Complementary and Synchronous Force Production in Joint Action

Abstract

If two people lift and carry an object, they not only produce complementary forces on the object but also walk in synchrony. The first study thus tested the hypothesis that complementary and synchronous strategies simultaneously facilitated the action coordination performed by two people in a task of periodic isometric force production. When the total force was visible, the correlation between forces produced by two participants was highly negative, and the relative phase angles were then distributed at the 0–20° phase region. These innovative findings indicate that one participant compensated for another’s force errors and produced forces in synchrony exclusively when the total force was visible. The joint action then controlled force more accurately than the individual action. The second study examined the new force-sharing patterns in joint action performed by two, three, or four participants, and it further tested whether an increase in the number of participants results in a decrease in individual contribution (social loafing). In both the three- and four-person tasks, the correlation between forces produced by two of the three or four participants was negative, and the remaining one or two participants produced intermediate forces. The errors of force and movement interval and force variabilities were smaller in four- and three-people groups than individuals. The second study thus eliminated the effect of social loafing. The third study examined development of a leader-follower relationship in joint action performed by participants with different skill levels. Participants with low force variability produced a stronger and earlier force than those with high force variability. If two people row a boat, they often call to each other to synchronize their strokes. It is anticipated that such a call promotes periodic joint action. The fourth study thus examined the effects of speech on both complementary and synchronous strategies in joint action. Although periodically uttering a syllable worsened complementary force production when the target was visible, it promoted synchronization of their performance to each other’s timing when the target was invisible. All motor control theories (i.e., uncontrolled manifold hypothesis) predict that as the error produced by participants increases, error compensation also increases. The fifth study thus tested the hypothesis that a load perturbation facilitates interpersonal compensation for force error. Two cooperative participants a and b produced a target force, and the force produced by another participant c increased or decreased the sum of the forces produced by participants a and b. The load perturbation facilitated interpersonal compensation for force error, but caused performance to deteriorate.