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

Abstract

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.

Modulation of Excitability in the Cerebral Cortex Projecting to Upper Extremity Muscles by Rotational Positioning of the Forearm

“…In addition, a significant decrease in ICF using paired-magnetic stimulation paradigm was elicited after changing shoulder position from horizontal adduction to abduction while intracortical inhibition was never observed. These findings suggest that afferent volleys from the shoulder influence corticomotor excitability of the hand both at spinal and cortical levels (Ginanneschi et al 2005).…”

“…On the other hand, MEP of ECR was significantly higher and lower at the static horizontal shoulder adduction and abduction, respectively (Ginanneschi et al 2006). In addition, MEP of ADM was higher at the static horizontal shoulder adduction (Ginanneschi et al 2005). This indicates that ADM and the wrist extensor, ECR, are synergistically coupled at the positions of horizontal shoulder adduction/abduction.…”

“…Therefore it seems likely that sensory inputs to the central nervous system originated from the muscles or joints related to forearm rotation mainly modulate the excitability of the motor cortex neurons distributing wrist extensors and flexors. On the other hand, the intrinsic hand muscle, abductor digiti minimi (ADM) showed changes in F-and H-wave amplitudes and intracortical facilitation (ICF) in the static shoulder positions of horizontal abduction-adduction (Ginanneschi et al 2005). This indicates that shoulder positions simultaneously influence the excitability of corticomotor neurons of ADM both in the motor cortex and spinal cord.…”

“…The function of the hand is achieved by the coordinated movements of wrist, forearm, elbow and shoulder (Paulignan et al 1990;Chieffi and Gentilucci 1993;Kalaska et al 1997;Ginanneschi et al 2005Ginanneschi et al , 2006. Coordinated muscle contraction of the whole upper limb is required for transportation of the hand to the target object by proximal joints and for manipulation of the hand for target tasks by distal joints (Scott 2000).…”

Position-dependent torque coupling and associated muscle activation in the hemiparetic upper extremity

“…In this report, we pose the question of whether these abnormal patterns are affected by altering afferent somatosensory feedback resulting from changes in static limb configuration. Motor output to distal upper extremity Ginanneschi et al 2005;Ginanneschi et al 2006) and lower extremity (Knikou and Rymer 2002a, b) muscles is known to be modulated by changes in static proximal joint postures in individuals without stroke. We designed a protocol where individuals with stroke and control subjects generated isometric single-and multi-joint torque combinations in four different arm configurations.…”