Anterior cruciate ligament (ACL) tears are common traumatic knee injuries causing joint instability, quadriceps muscle weakness and impaired motor coordination. The neuromuscular consequences of injury are not limited to the joint and surrounding musculature, but may modulate central nervous system reorganization. Neuroimaging data suggest patients with ACL injuries may require greater levels of visual-motor and neurocognitive processing activity to sustain lower limb control relative to healthy matched counterparts. Therapy currently fails to adequately address these nuanced consequences of ACL injury, which likely contributes to impaired neuromuscular control when visually or cognitively challenged and high rates of re-injury. This gap in rehabilitation may be filled by visual perturbation training, which may reweight sensory neural processing toward proprioception and reduce the dependency on vision to perform lower extremity motor tasks and/or increase visuomotor processing efficiency. This clinical commentary details a novel approach to supplement the current standard of care for ACL injury by incorporating stroboscopic glasses with key motor learning principles customized to target visual and cognitive dependence for motor control after ACL injury. Level of Evidence 5

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Background: The regulation of muscle force is a vital aspect of sensorimotor control, requiring intricate neural processes. While neural activity associated with upper extremity force control has been documented, extrapolation to lower extremity force control is limited. Knowledge of how the brain regulates force control for knee extension and flexion may provide insights as to how pathology or intervention impacts central control of movement.Objectives: To develop and implement a neuroimaging-compatible force control paradigm for knee extension and flexion.Methods: A magnetic resonance imaging (MRI) safe load cell was used in a customized apparatus to quantify force (N) during neuroimaging (Philips Achieva 3T). Visual biofeedback and a target sinusoidal wave that fluctuated between 0 and 5 N was provided via an MRI-safe virtual reality display. Fifteen right leg dominant female participants (age = 20.3 ± 1.2 years, height = 1.6 ± 0.10 m, weight = 64.8 ± 6.4 kg) completed a knee extension and flexion force matching paradigm during neuroimaging. The force-matching error was calculated based on the difference between the visual target and actual performance. Brain activation patterns were calculated and associated with force-matching error and the difference between quadriceps and hamstring force-matching tasks were evaluated with a mixed-effects model (z > 3.1, p < 0.05, cluster corrected).Results: Knee extension and flexion force-matching tasks increased BOLD signal among cerebellar, sensorimotor, and visual-processing regions. Increased knee extension force-matching error was associated with greater right frontal cortex and left parietal cortex activity and reduced left lingual gyrus activity. Increased knee flexion force-matching error was associated with reduced left frontal and right parietal region activity. Knee flexion force control increased bilateral premotor, secondary somatosensory, and right anterior temporal activity relative to knee extension. The force-matching error was not statistically different between tasks.Conclusion: Lower extremity force control results in unique activation strategies depending on if engaging knee extension or flexion, with knee flexion requiring increased neural activity (BOLD signal) for the same level of force and no difference in relative error. These fMRI compatible force control paradigms allow precise behavioral quantification of motor performance concurrent with brain activity for lower extremity sensorimotor function and may serve as a method for future research to investigate how pathologies affect lower extremity neuromuscular function.

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Context: Neuromuscular training (NMT) facilitates the acquisition of new movement patterns that reduce ACL injury risk; however, the neural mechanisms underlying these changes are unknown. Objective: Determine the relationship between brain activation and biomechanical changes following NMT with biofeedback. Study Design: Controlled Laboratory Study Setting: Research laboratory Participants: Final analyses included twenty high school female soccer athletes (15.7±0.95 years; 1.68±0.05 m; 59.91±5.62 kg). Main Outcome Measures: Ten participants completed 6 weeks of NMT augmented with real-time biofeedback (aNMT) to reduce knee injury risk movements, and 10 participants completed no training. aNMT was implemented with visual biofeedback that responded in real-time to injury-risk biomechanical variables. A drop vertical jump with 3D motion capture was used to assess injury risk neuromuscular changes before and after the six-week intervention. Pre to post brain activation changes were measured using functional magnetic resonance imaging (fMRI) during unilateral knee and multi-joint motor tasks. Results: Following aNMT, sensory (precuneus), visual-spatial (lingual gyrus), and motor planning (pre-motor) brain activity increased for knee specific movement and sensorimotor cortex activity for multi-joint movement decreased. Knee abduction moment during landing also decreased (4.66±5.45 Nm; p=0.02; g=0.82) in the aNMT group with no change in the control group (p>0.05). The training-induced increased brain activity for isolated knee movement was associated with decreases in knee abduction moment (r=.67, p=.036) and sensorimotor cortex activity for multi-joint movement (r=.87, p=.001). No significant change in brain activity was observed in the control group (p>0.05). Conclusions: The relationship between neural changes observed across tasks and reduced knee abduction suggests that aNMT facilitates recruitment of sensory integration centers to support reduced injury risk mechanics and improve sensorimotor neural efficiency for multi-joint control. Further research is warranted to determine if this training related multimodal neuroplasticity enhances neuromuscular control during more complex sport-specific activities.