Recruitment of extensor-carpi-radialis motor units by transcranial magnetic stimulation and radial-nerve stimulation in human subjects

The responses of 34 extensor-carpi-radialis motor units to graded transcran ial magnetic stimulation (TMS) and electrical stimulation of the radial ner ve were investigated in six human subjects. Simultaneously with the recordi ng of the single motor-unit discharges, motor-evoked potentials (MEPs) and H-reflexes evoked by the two types of stimulation were recorded by surface electrodes and expressed as a percentage of the maximal motor response (Mma x). Ten motor units were activated in the H-reflex when it was less than 5% of Mmax, but not in the MEP even when it was 15% of Mmax. The opposite was observed for three motor units. Eleven motor units were recruited by both stimuli, but with significantly different recruitment thresholds. Only ten motor units had a threshold similar to TMS and radial nerve stimulation. Fr om these observations, we suggest that caution should be taken when making conclusions regarding motor cortical excitability based on changes in the s ize of MEPs, even when it is ensured that there are no similar changes in b ackground EMG-activity or H-reflexes.