Mechanisms of Non-Contact ACL Injury

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The deleterious impact of non-contact ACL injuries on athlete health and performance has precipitated extensive efforts towards their prevention. In particular, recent research has focused on identifying underlying neuromuscular control predictors of ACL injury risk, as such factors are readily amenable to training, and in essence, preventable. Neuromuscular training programs continue to evolve out of this research, which attempt to modify what are considered abnormal and potentially hazardous movement strategies. Despite the ever-increasing complexity and number of these programs however, ACL injury rates, and the associated sex-disparity in these rates, have not diminished. Rather than accept this as inevitable, we suggest that current prevention programs evidently continue to exclude key components of the non-contact ACL injury mechanism. Large-scale prevention is thus unlikely unless the precise mechanism/s of injury can be successfully elucidated.

Non-contact ACL injuries most likely stem from a complex interaction of anatomical, hormonal and neuromuscular factors. An equally complex integrative research design thus appears necessary to clearly determine how these combined factors manifest within the injury. With this in mind, we continue to utilize a combination of state-of-the-art experimental, cadaveric and model simulation methods to more effectively understand the causal factors of non-contact ACL injury. Through such an approach, successful screening of subject rather than population-specific injury risk may also eventually be possible. This in turn would facilitate more effective prevention methods that promote successful neuromuscular adaptation within individual non-modifiable constraints. Such steps appear necessary, if non-contact ACL injuries are to be ultimately eradicated. Considering these facts therefore, our research into non-contact ACL injury mechanisms currently focuses on the following:

•  Temporal Effects of Neuromuscular Fatigue on Anterior Cruciate Ligament Injury Risk in the NFL Athlete

Non-contact anterior cruciate ligament (ACL) injury is a potentially traumatic sports related injury suffered by a disproportionately large number of athletes playing within the National Football League (NFL). This injury typically necessitates surgical reconstruction, followed by a difficult and lengthy rehabilitation program, culminating in the athlete being sidelined for an extended period. Further, despite ongoing advances in surgical repair techniques, few athletes return to their pre-injury competitive levels, drastically shortening their professional careers. Injury also predisposes the NFL athlete to a significant long-term risk of degenerative osteoarthritis, often pre-empting the need for total knee replacement. With these facts in mind, identification of the mechanisms of non-contact ACL injury and the means through which they can be prevented is paramount within the NFL. Altered or abnormal lower limb neuromuscular control during the landing, cutting and pivoting maneuvers synonymous with football, is increasingly suggested as a key risk factor of non-contact ACL injury. Specifically, it is purported that these neuromuscular abnormalities culminate in dynamic joint postures that compromise the integrity of the ACL. Football is a sport that necessarily requires sustained maximal physical effort, meaning that player fatigue is largely unavoidable. It may be that the fatigue effects cumulatively incurred throughout a football game increase the likelihood of performing a movement resulting in ACL injury. Unfortunately, this information is not yet available. Furthermore, the extent to which the potentially deleterious effects of fatigue, and hence ACL injury risk, may vary throughout the course of a single season is also unknown. With these facts in mind, the specific aims of this research project therefore are to:

  1. Determine the effects of neuromuscular fatigue on three-dimensional lower limb joint biomechanics during execution of sidestep cutting maneuvers.
  2. Determine whether these effects are dependent on the time of season

To achieve the above goals, we will use state of the art high-speed motion analysis and electromyography (EMG) techqniques to measure three-dimensional joint (hip, knee and ankle) motions and loads and muscle activation strategies of forty-five football athletes performing a series of anticipated and unanticipated jump-landing movements. Subjects will be tested on four separate occasions within a single season (31 weeks), incorporating, pre, within and post season sessions. At each tesing session, subjects will a series of dynamic landing movements both before and during exposure to a cumulative general fatigue protocol. The joint biomechanics and muscle EMG data obtained during each jump series, across each test session, will then be compared to precisely quantify the effects of decision making and how these effects depend on both fatigue, leg (dominant versus non-dominant) and time of season. We expect that the deleterious combined effects of fatigue and decision making will be more pronounced in the initial and latter stages of a single season, when athlete fitness levels are typically compromised. By teaching tailoring prevention strategies to within-season variations in athlete strength and conditioning, more successful program outcomes and hence, reduced injury rates, are likely.

•  Combined Effects of Neuromuscular Fatigue and Anticipation on Non-Contact ACL Injuries in Females

Non-contact anterior cruciate ligament (ACL) injury is a potentially traumatic sports related injury suffered by a disproportionately large number of female athletes. Hence, identification of the mechanisms of female non-contact ACL injury is paramount. Recent studies suggest that both neuromuscular fatigue and unanticipated movements may in isolation contribute to non-contact ACL injury risk. To date however, the combined impact of these factors has not been investigated. Sports participation often requires sustained maximal physical effort, meaning that player fatigue is largely unavoidable. Similarly, the random nature of sports activity necessarily precipitates execution of unanticipated movement responses. Considering fatigue manifests through both central and peripheral pathways therefore, poor reactions, decisions and hence movement responses appear feasible in the presence of fatigue. Considering these facts, the specific aims of this research project are to:

  1. Quantify differences in female lower limb (hip, knee and ankle) joint kinematics, kinetics and muscle activation patterns between anticipated and unanticipated single leg pivot landing tasks.
  2. Determine the extent to which these differences are influenced by neuromuscular fatigue.
  3. Determine whether fatigue induced neuromuscular control during unanticipated landings can be predicted by simple reaction time measures.

Forty female recreational athletes have been recruited for the purpose of this study. Specifically, they will have lower limb biomechanics and muscle EMG data recorded during a series of anticipated and unanticipated single leg landings via state-of-the-art motion analysis techniques, both before and during exposure to a generalized fatigue protocol. For unanticipated trials, subjects will be required to rapidly respond and adjust movement responses while in the air immediately prior to landing, based on a series of randomly activated light stimuli.


During the fatigue protocol, subjects will perform continuous series of three single leg squats followed immediately by a landing trial until squats can no longer be performed. We expect that females will demonstrate potentially hazardous knee motion, load and muscle activation strategies when performing unanticipated landings, particularly in the presence of fatigue. We further expect that specific muscle activation timings during simple reaction tasks will predict high risk knee load states, such as increased knee abduction torques. This result will have immediate implications for improved injury screening and prevention methods.

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