Prehension synergies during smooth changes of the external torque

Abstract

We studied characteristics of digit action and their co-variation patterns across trials (prehension synergies) during static holding of an object while the external torque could change slowly and smoothly. The subjects held in the air an instrumented handle with an attachment that allowed a smooth change in the external torque over about 12 s; the load was always kept constant. Series of trials were performed under three conditions: The torque could be zero throughout the trial or it could change slowly requiring a smooth change of the effort from a non-zero pronation value to zero (PR-0) or from a non-zero supination value to zero (SU-0). The handle was kept vertical at all times. Indices of variance and co-variation of elemental variables (forces and moments of force produced by individual digits) stabilizing such performance variables as total normal force, total tangential force, and total moment of force were computed at two levels of an assumed control hierarchy. At the upper level, the task is shared between the thumb and virtual finger (an imagined digit with the mechanical action equal to that of the four fingers), while at the lower level, action of the virtual finger is shared among the actual four fingers. We analyzed the total moment of force as the sum of the moments of force produced by the thumb and virtual finger and also as the sum of the moments of force produced by the normal forces and tangential forces. The results showed that the adjustments in the total moment of force were produced primarily with changes in the moment produced by the virtual finger and by changes in the moment produced by the normal forces. The normal force of the thumb at the final state (which was the same across conditions) was larger in the two conditions with changes in the external torque. The safety margin was significantly higher in the PR-0 condition, and it dropped with the decrease in the external torque. A co-contraction index was computed to reflect moment of force production by the fingers acting against the total moment produced by the virtual finger. It was higher for the SU-0 condition. Most variance indices dropped with a decrease in the external torque. The co-variation indices, however, remained unchanged over the trial duration. They showed signs of a trade-off between the two levels of the assumed hierarchy: Larger indices at the higher level corresponded to smaller indices at the lower level. This study and the previous one (Sun et al. 2011) document several previously unknown features of prehensile tasks. The results show that characteristics of digit action and interaction in such tasks depend not only on the magnitudes of external constraints but on a variety of other factors including time changes in the constraints and their history.

Interpersonal synergies: static prehension tasks performed by two actors

“…At that level, the VF force variance is, by definition, V ORT , and its large amounts bring the synergy index down. It is possible to have strong synergies at both levels as shown in experiments for synergies stabilizing tangential force during one-hand prehensile tasks (Gorniak et al 2009; Sun et al 2011). Nevertheless, the overall tendency for synergy indices computed at the two levels is to compete with each other (e.g., Wu et al 2012; Singh et al 2014).…”

“…Several earlier studies documented trade-offs between synergies at different levels of a hierarchical system involved in the control of prehensile actions (Gorniak et al 2007, 2009a; Sun et al 2011; Wu et al 2012). These findings could be related to the method of computation of the synergy index.…”

Seven Properties of Self-Organization in the Human Brain

“…The human hand has evolved [87] as a function of active constraints [88][89][90][91][92][93][94][95][96][97][98][99][100] and is in harmony with other sensory systems such as the visual and auditory brain [97,99,101]. Grip force profiles are a direct reflection of complex low-level, cognitive, and behavioral synergies this evolution has produced [87][88][89][90][91][92][93][94][95][96][97][98][99][100][101]. The state of the art in experimental studies on grip force control for lifting and manipulating objects [88,89,91,93,94] provides insight into the contributions of each finger to overall grip strength and fine grip force control.…”

Analytical Inverse Optimization in Two-Hand Prehensile Tasks

“…For example, phenomena of hysteresis have been reported over many studies of motor behavior; in particular, subjects grip objects differently depending on the direction of slow change in the object’s weight or external torque (Sun et al 2011a,b). In our study, similar combinations of constraints could be achieved with different rates of change of the constraints, zero, positive, or negative.…”