and remained well below (3-4cm) the control group after 9 months. Whilst peak power, as measured by dual force plates followed a similar pattern and remained well below (3-4W/kg) the control group after 9 months. There were also significant interlimb asymmetries in the ACLR group during the eccentric (preload and deceleration phases) and concentric (jump phase) which decreased the further out from surgery the participants were, yet significant differences remained after 9 months. Indicating ACLR players were employing an offloading strategy to protect the injured knee. Furthermore, the uninjured limb was the dominant force producer. Both findings are consistent with other similar research indicating the changes occur due to altered nervous system function, strength deficits, reduced range of motion and fear of reinjury. The over reliance on the dominant limb for peak force production is important as it results in greater torque and stress loading of the knee and if the musculature is unable to dissipate the force effectively during a jump landing it may contribute to higher risk of injury, it may also lead to fatigue and potential injury of the uninjured knee.

The results suggest that even after 9 months soccer players who have undergone ACLR are showing power, strength, and asymmetries between limbs with lower power and jump height figures compared to healthy controls. Knowing that strength deficits are associated with reinjury rates in a variety of lower limb injuries, it would suggest that potentially players are being returned to play before they are ready, increasing their risk of reinjury (11,12).

Although this study has its weaknesses the results are consistent with those in similar studies looking a landing patterns in participants following ACLR helping to build on the available evidence that limb kinetic asymmetries exist in ACLR patients up to and greater than 9 months (1,6,13). These findings together suggest that longer recovery times are warranted, and individuals should only be returned to play following ACLR when they have limb asymmetries within a tolerable limit. Read et.al recommend benchmark goals such as, a jump height of 33-35cm and a concentric impulse asymmetry of no more than 2.5-3.1%. They also provide quartile figures, enabling clinicians to establish whether an individual is progressing quickly or slowly with their rehabilitation based on the CMJ metrics. They also point out that inter limb differences are task, variable and physical quality specific, meaning limb differences will occur across different tests and variables in the same task therefore using one asymmetry metric such as a single leg hop test with a <10% asymmetry isn’t an appropriate guide to determine progression and return to play status. A combination of tasks should be used including single leg hop, isokinetic strength, CMJ analysing the different variables in the task ie. height, power etc.

ACL injuries are devastating knee injuries and it is paramount to reduce the risks of reinjury that athletes are not rushed back prematurely, furthermore evidence shows a 50% reduction in reinjury rates for every month return to sport is delayed up to 9 months post-surgery (5). Focusing on reducing lower limb strength asymmetry, especially improving quadriceps strength is vitally important in helping to monitor an athletes ACLR progress and in determining when they are ready to return safely to sport. Consideration should also be given to ACL injury prevention programs which have been shown to reduce the risk of ACL injures by 53%, with any program consisting of strength, plyometrics, agility, balance and flexibility exercises (2).

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