Exp Brain Res (2005) 161: 374–382 DOI 10.1007/s00221-004-2084-x RESEARCH ARTICLE F. Ginanneschi . F. Del Santo . F. Dominici . F. Gelli . R. Mazzocchio . A. Rossi Changes in corticomotor excitability of hand muscles in relation to static shoulder positions Received: 16 March 2004 / Accepted: 26 July 2004 / Published online: 23 October 2004 # Springer-Verlag 2004 Abstract We examined whether the recruitment proper- ties 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° adduction to 30° abduction with respect to the neutral position at 0°, while elbow and wrist joints were constrained statically at 90° and 180° respectively. We found that 30° abduction of the shoulder significantly depressed MEP size and prolonged MEP latency in comparison with 30° shoulder adduction. The neutral shoulder angle position (at 0°) significantly reduced MEP size but had no effect on MEP latency in comparison with 30° 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° abduction than at 30° adduction of the shoulder. However, the threshold value did not differ significantly between the two positions. To differentiate excitability changes at cortical versus sub- cortical sites, intracortical inihibition (ICI) and intracor- tical facilitation (ICF) were assessed using a paired- magnetic pulse paradigm. A significant decrease in ICF was observed after changing shoulder position from 30° adduction to 30° 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° shoulder abduction in comparison with 30° 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. Keywords Input–output properties . Corticospinal pathway . Transcranial magnetic stimulation . Proximal– distal arm influences Introduction Coordinated activity between the shoulder and the hand is required in a large number of everyday activities (Jeannerod et al. 1995). For example, the manual prehen- sion of an object is a movement that involves both proximal and distal joint segments (Kalaska et al. 1997). A large body of evidence suggests that motor cortex controls the different limb segments as a whole rather than individually (Scott 2000). In rhesus macaques, a motor cortical region containing neurons that specify functional synergies of distal and proximal muscle has been identified (Park et al. 2001). In humans, activation of shoulder, elbow, and wrist muscles in pointing movements appears to involve common motor cortical circuits (Devanne et al. 2002). In addition, it is widely recognised that the directional preference of cells in primary motor cortex is influenced by the starting posture or geometry of the arm and is not fixed to the absolute spatial direction of hand movement (Kalaska and Drew 1993). The primary motor cortex possess the neural substrates for synergies (Devanne et al. 2002) and it could contribute to the mechanisms required to specify the arm muscle recruitment patterns as a function of starting postures (Scott 2000). However, there is also considerable evidence that activation of afferents registering static changes in joint position can influence spinal reflex excitability F. Ginanneschi . F. Del Santo . F. Dominici . F. Gelli . R. Mazzocchio . A. Rossi (*) Sezione di Neurofisiologia Clinica, Dipartimento di Scienze Neurologiche e del Comportamento, Universita’ degli Studi di Siena, Viale Bracci, 53100 Siena, Italy e-mail: rossiale@unisi.it Fax: +39-577-40327