Modification of Spastic Stretch Reflexes at the Elbow by Flexion Synergy Expression in Individuals With Chronic Hemiparetic Stroke

Abstract

Objective: To systematically characterize the effect of flexion synergy expression on the manifestation of elbow flexor stretch reflexes poststroke, and to relate these findings to elbow flexor stretch reflexes in individuals without neurologic injury.

Design: Controlled cohort study.

Setting: Academic medical center.

Participants: Participants (N=20) included individuals with chronic hemiparetic stroke (n=10) and a convenience sample of individuals without neurologic or musculoskeletal injury (n=10).

Interventions: Participants with stroke were interfaced with a robotic device that precisely manipulated flexion synergy expression (by regulating shoulder abduction loading) while delivering controlled elbow extension perturbations over a wide range of velocities. This device was also used to elicit elbow flexor stretch reflexes during volitional elbow flexor activation, both in the cohort of individuals with stroke and in a control cohort. In both cases, the amplitude of volitional elbow flexor preactivation was matched to that generated involuntarily during flexion synergy expression.

Main outcome measures: The amplitude of short- and long-latency stretch reflexes in the biceps brachii, assessed by electromyography, and expressed as a function of background muscle activation and stretch velocity.

Results: Increased shoulder abduction loading potentiated elbow flexor stretch reflexes via flexion synergy expression in the paretic arm. Compared with stretch reflexes in individuals without neurologic injury, paretic reflexes were larger at rest but were approximately equal to control muscles at matched levels of preactivation.

Conclusions: Because flexion synergy expression modifies stretch reflexes in involved muscles, interventions that reduce flexion synergy expression may confer the added benefit of reducing spasticity during functional use of the arm.

Keywords: Motor disorders; Muscle spasticity; Neurological rehabilitation; Rehabilitation; Robotics; Stroke.

Copyright © 2017 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.

Figures

Fig 1

Experimental setup, protocol. (A) Schematic representation of the Arm-Coordination and Training 4-D robotic device. (B) Participants either lifted the limb against an applied force (causing flexion synergy expression) or volitionally flexed the elbow while EE perturbations were delivered. Shown: stretch reflex elicited in response to a 270°/s perturbation. Abbreviations: And. vel., angular velocity; EMG, electromyography.

Fig 2

Flexion synergy expression at the elbow. Because SABD loading in the paretic arm increases poststroke, the BIC is involuntarily activated. Data are collected from the 2-second lift and hold period that preceded perturbation, as shown in figure 1B. Data are normalized to the BIC EMG generated during each participant’s isometric maximum voluntary EF contraction. Abbreviations: EMG, electromyography; max, maximum; s.e.m., standard error of the mean.

Fig 3

Single participant flexion synergy/stretch reflex results. As SABD load level or perturbation velocity increases, BIC reflex amplitude increases. Each subplot represents the baseline corrected BIC EMG at the combination of SABD load level and perturbation velocity indicated by the axes grid. SABD load level increases down the rows and is indicated by the right-side y-axis label; perturbation velocity increases left to right across the columns and is indicated by the top x-axis label. Abbreviations: EMG, electromyography; max, maximum. Adapted with permission from Institute of Electrical and Electronics Engineers.

Fig 4

Group average flexion synergy/stretch reflex results. Across the stroke cohort (n=10), BIC EMG increases as SABD load or perturbation angular velocity increases. Each colored rectangle within a panel represents the group average evoked BIC EMG at the given SABD load and perturbation velocity combination. The color data from both panels is interpreted by the scale bar at right, with cooler colors representing lower relative EMG activity and warmer colors representing higher EMG activity. The left panel represents SLR responses; the right panel, LLR responses. SABD load level increases down the rows and is indicated for both panels by the left-side y-axis label; perturbation velocity increases left to right across the columns and is indicated for both panels by the bottom left x-axis label. Color intensity: normalized BIC EMG magnitude (% max); each block represents the across-participant mean BIC EMG amplitude at a given SABD load and perturbation velocity. Abbreviations: EMG, electromyography; max, maximum.

Fig 5

Preactivation normalizes stretch-evoked BIC EMG between stroke and control participants. (A) Under relaxed conditions, stretch reflexes are exaggerated poststroke compared with controls; (B) once preactivated (approximately 20% max BIC EMG), reflex magnitude is comparable. Color code and gray identifiers indicate matched participants. EMG records: average response per participant to 270°/s perturbation. (C) Mean LLR responses to a 270°/s perturbation are shown for the stroke cohort during flexion synergy expression and during volitional background muscle activation; results are displayed as a function of background BIC activation level. Abbreviations: EMG, electromyography; max, maximum; s.e.m., standard error of the mean.

Fig 6

Group-average volitional preactivation/stretch reflex results. Across the stroke cohort (top panel) and the control cohort (bottom panel), stretch-evoked BIC EMG increases as volitional preactivation (subplot rows) or perturbation angular velocity (subplot columns) increases. Although reflex amplitude remains exaggerated in the relaxed BIC poststroke, reflexes become indistinguishable across groups once preactivated. Color intensity: normalized BIC EMG magnitude; each block represents the across-participant mean BIC EMG amplitude for a given preactivation. Abbreviations: EMG, electromyography; max, maximum.