Differential neurological adaptations during rehabilitation and at the time of return to sports in individuals with anterior cruciate ligament reconstruction: A systematic review and meta-analysis.

BACKGROUND: Individuals with anterior cruciate ligament reconstruction (ACLR) demonstrated neurological adaptations, but it remains unclear how these changes evolve from rehabilitation to return to sport (RTS) and how they affect quadriceps neuromuscular weakness. Therefore, this review aims to: (a) compare the activation in the cerebral cortex as well as the corticospinal and spinal tract of quadriceps between individuals with ACLR and healthy controls; (b) determine the effects of the neurological alterations on quadriceps neuromuscular function during rehabilitation and RTS. METHODS: A systematic search was conducted across PubMed, Web of Science, Medline, and SPORTDiscus. Risk of bias was assessed by the Methodological Index for Non-randomized Studies checklist and Egger's test. The evidence certainty was evaluated by the Grading of Recommendations Assessment, Development, and Evaluation guidelines. Participant characteristics, time of data collection, the stage of rehabilitation (<12 months post-ACLR) or RTS (≥12 months post-ACLR), data on neurological measures (cortical activation, corticospinal and spinal excitability), data on quadriceps neuromuscular function (strength, rate of torque development (RTD), central activation ratio (CAR)), and relevant results in individual studies were extracted and synthesized in a narrative or quantitative format. RESULTS: Twenty studies involving 504 individuals with ACLR and 362 healthy controls were included, with certainty levels rated low to very low. Compared to healthy controls, individuals with ACLR exhibited increased active/resting motor threshold (AMT/RMT) in the nonsurgical limb only (standardized mean difference (SMD) = 0.64, p = 0.009), comparable interval intracortical facilitation and short interval intracortical inhibition, bilateral increased motor evoked potential (MEP) amplitudes (surgical: SMD = 0.71, p = 0.02; nonsurgical: SMD = 0.32, p = 0.03), and comparable H-reflex/M-wave ratio (H/M ratio), with the MEP amplitudes negatively associated with quadriceps strength during rehabilitation. At RTS, individuals with ACLR presented greater activation in frontal and parietal regions in the brain cortex contralateral to the surgical limb during knee flexion/extension movements, increased AMT/RMT (SMD = 0.73, p = 0.003) and H/M ratio (SMD = 0.33, p = 0.0009) in the surgical limb, with a negative association between AMT/RMT and quadriceps CAR. CONCLUSION: Individuals with ACLR demonstrate bilateral corticospinal maladaptations in the quadriceps during rehabilitation (<12 months post-ACLR), and the alterations in the cortical, corticospinal, and spinal excitability in the surgical limb remain unresolved at the time of RTS (≥12 months post-ACLR). These neurological alterations are associated with quadriceps neuromuscular function, which underscores the importance of incorporating bilateral neurophysiological interventions and assessments into rehabilitation protocols and RTS criteria to improve the outcomes of ACLR.