In our latest invited narrative review, published in Musculoskeletal Science and Practice, our team investigates the neuroplastic adaptations that occur within the efferent motor pathways in response to common knee pathologies, specifically ACL injuries, anterior knee pain, and osteoarthritis. While these conditions differ clinically, they share a central feature: persistent impairments in neuromuscular function that lead to long-term challenges in daily activities and quality of life. By comparing the unique neurophysiological changes in each condition, we aimed to uncover insights into how neural adaptations reflect the specific demands and clinical characteristics of each pathology.

Why Study Efferent Pathway Plasticity in Knee Injuries?

Knee injuries are commonly viewed as mechanical issues, but this perspective only addresses part of the picture. The knee is richly connected to the brain and spinal cord through a network of motor and sensory pathways. After an injury, neural adaptations in these pathways can drive chronic dysfunction in the quadriceps and hamstrings, limiting the muscle’s ability to generate force and respond to movement demands. Such neural adaptations are not uniform across all knee injuries; instead, each pathology—whether an acute injury like an ACL tear or a chronic degenerative condition like osteoarthritis—seems to create its own "signature" pattern of neuroplastic change.

These adaptations are critical for clinicians to understand, as they could explain why traditional rehabilitation often fails to fully restore function, even after extensive physical therapy. Our review synthesizes findings from recent neuroimaging and neurophysiology studies to illustrate how these pathways are altered and to highlight the need for targeted interventions.

Key Findings: Comparing Neural Pathways Across Pathologies

Our review revealed both commonalities and key differences in how the brain and motor pathways respond to knee pathologies.

1. **ACL Injuries**: For individuals with ACL injuries, the literature points to significant changes in the corticomotor pathways, including persistent inhibition within the motor cortex and reduced excitability in the corticospinal tract. These changes can interfere with voluntary muscle activation, limiting the effectiveness of traditional strength training. ACL reconstructions often fail to reverse these changes fully, which can lead to prolonged deficits in force production and increase the risk of reinjury.

2. **Anterior Knee Pain (AKP)**: In contrast, individuals with anterior knee pain often show alterations in motor planning regions rather than the primary motor cortex alone. For instance, the pre-motor area shows hypoactivation in tasks involving lower-limb motor control, suggesting that anterior knee pain disrupts not just muscle activation but the anticipatory motor patterns needed for stability and coordination. This difference might explain why AKP often leads to adaptations that favor muscle protection and bracing, contributing to chronic pain and reduced functionality.

3. **Osteoarthritis (OA)**: In knee osteoarthritis, motor adaptations appear to favor joint preservation, with inhibitory processes that likely reduce quadriceps activation to minimize load on the damaged joint. However, these adaptations come at a cost, leading to further muscle atrophy and reduced dynamic stability over time. Patients with OA also show distinct deficits in corticomuscular coherence, which is associated with impaired force control during gait and other weight-bearing activities.

Clinical Implications: A Call for Targeted Rehabilitation Approaches

Our review underscores the potential benefits of developing condition-specific rehabilitation protocols that address the unique neural changes associated with each pathology. Standard strength training may not be sufficient to reverse the inhibitory effects seen in ACL injuries, while anterior knee pain might benefit from therapies focused on motor planning and sensorimotor integration. In osteoarthritis, interventions that target cortical excitability and enhance corticomuscular coherence could improve patients’ motor control and support joint health over time.

Future Directions

This review also highlights several promising directions for future research. There remains a significant gap in our understanding of how to measure and address these neural changes clinically. Developing reliable, accessible neurophysiological assessments—such as motor-evoked potentials or measures of corticomuscular coherence—could allow clinicians to monitor neural function and adjust interventions accordingly. Further, the integration of neurostimulation techniques, like transcranial magnetic stimulation (TMS) or neuromuscular biofeedback, could provide a means of enhancing cortical activation and promoting adaptive plasticity during rehabilitation.

Closing Thoughts

Our goal with this review was to provide a comprehensive look at how different knee pathologies influence the motor pathways that underlie muscle function. By recognizing the unique neurophysiological signatures of ACL injuries, anterior knee pain, and osteoarthritis, we hope to inspire new approaches in rehabilitation that go beyond conventional strengthening exercises. Targeted, neuroscience-informed interventions hold the promise of enhancing recovery, restoring function, and improving the long-term outcomes for patients across these challenging knee conditions.

For a deeper dive into our findings, read the full review in Musculoskeletal Science and Practice. It is free for download.