Cortico-motoneuronal output to intrinsic hand muscles is differentially influenced by static changes in shoulder positions

We investigated whether shoulder position influenced the recruitment properties of the abductor digiti minimi muscle (ADM) and first dorsal interosseous muscle (FDI). ADM and FDI motor evoked potentials (MEPs) in response to transcranial magnetic stimulation (TMS) were obtained in seven healthy volunteers at two different static positions of the shoulder joint (30° adduction vs 30° abduction) while the arm was passively supported at shoulder level (90° in the horizontal plane) and the elbow joint was fixed at 90°. ADM and FDI voluntary activity was also examined during (1) externally paced finger abductions at 2 Hz in the two different shoulder positions (EMG_ADM and EMG_FDI was back-averaged time-locked to the end of finger abduction) and (2) maximal voluntary abduction of the little finger and the index finger. Maximal EMG power and force were analysed in the two shoulder positions. H-reflexes from ADM and FDI were also obtained in two subjects. The ADM stimulus-response curve to TMS showed that the slope and plateau level were significantly reduced with the shoulder at 30° abduction. In contrast, the FDI stimulus-response curve to TMS was not influenced by shoulder position. The back-averaged EMG_ADM showed a significant decrease in peak amplitude and area with the shoulder at 30° abduction, while no change in EMG_FDI was observed under the same condition. Similarly, maximal EMG_ADM and force exertion by the little finger were significantly reduced with the shoulder at 30° abduction, while no such effect was observed for FDI. ADM H-reflex, but not FDI, was also decreased with shoulder abduction. These results indicate that the corticospinal pathway to ADM is less accessible to TMS and to voluntary command when the shoulder is placed at 30° abduction. In contrast, activation of FDI, whether by TMS or by volition, is not influenced by shoulder position. This finding suggests that there are differences in the corticospinal innervation to ADM and FDI, possibly due to the different role of these muscles in hand function.