Apr 24, 2019

Neural Basis of ACL Injury and Clinical Strategies for Rehabilitation

Criss CR, BS, Faltus J DPT, MS, SCS, ATC ,CSCS, Grooms D PhD, ATC, CSCS

Anterior cruciate ligament (ACL) rupture is one of the most common sports-related injuries.¹ Standard treatment approaches for ACL injury is typically reconstruction followed by long periods of rehabilitation and reconditioning. Despite improvements in surgical reconstruction techniques, and implementation of multicomponent rehabilitation programs, strength deficits, asymmetric motor mechanics and elevated reinjury rates still exist beyond the timeline of typical therapy.²

Neuroplasticity after ACL injury

Growing evidence suggests central nervous system adaptations following ACL injury may be the missing link to explain the prolonged deficits associated with injury. ACL injury results in damage to local mechanoreceptors, which provide proprioceptive information to the nervous system. Thus, ACL injury not only results in structural joint disruption, but disruption of the neural afferent input from the knee as well. The loss of peripheral afferent information leads to compromised motor output. Recent evidence has demonstrated brain changes that contribute to motor control such as cognitive processing, sensory integration and attention regulation are present in those with ACL reconstruction³. Furthermore, functional magnetic imaging has shown brain activity pattern differences for knee joint sensorimotor control in key regions responsible for sensory-motor and visual-motor processing after injury. ⁴ These results support previous work that injury reduces proprioceptive and automatic control activity and increase reliance on visual feedback and attention for knee motor control.⁵ This change in feedback processing is theorized to be due to sensory reweighting, where utilization of one sensory system is amplified due to the loss or disruption of another. While using a higher level of attentional focus and visual feedback for knee control may be adequate for daily functional activities, these motor control mechanisms may break down during high-intensity, visually demanding environments such as those in athletics.

Transitioning Traditional Therapy to Neuroplastic Therapy

New techniques are needed to address the neuroplasticity (brain changes) associated with injury as conventional therapy focuses on the local joint or muscles in isolation. Clinicians can encourage utilization of the remaining proprioceptive inputs to improve sensorimotor control andor train sensory-visual strategies to be more efficient as a compensation for the lost ligament mechanoreceptors. Traditional rehabilitation strategies emphasize regaining function with most exercises prescribed with an internal focus of attention, where full attention is directed toward the affected muscle or joint. These instructions may include “contract your quad muscles” during terminal extension exercise to increase the quadriceps muscle activity or “do not let your knee buckle” during a bodyweight squat exercise to avoid excessive knee valgus.⁶ While these traditional cueing strategies may be necessary to restore muscle function and normalize movement patterns, they do not help transition an athlete to challenging, high-level sport activities requiring movement integration with constantly changing visual stimuli. Therefore, rehabilitation strategies that integrate external focus of attention, which shifts attention toward the environment and encourage more automatic motor control to maintain joint-to-joint integrity, should be implemented during training progressions to increase motor transfer to sport.

Early Phases of Rehab

The early phases of ACL rehab typically emphasize improving quadriceps muscle strength and exercises are commonly initiated in double leg weight-bearing and gradually progressed into single leg stance with an emphasis on improving stability. Exercises performed at this stage include assisted squats, Romanian deadlifts, and lunge variations to cones. Integrating external focus of attention during these early stages can be achieved by incorporating a metronome (60 beats per second) to avert attention to auditory cues. For example, by introducing a metronome as an ancillary component to assisted squats, an individual may focus on repetitive pace matching rather than strictly the task itself.

Plyometric & Sport Specific Integration

In order to improve dynamic stability while optimizing force attention and production capability, plyometric and sports-specific activities can be gradually introduced into a rehabilitation regimen. During these phases of rehab, the use of external cueing promotes perceptual awareness while restoring motor skills. Specifically, during plyometric exercises, clinicians can add landing sound instruction. Studies have reported that when patients are instructed to listen to impact sound and achieve soft landing during a drop-landing task, vertical ground reaction forces decrease, and landing mechanics improve through increased ankle and knee excursion.⁷ Furthermore, sound-intensity feedback may reduce impact forces and loading rates during running activity progressions.⁸ Either strategy deviates from the typical internal focus of attention such as “land with your knees in line with your toes” to external focus of attention like “land soft and quiet”.

Another method to improve sensorimotor adaptations is integrating visual tracking and/or stroboscopic glasses into sport-specific exercises. Such therapies may enhance perceptual and attentional ability by deviating visual attention away from the body and pushing it to the external environmental. Furthermore, visual knockdown from stroboscopic glasses may force the central nervous system to rely more heavily on proprioceptive systems for motor control. By reverting to proprioceptive systems for feedback and feedforward control, it may free up visual resources to direct toward athletic performance and external environmental stimuli.

Current rehabilitation programs that use strictly internal focus cueing strategies may limit patient outcomes and transfer to sports. Applying external focus strategies to rehabilitation protocols may improve ACL knee motor control, and transferability to sport. The extent of deficits such as attentional control and cognitive processing for joint motor control likely varies from patient to patient. Therefore, clinicians should attempt to tailor their training programs to patient needs.

References

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3. Baumeister J, Reinecke K, Weiss M. Changed cortical activity after anterior cruciate ligament reconstruction in a joint position paradigm: an EEG study. Scand J Med Sci Sports. 2008;18(4):473-484. doi:10.1111/j.1600-0838.2007.00702.x
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