The organization of intralimb and interlimb synergies in response to different joint dynamics

Abstract

We sought to understand differences in joint coordination between the dominant and nondominant arms when performing repetitive tasks. The uncontrolled manifold approach was used to decompose the variability of joint motions into components that reflect the use of motor redundancy or movement error. First, we hypothesized that coordination of the dominant arm would demonstrate greater use of motor redundancy to compensate for interaction forces than would coordination of the nondominant arm. Secondly, we hypothesized that when interjoint dynamics were more complex, control of the interlimb relationship would remain stable despite differences in control of individual hand paths. Healthy adults performed bimanual tracing of two orientations of ellipses that resulted in different magnitudes of elbow interaction forces. For the dominant arm, joint variance leading to hand path error was the same for both ellipsis orientations, whereas joint variance reflecting the use of motor redundancy increased when interaction moment was highest. For the nondominant arm, more joint error variance was found when interaction moment was highest, whereas motor redundancy did not differ across orientations. There was no apparent difference in interjoint dynamics between the two arms. Thus, greater skill exhibited by the dominant arm may be related to its ability to utilize motor redundancy to compensate for the effect of interaction forces. However, despite the greater error associated with control of the nondominant hand, control of the interlimb relationship remained stable when the interaction moment increased. This suggests separate levels of control for inter- versus intra-limb coordination in this bimanual task.

Fig. 1

Illustration of the experimental setup: a view from the right side; b the arrangement of the ellipse template. Arrows indicate hand movement direction. See text for details

Fig. 2

Illustration of joint angle calculation for the dominant arm: scapular abduction–adduction; shoulder horizontal abduction–adduction; elbow flexion–extension; wrist flexion–extension. The same arrangement applies to the nondominant arm

Fig. 3

Hand path from one representative subject during (a) contralateral and (b) ipsilateral elliptical tracing; (c) constant error and (d) hand path standard deviation for each tracing condition. Error bars are standard errors of the mean (SEM)

Fig. 4

Example components of the individual moments at the shoulder (a, b) and elbow (c, d) during contralateral and ipsilateral tracing. Data from a representative subject is shown. Positive values indicate joint abduction or extension; negative values indicate joint adduction or flexion

Fig. 5

Mean percent of movement cycle during which (a) muscle and (b) interaction moments assisted NM at the shoulder and elbow joints. Interval is expressed as a percentage of the cycle period. Error bars are SEM

Fig. 6

Mean (a) muscle and (b) interaction moment impulses at the shoulder and elbow joints. Error bars are SEM

Fig. 7

Mean components of joint configuration variance related to control of the vectorial distance between the hands, the nondominant and dominant hands’ paths. GEV is represented by open (contralateral) or black (ipsilateral) bars; NGEV for all control hypotheses is indicated by the light gray bar immediately to the right of the bars representing GEV. Error bars are SEM. *P < 0.05

Fig. 8

Shoulder–elbow angle–angle plots to illustrate coordination differences when tracing the contralaterally versus ipsilaterally oriented ellipses. Arrows next to the joint paths indicate the direction of joint movements