Pii: S0304-3940(00)01709-2

A relation between corticospinal excitability and background voluntary muscular activity was investigated at low and high transcranial magnetic stimulation (TMS) intensities while grasping a stationary object with different widths (2 and 8 cm) using a precision grip. The muscle activity was recorded from the ®rst dorsal interosseous muscle. Regression analysis revealed that there was a linear relation between the motor evoked potential (MEP) and background muscular activity for both grip widths at each of the TMS intensities. At the low TMS intensity, the slope of the regression lines was similar for the 2 and 8 cm grip widths. It was, however, different when the TMS intensity was high. The results suggested that sensitivity modulation of bias level (input) facilitation occurred with wider grip width. The results of this study would re ̄ect to quantitative aspects of the relation between the synaptic drive to the motoneuron pool and the resulting efferent activity characterized, i.e. input±output relations of the human corticospinal pathway dependent on the occasion demand. q 2001 Elsevier Science Ireland Ltd. All rights reserved.