Mild Stroke Affects Pointing Movements Made in Different Frames of Reference

Abstract

Background: Motor performance is a complex process controlled in task-specific spatial frames of reference (FRs). Movements can be made within the framework of the body (egocentric FR) or external space (exocentric FR). People with stroke have impaired reaching, which may be related to deficits in movement production in different FRs.

Objective: To characterize rapid motor responses to changes in the number of degrees of freedom for movements made in different FRs and their relationship with sensorimotor and cognitive impairment in individuals with mild chronic stroke.

Methods: Healthy and poststroke individuals moved their hand along the contralateral forearm (egocentric task) and between targets in the peripersonal space (exocentric task) without vision while flexing the trunk. Trunk movement was blocked in randomized trials.

Results: For the egocentric task, controls produced the same endpoint trajectories in both conditions (free- and blocked-trunk) by preserving similar shoulder-elbow interjoint coordination (IJC). However, endpoint trajectories were dissimilar because of altered IJC in stroke. For the exocentric task, controls produced the same endpoint trajectories when the trunk was free or blocked by rapidly changing the IJC, whereas this was not the case in stroke. Deficits in exocentric movement after stroke were related to cognitive but not sensorimotor impairment.

Conclusions: Individuals with mild stroke have deficits rapidly responding to changing conditions for complex reaching tasks. This may be related to cognitive deficits and limitations in the regulation of tonic stretch reflex thresholds. Such deficits should be considered in rehabilitation programs encouraging the reintegration of the affected arm into activities of daily living.

Keywords: kinematics; motor control; motor equivalence; stroke; upper limb.

Figure 1.

Experimental setup: The participant sat on an armless chair and wore a harness with an electromagnet plate that could be locked to an electromagnet attached to the chair back to prevent trunk flexion. Participants performed reaching along the contralateral arm in the egocentric condition (A, B), or between 2 targets in the exocentric (C, D) condition, while leaning the trunk forward with eyes closed. (A, C) When the electromagnet was not activated, the participant leaned the trunk forward while reaching: free-trunk condition. (B, D) When the electromagnet was activated, only arm movement could occur: blocked-trunk condition. Modified from Ghafouri et al.

Figure 2.

Mean trajectory (A, D) and shoulder-elbow interjoint coordination traces (B, C, E, F) of a representative healthy (A-C) and stroke (D-F) participant performing the egocentric task. Ellipses represent standard error values. Ellipse spacing represents time, where more tightly spaced ellipses indicate slower movements. Free-trunk traces are shown in black, and blocked-trunk traces are shown in gray. Black circles: initial positions; white circles: divergence points.

Figure 3.

Mean trajectory (A, D) and shoulder-elbow interjoint coordination traces (B, C, E, F) of a representative healthy (A-C) and stroke (D-F) participant performing the exocentric task. Ellipses represent standard error values. Ellipse spacing represents time, where more tightly spaced ellipses indicate slower movements. Free-trunk traces are shown in black and blocked-trunk traces are shown in gray. Black circles: initial positions; white circles: divergence points.

Figure 4.

Slopes (A, B) and divergence points (C, D) of trajectories and shoulder-elbow interjoint coordination. (A, B): Mean (SD) values are shown for free (black bars) and blocked (white bars) conditions. (C, D): Divergence points for each pair of trajectories and interjoint coordination traces are shown for healthy (black bars) and stroke (white bars) groups. Statistically significant differences are shown with asterisks (*). Abbreviations: Sh, Shoulder; Fl, Flexion; El, Elbow; Ab, Abduction.

Figure 5.

Schematic diagram of the referent arm-trunk configuration for a movement made in the external frame of reference (FR). The hand traces the referent hand trajectory (solid line) between the initial position R_i (open body configuration) and the final position R_f (gray body configuration). The actual hand trajectory (dashed line) deviates from the referent trajectory because of the interaction of the limb with the environment (ie, gravity, inertial forces). Muscle forces emerge as a result of the deflection of the actual from the referent hand trajectory.