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

Abstract

To better understand how arm weight support (WS) can be used to alleviate upper limb impairment after stroke, we investigated the effects of WS on muscle activity, muscle synergy expression, and corticomotor excitability (CME) in 13 chronic stroke patients and 6 age-similar healthy controls. For patients, lesion location and corticospinal tract integrity were assessed using magnetic resonance imaging. Upper limb impairment was assessed using the Fugl-Meyer upper extremity assessment with patients categorised as either mild or moderate-severe. Three levels of WS were examined: low=0, medium=50 and high=100 % of full support. Surface EMG was recorded from 8 upper limb muscles, and muscle synergies were decomposed using non-negative matrix factorisation from data obtained during reaching movements to an array of 14 targets using the paretic or dominant arm. Interactions between impairment level and WS were found for the number of targets hit, and EMG measures. Overall, greater WS resulted in lower EMG levels, although the degree of modulation between WS levels was less for patients with moderate-severe compared to mild impairment. Healthy controls expressed more synergies than patients with moderate-severe impairment. Healthy controls and patients with mild impairment showed more synergies with high compared to low weight support. Transcranial magnetic stimulation was used to elicit motor-evoked potentials (MEPs) to which stimulus-response curves were fitted as a measure of corticomotor excitability (CME). The effect of WS on CME varied between muscles and across impairment level. These preliminary findings demonstrate that WS has direct and indirect effects on muscle activity, synergies, and CME and warrants further study in order to reduce upper limb impairment after stroke.

Alterations in intermuscular coordination underlying isokinetic exercise after a stroke and their implications on neurorehabilitation

“…When the number of synergies in the stroke group was four, two out of five synergies were merged into one deltoid synergy. Deltoid muscle coupling was also observed during other mechanically constrained tasks for which the number of muscle synergies in the affected limbs of stroke participants was equal or similar to those in their unaffected limbs or the intact limbs of the control participants (12,(14)(15)(16). According to a previous study on free gait, a smaller number of synergies underlays the activation of lower limb muscles of stroke survivors compared with that of the control participants, but a comparable composition of synergies was observed in the stroke and the control groups when their numbers of synergies were matched (17).…”

“…Unfortunately, no study has compared the characteristics of upper limb intermuscular coordination across varying biomechanical task conditions in patients with a stroke. Instead, intermuscular coordination has been evaluated for a wide range of tasks in various studies of heterogeneous design (Table 1) (10)(11)(12)(13)(14)(15)(16). By comparing their findings, we discussed how the intermuscular coordination post stroke was affected by the presence of the task constraints.…”

“…Previous studies focusing on post-stroke patients suggest that the characteristics of intermuscular coordination, underlying the movement performed in support of external devices, can be different from those evaluated in free space after a stroke. The number and composition of muscle synergies during assisted horizontal or three-dimensional reaching tasks were comparable between stroke survivors and neurologically intact individuals; however, increased coupling of the deltoid muscles affected the composition of the shoulder-related synergies in the stroke group (12,14). When the three-dimensional reaching task of the affected limb was supported by weight supporters or therapeutic robots, the expression of the abnormal synergistic pattern of the shoulder muscles including deltoid muscles was reduced compared to when no assistance was provided (13,14).…”

“…The number and composition of muscle synergies during assisted horizontal or three-dimensional reaching tasks were comparable between stroke survivors and neurologically intact individuals; however, increased coupling of the deltoid muscles affected the composition of the shoulder-related synergies in the stroke group (12,14). When the three-dimensional reaching task of the affected limb was supported by weight supporters or therapeutic robots, the expression of the abnormal synergistic pattern of the shoulder muscles including deltoid muscles was reduced compared to when no assistance was provided (13,14). The motor deficit during the assisted reaching task after a stroke was mainly caused by the alteration of the activation coefficients of muscle synergies instead of the number and the composition of muscle synergies (12).…”

“…Different mechanisms of post-stroke changes in intermuscular coordination underlying shoulder and elbow joints have been reported for the following task constraints: free movement (10,11), constrained (particularly, assisted) movement (12)(13)(14), and static strengthening (15,16) (see Table 1).…”