Functional-Structural Plasticity Associated With the Duration of Sports Participation in Lower Limb Amputees: A Multimodal Neuroimaging Study.

Structural and functional brain reorganization can occur after long-term physical activity and lower limb amputation (LLA). A previous study suggested that activation of the primary motor cortex (M1) ipsilateral to the amputated leg during rectus femoris contraction is associated with the amount of sports participation in individuals with LLA. However, the structural basis of ipsilateral M1 activation remains unclear. The aim of this study was to investigate whether ipsilateral M1 activation is associated with white matter microstructure in descending motor pathways. We hypothesized that ipsilateral M1 activation would be related to microstructural properties of the corticoreticular tract (CRT) rather than the corticospinal tract (CST). Twenty-three individuals with LLA who had participated in sports for varying durations underwent functional magnetic resonance imaging (fMRI) and diffusion-weighted imaging. During fMRI, percent signal change (PSC) during rectus femoris contraction in the amputated leg was quantified. White matter microstructure was assessed using fixel-based analysis (FBA). Ipsilateral M1 PSC during contraction of the amputated leg was positively correlated with fiber cross-section (FC) in the CRT within the same hemisphere, whereas no significant correlation was observed for the CST. No significant correlations were found in control analyses using PSCs during contraction of the rectus femoris in the nonamputated leg, during motor imagery of ankle movements of the amputated leg, or in the visual cortex. These findings indicate a task- and hemisphere-specific functional-structural association between ipsilateral M1 activation and the CRT. Although FC in the CRT was not directly correlated with the amount of sports participation, it may constrain the extent of ipsilateral M1 functional reorganization associated with long-term sports involvement. Our results highlight a potential role of the CRT, in addition to the CST, in motor plasticity and rehabilitation in individuals with LLA.