Rectus Femoris Corticomotor Symmetry in Healthy Individuals

Abstract

Transcranial magnetic stimulation (TMS) involves non-invasive magnetic stimulation of the human brain. TMS can explore the excitability and map motor representation of skeletal muscles within the primary motor cortex. There is a lack of motor mapping studies in lower limb muscles, and few conducted in healthy cohorts. It is of clinical relevance to characterize motor representation areas of muscles in healthy individuals to allow researchers to quantify the extent of changes seen in pathological populations Purpose: The purpose of this study was to determine the size and center of the rectus femoris motor area in healthy individuals. METHODS: Ten healthy subjects (5M/5F) aged 27.3±6.46 years had the motor representation of the rectus femoris muscle of both limbs determined using TMS. Subjects attended two 45min testing sessions, an average 17d apart. Active motor threshold (AMT) was established at ∼10% maximum contraction with mapping stimulus intensity adjusted at 120% AMT. Four stimuli were given 5s apart while participant contracted rectus femoris at ∼10% of max, starting at the identified optimal site and then moving in 2cm steps in anterior, then posterior directions until a MEP could no longer be elicited. This pattern was repeated for lateral, then medial sites (1cm steps) until all map borders had been determined. Paired t-tests (P<0.05) were used to assess symmetry. RESULTS: No significant difference in AMT were found between hemispheres (55.6±6.85% RH vs 52.2±6.49% LH, P=0.101).The mean AP-CoG position in the RH was 0.15±0.54cm anterior from the vertex, and 0.20±0.67cm anterior in the LH. The mean ML-CoG position in the RH was 2.35±0.62cm lateral from the vertex, and 2.45±0.52cm lateral in the LH. There were no significant differences between hemispheres (AP-CoG P=0.760, and ML-CoG P=0.257). Surface area representation at 66% of map MEP amplitude was 5.92±4.78cm for the RH and 8.24±8.63cm for the LH. No significant differences were found between hemispheres (P=0.130). CONCLUSIONS: In healthy individuals with no history of neuromuscular pathology, rectus femoris corticomotor excitability and the motor map representation were found to be symmetrical between hemispheres.