A summary of:
Guiard, Y., 1987. Asymmetric division of labor in human skilled bimanual action: The kinematic chain as a model. Journal of Motor Behavior, 19(4), 486-517 (online article).
This paper proposes a kinematic chain model for describing the relationship and functioning of the hands in asymmetric bi-manual tasks. As shown in the illustration above, these are tasks such as playing a violin where both hands are engaged in the task, but carry out different functions. They may be contrasted with unimanual tasks, such as tooth-brushing, where one hand is primarily engaged in the action while the other hand remains passive symmetric bi-manual tasks, such as weight-lifting where both hands are engaged, but undertake the same task. The author states that previous research had focussed mainly on unimanual tasks and that though bi-manual tasks had recently become a topic of interests for experimental psychologists, the focus there was mainly on symmetric bi-manual tasks. The area of bi-manual asymmetric tasks had, by contrast received less attention from researchers and is the focus for this paper.
The author goes on to show that in bi-manual tasks such as hand-writing, or swinging a golf-club, the relationship between the functions of the hands is not simply that one hand is skilled and the other unskilled, but that one hand delineates ‘frames’ on a macrometric scale within which the other hand can insert ‘contents’ on a micrometric scale. In the case of a right-handed person hand-writing, this refers to the way that the left-hand positions and steadies the paper for the right-hand to make marks on it. This positioning is not like a clamp rigidly holding the paper still, but a ‘plastic stability’, where the position of the paper is constantly adjusted to fit with the unfolding task. So where the right-hand (of a right-hander) exhibits fine temporal and spatial resolution, the left-hand provides the coarser temporal and spatial framings for this.
As an abstract model of this relationship, the author proposes a kinematic chain, where each arm is modeled as an abstract motor and their functioning is chained such that the coarser temporal and spatial actions of one arm (the left arm of a right-hander) feed in to the finer temporal and spatial actions of the other (the right arm of a right-hander). To understand what this means, it’s first easier to think of a single arm, like a robot arm that is driven by one motor at the shoulder and another at the wrist. The motor at the shoulder will be the best for providing the big positioning movements of the hand, while the one at the wrist will be the best for finer positioning movements.