Task‐induced modulation of motor evoked potentials in upper‐leg muscles during human gait: a TMS study

Abstract

The aim of this study was to determine the relative involvement of the corticospinal (CS) pathway in voluntarily controlled walking compared to unconstrained walking. In the voluntarily controlled walking condition, subjects had to walk at the same speed as in unconstrained walking with a mechanical constraint, which is known to affect specifically the upper-leg muscles. The motor cortex was activated transcranially using a focal magnetic stimulation coil in order to elicit motor evoked potentials (MEPs) in the rectus femoris (RF) and the biceps femoris (BF). The magnetic stimulation was delivered at the end of the swing (at 90% of the cycle duration), when the EMG backgrounds were similar in the two experimental conditions. For each subject in each condition, MEPs were measured for several stimulus intensities in order to establish the input/output (I/O) curve (MEPs amplitude plotted against stimulus strength). The results showed a significant increase in the MEPs amplitude of both the RF and BF in voluntarily controlled walking compared to unconstrained walking, which is the first evidence of cofacilitation of MEPs in antagonist upper-leg muscles during human gait. In conclusion, although a lot of studies have emphasized a privileged input of the corticospinal pathway to the distal lower-leg muscles, this study shows that, if a locomotory task requires fine control of the proximal upper-leg muscles, a selective facilitation of MEPs is observed in these muscles.

Task-specific changes in motor evoked potentials of lower limb muscles after different training interventions

“…For such immediate effects, transcranial magnetic stimulation (TMS) studies identified predominantly cortical plasticity such as increased motor cortex excitability and enlargement of motor cortical representation (Classen et al, 1998;Pascual-Leone et al, 1994, 1995. Analogous to hand function, gait and postural control are highly adaptable and underlie specific cortical control (Bonnard et al, 2002;Camus et al, 2004;Christensen et al, 2001;Schubert et al, 1999;Taube et al, 2006). Likewise, short-term motor skill training in the leg muscles induces increased motor cortex excitability, which is consistent with findings in the upper limb suggesting a similar underlying principle (Perez et al, 2004).…”

Training with robot-applied resistance in people with motor-incomplete spinal cord injury: Pilot study

“…In addition, the improvement in locomotor control in people with m-iSCI is associated with increased corticospinal drive to leg muscles [69][70]. Thus, given the role of the primary motor cortex in the control of skilled locomotion [64][65][66], as well as evidence that corticospinal excitability is modulated with locomotor adaptations [71][72][73] and associated with locomotor recovery following SCI [69][70][74][75], it is possible that the improvements in skilled walking following Loko-R training was associated with enhanced cortical excitability secondary to greater active engagement during training.…”

Noninvasive Neuromodulation in Poststroke Gait Disorders

“…MEP facilitation seems to be greater during precision stepping tasks and selective for lower-leg or upper-leg muscles. 34,40 Unlike for the upper limb, the somatotopy of the legs is located deep, adjacent to the midline; therefore, for technical limitations, the transcallosal connections between M1 of the 2 legs cannot be studied by TMS. However, Goodwill et al 41 observed an increase in strength of the left leg following unilateral training of the right lower limb.…”