Increases in corticospinal tract function by treadmill training after incomplete spinal cord injury

In this study, we examined if several months of intensive locomotor training increases the function of spared corticospinal tract pathways after chronic spinal cord injury (SCI) in association with the recovery of locomotor function. Transcranial magnetic stimulation (TMS) at incrementing levels of intensity was applied over the motor cortex supplying either the tibialis anterior or vastus lateralis muscles, and the resulting peak-to-peak amplitude of the motor-evoked potentials (MEPs) were measured to obtain a recruitment curve both before and after training. In the majority of subjects (7/8), 3-5 mo of daily intensive training increased the responses to TMS in at least one of the leg muscles tested (9/13). On average, across all muscles tested MEP(max), which was evoked at high stimulation intensities, increased by 46% and MEP(h), which was evoked at intermediate stimulation intensities, increased by 45% (both significantly different from 0), indicating an increase in corticospinal tract connectivity from training. The slope of the sigmoid function fit to the recruitment curve increased by 24% after training (significantly different), indicating an expansion and/or increased excitability of corticospinal circuits supplying muscles to the lower leg. We also observed that the average duration of the silent period measured at MEP(max) increased after training from 130 to 178 ms, suggesting that training had effects on cortical circuits thought to mediate this long-lasting inhibition. The percentage increase in MEP(max) was positively and significantly correlated to the degree of locomotor recovery as assessed by the WISCI II score, the distance a subject could walk in 6 min, and the amplitude of the locomotor EMG activity, suggesting that the corticospinal tract, in part, mediated the functional locomotor recovery produced from training.