Changes in corticospinal excitability following adaptive modification to human walking

Locomotor adaptations to a novel environment can be measured through changes in muscle activity patterns and lower limb kinematics. The location and mechanisms underlying these adaptive changes are unknown. The purposes of the current study were (1) to determine whether corticospinal tract (CST) excitability is altered by resisted walking and (2) to ascertain whether changes in cortical excitability are muscle specific. Forty healthy participants walked with a robotic gait device (Lokomat) that applied a velocity-dependent resistance against hip and knee movements during walking. CST excitability was assessed by quantifying motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation immediately before and after baseline and resisted walking. MEPs were measured in either the biceps femoris (BF) or the rectus femoris (RF). Recruitment curves were collected by stimulating in 5 % increments from 105 to 145 % of active motor threshold. Results demonstrated a significant increase in MEP amplitude in the BF following baseline walking in the Lokomat. The RF did not demonstrate these changes. There was no further change in MEP size following resisted walking in either muscle group. These results suggest that locomotion increases CST excitability in a muscle-specific fashion. As such, it may be important for determining how to enhance the central nervous system's ability to integrate adaptive strategies during walking.