Structural brain changes after 4 weeks of unilateral strength training of the lower limb
Strength training enhances muscular strength and neural drive but the underlying neuronal mechanisms remain unclear. This study used magnetic resonance imaging (MRI) to identify possible changes in corticospinal tract (CST) microstructure, cortical activation and subcortical structure volumes following unilateral strength training of the plantar flexors. Mechanisms underlying cross education of strength in the untrained leg were also investigated. Young, healthy adult volunteers were assigned to training (n=12) or control (n=9) groups. The 4 weeks of training consisted of 16 sessions of 36 unilateral isometric plantar flexions. Maximum voluntary isometric contraction torque (MVC) was tested pre and post training. MRI investigation included a T1-weighted scan, diffusion tensor imaging (DTI) and functional MRI (fMRI). Probabilistic fiber tracking of the CST was performed on the DTI images using a two-ROI approach. Fractional anisotropy and mean diffusivity (MD) were calculated for the left and right CST in each individual before and after training. Standard fMRI analyses and volumetric analyses of subcortical structures were also performed. MVC significantly increased in both the trained and untrained legs of the training group, but not the control group. A significant decrease in MD was found in the left CST following strength training of the right leg. No significant changes were detected in the right CST. No significant changes in cortical activation were observed following training. A significant reduction in left putamen volume was found after training. This study provides the first evidence for strength training-related changes in white matter and putamen in the healthy adult brain.