Posture interacts with arm weight support to modulate corticomotor excitability to the upper limb

Abstract

The use of arm weight support (WS) to optimize movement quality may be an avenue for improved upper limb stroke rehabilitation; however, the underlying neurophysiological effects of WS are not well understood. Rehabilitation exercises may be performed when sitting or standing, but the interaction of posture with WS has not been examined until now. We explored the effect of posture with WS on corticomotor excitability (CME) in healthy adults. Thirteen participants performed static shoulder abduction in two postures (sitting and standing) at three levels of WS (0, 45, and 90 % of full support). Transcranial magnetic stimulation of primary motor cortex was used to elicit motor-evoked potentials (MEPs) in eight upper limb muscles. Stimulus-response (SR) curves were fitted to the MEP data using nonlinear regression. Whole-body posture interacted with WS to influence tonic activity and CME in all muscles examined. SR curve parameters revealed greater CME when standing compared to sitting for upper arm muscles, but lower CME to the shoulder, forearm, and hand. Distal to the shoulder, tonic activity and CME were modulated independent of any explicit differences in task requirements. Overall, these results support a model of integrated upper limb control influenced by whole-body posture and WS. These findings have implications for the application of WS in settings such as upper limb rehabilitation after stroke.

Effects of arm weight support on neuromuscular activation during reaching in chronic stroke patients

“…The number of synergies expressed by the moderate-severe impairment group did not respond to changes in WS, likely because of neuroanatomical constraints on available substrates for motor control. In the control group, WS affected CME (MEP area) measured in all muscles except ECR, similar to previous studies with WS and younger participants (Runnalls et al, 2014;2017).…”

“…A similar argument could explain why ECR CME did not respond to WS in the control and mild groups (Table 4). In contrast, previous experiments with healthy adults found that ECR CME was modulated by WS (Runnalls et al, 2014;2017). Although it is unknown which factors may account for the discrepancy between studies, the present findings provide evidence in support of an indirect modulation of neural excitability distinct from changes in muscle activity.…”

“…The observed modulation of CME with WS in the BB and TB of healthy adults is consistent with previous experiments (Runnalls et al, 2014;2017). Considering again the task did not require any activity at the elbow, the up-regulation of CME with less WS is likely an indirect effect of altered voluntary drive to proximal muscles mediated through neural linkages.…”

“…Corticomotor excitability (CME) across the upper limb is modulated with the amount of WS in healthy adults (Runnalls et al, 2014;2017). The reduction of antigravity torques required for shoulder abduction has indirect effects on other upper limb muscles through putative neural linkages.…”

Effect of Standing on a Standardized Measure of Upper Extremity Function

“…Although many functional tasks are performed in standing (e.g., meal preparation at the kitchen counter), most measurement of the upper extremity function in clinical and research settings occurs in sitting. Research suggests that the performance of skilled arm and hand movements in standing differs from the same movements in sitting (Berrigan et al, 2006; Cuisinier et al, 2005), requires precise coordination between upper extremity movement and postural control (Chen et al, 2016; Pozzo et al, 2002), and is associated with changes in corticomotor excitability for upper extremity muscles (Kantak et al, 2013; Runnalls et al, 2017). The impact of standing on the control of upper extremity tasks may be magnified in individuals with neurologic conditions such as stroke, who often present with postural control deficits (Belgen et al, 2006; Harris et al, 2005; Jorgensen et al, 2002; van de Port et al, 2006).…”