Using transcranial magnetic stimulation to map the cortical representation of lower-limb muscles

OBJECTIVE: To evaluate the extent to which transcranial magnetic stimulation (TMS) can identify discrete cortical representation of lower-limb muscles in healthy individuals. METHODS: Motor evoked potentials were recorded from resting vastus medialis, rectus femoris, vastus lateralis, medial and lateral hamstring, and medial and lateral gastrocnemius muscles on the right leg of 16 young healthy adults using bipolar surface electrodes. TMS was delivered through a 110-mm double-cone coil at 63 sites over the left hemisphere. Location and size of cortical representation and number of discrete peaks were quantified. RESULTS: Within the quadriceps group there was a main effect of muscle on anterior-posterior centre of gravity (p = 0.010), but the magnitude of the difference was small. There was also a main effect of muscle on medial–lateral hotspot (p = 0.027) and map volume (p = 0.047), but no post-hoc tests were significant. The topography of each lower-limb muscle was complex and variable across individuals. CONCLUSIONS: TMS delivered with a 110-mm double-cone coil could not reliably identify discrete cortical representations of resting lower-limb muscles when responses were measured using bipolar surface electromyography. SIGNIFICANCE: The characteristics of the cortical representation provide a basis against which to evaluate cortical reorganisation in clinical populations.