Changes in corticomotor excitability of hand muscles in relation to static shoulder positions.

We examined whether the recruitment properties of the corticospinal pathway to intrinsic hand muscles are influenced by variations of the shoulder joint angle. Abductor digiti minimi (ADM) motor evoked potentials (MEPs) in response to transcranial magnetic stimulation were examined during different static positions of the shoulder joint in the horizontal plane from 30 degrees adduction to 30 degrees abduction with respect to the neutral position at 0 degrees, while elbow and wrist joints were constrained statically at 90 degrees and 180 degrees respectively. We found that 30 degrees abduction of the shoulder significantly depressed MEP size and prolonged MEP latency in comparison with 30 degrees shoulder adduction. The neutral shoulder angle position (at 0 degrees ) significantly reduced MEP size but had no effect on MEP latency in comparison with 30 degrees shoulder abduction. The input-output relationship between MEP size and stimulus intensity was sigmoidal. The plateau value and maximum slope were significantly lower at 30 degrees abduction than at 30 degrees adduction of the shoulder. However, the threshold value did not differ significantly between the two positions. To differentiate excitability changes at cortical versus subcortical sites, intracortical inhibition (ICI) and intracortical facilitation (ICF) were assessed using a paired-magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30 degrees adduction to 30 degrees abduction. In contrast, no variation in the amount of ICI occurred in relation to the same changes in shoulder position. ADM F-waves elicited by electrical stimulation of the ulnar nerve at the wrist were significantly decreased at 30 degrees shoulder abduction in comparison with 30 degrees adduction. A similar pattern was observed in one subject in whom the H-reflex could be exceptionally elicited in ADM. We conclude that shoulder position influences the recruitment efficiency (gain) of the corticospinal volleys to motoneurons of intrinsic hand muscles. It is proposed that activity of peripheral receptors signalling static shoulder position influences corticomotor excitability of hand muscles both at the cortical and at the spinal level. This modulation may be functionally relevant when reaching to grasp objects.