Neuromuscular & biomechanical control in athletes returning to sport after anterior cruciate ligament reconstruction
Anterior cruciate ligament (ACL) injuries are common during dynamic sports activities in the young, active population and often have important short and long-term physical, psychological and professional consequences, resulting in a lengthy absence from sports and a high economic cost for society. Unfortunately, the success rates for return to sport (RTS) still remain low with only 55% of the athletes who underwent an ACL reconstruction (ACLR) returning to competitive sports. Furthermore, the risk of sustaining a second ACL injury is very high, with re-injury rates of up to 15-25% in the first 2 years after RTS. Finally, the risk for early development of post-traumatic knee osteoarthritis (PTOA) is increased, with almost half of the ACLR patients that have signs of osteoarthritis 10-20 years after reconstruction. These low success rates suggest that postoperative rehabilitation and RTS criteria can still be improved. In the current RTS approach, time post-surgery, recovery of strength, and recovery of hop-distance are used to warrant safe RTS. These RTS criteria are unable to detect remaining neuromuscular and biomechanical deficits, which might predispose ACLR athletes for re-injuries and PTOA. Furthermore, current RTS testing is performed in a controlled and fully anticipated environment, while athletes that return-to-sport will be confronted with several sport-specific challenges such as fatigue and environmental challenges that might further jeopardize neuromuscular and biomechanical control. Therefore, the aim of this doctoral thesis was to evaluate whether ACLR athletes still show biomechanical and neuromuscular alterations at the time of RTS and 6 months later, and this during situations that involve sport-related challenges. This aim was achieved through five studies.
In study 1, we found that ACLR athletes demonstrate neuromuscular and biomechanical alterations during single leg landings at the time of RTS. At first sight, these compensations seem to be part of a protective strategy. First of all, they increase the activation of the hamstrings (which is an ACL agonist) and second, they reduce the general loading of their knee (e.g. reduced knee flexion moment). Important to mention, is that these alterations were found across 5 single leg landings tasks and thus seem to be ‘task-independent’ compensations which makes it very likely that they will also occur during sport and ADL activities.
In study 2, we found that the neuromuscular and biomechanical alterations identified at time of RTS did not resolve when ACLR athletes had returned to sport for six months. Therefore, additional interventions are needed to normalize these alterations prior to return to sport. Furthermore, explorative analyses showed that prolonged increased hamstrings activation might be related to self-reported instability at time of RTS and that prolonged decreased knee flexion moments might be related to insufficient psychological readiness at time of RTS. Future studies should further assess the role of these potential underlying causes as these insights will help researchers and clinicians developing interventions to solve neuromuscular and biomechanical alterations in ACLR athletes.
In study 3, we investigated whether neuromuscular control was jeopardized when ACLR and uninjured athletes were confronted with sport-related challenges. We found that uninjured athletes adapt their neuromuscular activation to different environmental challenges, but ACLR athletes show an almost unadjusted protective strategy of increased hamstrings activation independently of environmental challenges. This might be a compensation for altered proprioceptive input due to mechanoreceptor damage. However, under cognitively challenging circumstances, the protective strategy of the ACLR athletes was jeopardized confirming that underlying neurocognitive limitations contribute to altered neuromuscular control in ACLR athletes.
In study 4, we assessed how uninjured athletes respond to a fixed-demand protocol that simulates match related fatigue. We found that resistance to fatigue had a major influence of how someone responds after such protocol. More specifically, we found that being less resistant to fatigue was related with less optimal landing patterns. Based on this finding we can conclude that prevention and rehabilitation programs should implement training to improve fatigability.
In study 5, we investigated if ACLR athletes were more vulnerable to fatigue than uninjured controls by using the same fatigue protocol as in study 4. We found that overall, ACLR athletes and uninjured athletes have similar biomechanical and neuromuscular responses to fatigue. However, in some tasks, we did find fatigue enhanced landing deficits in ACLR athletes, suggesting that task demands determine whether ACLR athletes can cope with fatigue or not.
In conclusion, this project showed that ACLR athletes return to sport with remaining neuromuscular and biomechanical compensations. The finding that these alterations are seen across different tasks, across different environmental challenges, and even during follow-up sessions three and six months after RTS, indicates that these are robust compensations that are very likely to also occur during sports and ADL activities. It seems thus important to detect and address these alterations before those athletes return to sport. Especially, since there is initial evidence that they influence re-injury risk in the first year(s) after RTS and in addition, if these deficits will persist for several years, they might also accelerate the development of PTOA. Furthermore we found that neuromuscular patterns were jeopardized under sport-related challenges, and would therefore advise clinicians to implement such sport-related challenges in RTS test batteries and confront patients with challenging situations in the last stage of the rehabilitation to prepare them for returning to the dynamic environment of sports.