Impact of unilateral and bilateral impairments on bimanual force production following stroke

Abstract

Large bilateral asymmetry and task deficits are typically observed during bimanual actions of stroke survivors. Do these abnormalities originate from unilateral impairments affecting their more-impaired limb, such as weakness and abnormal synergy, or from bilateral impairments such as incoordination of two limbs? To answer this question, 23 subjects including 10 chronic stroke survivors and 13 neurologically intact subjects participated in an experiment where they produced bimanual forces at different hand locations. The force magnitude and directional deviation of the more-impaired arm were measured for unilateral impairments and bimanual coordination across locations for bilateral impairments. Force asymmetry and task error were used to define task performance. Significant unilateral impairments were observed in subjects with stroke; the maximal force capacity of their more-impaired arm was significantly lower than that of their less-impaired arm, with a higher degree of force deviation. However, its force contribution during submaximal tasks was greater than its relative force capacity. Significant bilateral impairments were also observed, as stroke survivors modulated two forces to a larger degree across hand locations but in a less coordinated manner than control subjects did. But only unilateral, not bilateral, impairments explained a significant amount of between-subject variability in force asymmetry across subjects with stroke. Task error, in contrast, was correlated with neither unilateral nor bilateral impairments. Our results suggest that unilateral impairments of the more-impaired arm of stroke survivors mainly contribute to its reduced recruitment, but that the degree of its participation in bimanual task may be greater than their capacity as they attempt to achieve symmetry.NEW & NOTEWORTHY We studied how unilateral and bilateral impairments in stroke survivors affect their bimanual task performance. Unilateral impairments of the more-impaired limb, both weakness and loss of directional control, mainly contribute to bimanual asymmetry, but stroke survivors generally produce higher force with their more-impaired limb than their relative capacity. Bilateral force coordination was significantly impaired in stroke survivors, but its degree of impairment was not related to their unilateral impairments.

Keywords: bimanual coordination; decision-making; force distribution; stroke; upper extremity.

Graphical abstract

Figure 1.

Experimental setup. A: custom-made apparatus that placed two load cells at 10 locations. B: 10 target locations determined by anthropometric measurements and voluntary reaching distance of each subject. The target force and bimanual force produced by subjects were displayed on the computer screen.

Figure 2.

Representative cases of bimanual force production. A: control subject (C5). B: subject with stroke (S4). Control subjects produced a similar level of forces with their two arms during the maximum voluntary force production and during the target force production (A1–A3). In contrast, subjects with stroke produced significantly smaller forces with their more-impaired arm during maximal force production (B1). However, at the same target location, when they produced submaximal forces, the contribution of the more-impaired arm was generally higher (B2). But the force produced by the more-impaired arm often decreased significantly when the target location changed (B3).

Figure 3.

Task performance and force asymmetry: A and B: overall force ratios (OFRs) and target force ratios (TFRs) during maximal force production (A1: OFR_max, A2: TFR_max) or task performance (B1: OFR, B2: TFR). C: force deviation of the two arms (C1: more-impaired/nondominant, C2: less-impaired/dominant). D: force error during task performance.

Figure 4.

Changes in force contribution of the less-preferred limb across target locations. A: modulation range (%) of the force produced by less-preferred limbs (A1: control; A2: stroke). B: representative plots of relative force contribution of nondominant/more-impaired arms (TFR) at five target locations in the frontal plane, averaged across anterior directions (control subject C3; stroke subject S8). The modulation ranges of subjects with stroke were significantly greater than those of control subjects except the lateral force of the more-impaired side. The force contribution of more-impaired arm increased at the targets located in the more-impaired side. TFR, target force ratio.

Figure 5.

Representative relationships between the force magnitude of the two limbs. A: control subject (C8): dominant and nondominant side forces. B: stroke survivor (S7): less-impaired and more-impaired side forces.

Figure 6.

Correlation between target force contribution (TFR) and unilateral impairments of the more-impaired/nondominant limb. A: control subjects. B: stroke survivors. Maximum force ratio (OFR: 1st column); force magnitude of nondominant (FND: control) or more-impaired (FMI: stroke) limb (2nd column); and force deviation of nondominant (θND: control) or more-impaired (θMI: stroke) limb (3rd column). In each graph, 39 data points (13 control subjects × 3 force directions) or 30 points (10 stroke subjects × 3 force directions) were included. Contribution of nondominant arm (TFR) of control subjects was not affected by any of the factors examined, but remained similar to that of dominant arm across conditions (TFR ≈ 1). TFR of stroke survivors, on the other hand, was found to significantly correlate with the relative force capacity (OFRmax) and force deviation of their more-impaired arm (θMI). OFR, overall force ratio.