Corticospinal tract structure and descending motor output following anterior cruciate ligament reconstruction: an exploratory multimodal neuroimaging assessment.

Background: Functional and structural differences in the central nervous system, including higher motor cortex activation, lower corticospinal excitability, and smaller corticospinal tract volume have been proposed as potential contributors to persistent quadriceps strength deficits following anterior cruciate ligament reconstruction (ACLR). However, prior work has largely examined these differences using isolated single modality neuroimaging approaches, limiting integration across brain activation, corticospinal excitability, and neural structure. Therefore, our primary aim was to compare neurophysiological and neurostructural outcomes between individuals with ACLR and uninjured individuals (CON) and to explore whether corticospinal tract (CST) volume conditioned the association between (1) cortical activation and corticospinal excitability and (2) corticospinal excitability and quadriceps strength. Methods: Nineteen right-foot dominant individuals, including 10 with a history of unilateral ACLR and 9 CON participated in the current study. Brain activation in primary motor cortex while performing a supine knee flexion-extension task, CST volume, and corticospinal excitability was measured by functional magnetic resonance imaging, diffusion tensor imaging, and transcranial magnetic stimulation, respectively. Group comparison and correlation analyses were conducted to evaluate the difference in quadriceps strength, brain activation, CST volume, and corticospinal excitability between the two groups and to measure association between these variables for each group, respectively. Exploratory moderation analyses were conducted to evaluate association between motor cortex activation, corticospinal excitability and MVIC, while determining whether this association is moderated by the structural volume of the CST. Results: Compared with controls, ACLR participants demonstrated higher active motor threshold, lower motor evoked potential (MEP) amplitude, and lower CST volume. CST volume was positively associated with MEP amplitude in the ACLR group. Exploratory moderation analyses yielded nominal interaction effects suggesting that CST volume may condition the relationships between task-related primary motor cortex activation and MEP amplitude, and between MEP amplitude and quadriceps strength in the ACLR group. Comparable interaction effects were not statistically detectable in controls. Conclusion: Individuals with ACLR demonstrated neurophysiological and neurostructural differences despite comparable quadriceps strength. Exploratory findings raise the hypothesis that CST volume may influence cross-modal relationships among brain activation, corticospinal excitability, and quadriceps strength after ACLR, highlighting the potential value of integrating structural and functional measures when evaluating the descending motor system. These findings provide a foundation for larger longitudinal investigations examining how CST structure interacts with neurophysiological function and motor recovery following ACLR.