Motor cortical function and the precision grip

While task-dependent changes in motor cortical outputs have been previously reported, the issue of whether such changes are specific for complex hand tasks remains unresolved. The aim of the present study was to determine whether cortical inhibitory tone and cortical output were greater during precision grip and power grip. Motor cortex excitability was undertaken by using the transcranial magnetic stimulation threshold tracking technique in 15 healthy subjects. The motor-evoked potential (MEP) responses were recorded over the abductor pollicis brevis (APB), with the hand in the following positions: (1) rest, (2) precision grip and (3) power grip. The MEP amplitude (MEP amplitude REST 23.6 +/- 3.3%; MEP amplitude PRECISION GRIP 35.2 +/- 5.6%; MEP amplitude POWER GRIP 19.6 +/- 3.4%, F = 2.4, P < 0.001) and stimulus-response gradient (SLOPEREST 0.06 +/- 0.01; SLOPEPRCISION GRIP 0.15 +/- 0.04; SLOPE POWER GRIP 0.07 +/- 0.01, P < 0.05) were significantly increased during precision grip. Short interval intracortical inhibition (SICI) was significantly reduced during the precision grip (SICI REST 15.0 +/- 2.3%; SICI PRECISION GRIP 9.7 +/- 1.5%, SICI POWER GRIP 15.9 +/- 2.7%, F = 2.6, P < 0.05). The present study suggests that changes in motor cortex excitability are specific for precision grip, with functional coupling of descending corticospinal pathways controlling thumb and finger movements potentially forming the basis of these cortical changes.