Prehension Synergies in the Grasps With Complex Friction Patterns: Local Versus Synergic Effects and the Template Control

Niu, Xianjun; Latash, Mark L.; Zatsiorsky, Vladimir M.

doi:10.1152/jn.00058.2007

Cited by 23 publications

(14 citation statements)

References 32 publications

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“…Opposite changes of the normal and tangential forces produced by a digit have also been reported previously. For example, when the friction at the area of contact decreased, the normal force increased and the tangential force decreased (Niu et al 2007). In our study, the two forces also changed in opposite directions, the tangential forces increased during fatigue and the normal forces decreased.…”

Section: Discussionmentioning

confidence: 99%

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

2013

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We investigated the effects of exercise-induced fatigue of a digit on the biomechanics of a static prehension task. The participants were divided into two groups. One group performed the fatiguing exercise using the thumb (group-thumb) and the second group performed the exercise using the index finger (group-index). We analyzed the prehensile action as being based on a two-level hierarchy. Our first hypothesis was that fatigue of the thumb would have stronger effects at the upper level (action shared between the thumb and all four fingers combined – virtual finger) and fatigue of the index finger would have stronger effects at the lower level of the hierarchy (action of the virtual finger shared among actual fingers). We also hypothesized that fatigue would cause a decrease in the normal force applied by the exercised digit and correspondingly lead to a decrease in the normal force applied by the opposing digit(s). Our third hypothesis was that fatigue would leave the tangential forces unaffected. Fatigue led to a significant drop in the normal force of both exercised and non-exercised (opposing) digits. The tangential forces of the exercised digits increased after fatigue. This led to a drop in the safety margin in the group-thumb, but not group-index. As such, the results supported the first two hypotheses but not the third hypothesis. Overall, the results suggested that fatigue triggered a chain reaction that involved both forces and moments of force produced by individual digits leading to a violation of the principle of superposition. The findings are interpreted within the framework of the referent configuration hypothesis.

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Section: Discussionmentioning

confidence: 99%

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

2013

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“…reflecting a preference by the controller that may not have an obvious mechanical interpretation (Latash et al 2001; Niu et al 2007). Moreover, sometimes synergies in similar tasks are seen in some subjects but not in others; they can also emerge in the process of practice (Latash et al 2002a; Kang et al 2004; Scholz et al 2003).…”

Section: Discussionmentioning

confidence: 99%

Prehension synergies and control with referent hand configurations

et al. 2009

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We used the framework of the equilibrium-point hypothesis (in its updated form based on the notion of referent configuration) to investigate the multi-digit synergies at two levels of a hypothetical hierarchy involved in prehensile actions. Synergies were analyzed at the thumb–virtual finger (VF) level (VF is an imaginary digit with the mechanical action equivalent to that of the four actual fingers) and at the individual finger level. The subjects performed very quick vertical movements of a handle into a target. A load could be attached off-center to provide a pronation or supination torque. In a few trials, the handle was unexpectedly fixed to the table and the digits slipped off the sensors. In such trials, the hand stopped at a higher vertical position and rotated into pronation or supination depending on the expected torque. The aperture showed non-monotonic changes with a large, fast decrease and further increase, ending up with a smaller distance between the thumb and the fingers as compared to unperturbed trials. Multi-digit synergies were quantified using indices of co-variation between digit forces and moments of force across unperturbed trials. Prior to the lifting action, high synergy indices were observed at the individual finger level while modest indices were observed at the thumb–VF level. During the lifting action, the synergies at the individual finger level disappeared while the synergy indices became higher at the thumb–VF level. The results support the basic premise that, within a given task, setting a referent configuration may be described with a few referent values of variables that influence the equilibrium state, to which the system is attracted. Moreover, the referent configuration hypothesis can help interpret the data related to the trade-off between synergies at different hierarchical levels.

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“…This is especially true for the normal forces. For zero external torque the normal forces

F_{i}^{n}

are known to scale with the load force (Niu et al 2007; Westling and Johansson 1984). It is reasonable to assume that a particular force sharing patterns result from minimization of a certain objective function of the normal and tangential forces.…”

Section: Applicationsmentioning

confidence: 99%

An analytical approach to the problem of inverse optimization with additive objective functions: an application to human prehension

et al. 2009

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We consider the problem of what is being optimized in human actions with respect to various aspects of human movements and different motor tasks. From the mathematical point of view this problem consists of finding an unknown objective function given the values at which it reaches its minimum. This problem is called the inverse optimization problem. Until now the main approach to this problems has been the cut-and-try method, which consists of introducing an objective function and checking how it reflects the experimental data. Using this approach, different objective functions have been proposed for the same motor action. In the current paper we focus on inverse optimization problems with additive objective functions and linear constraints. Such problems are typical in human movement science. The problem of muscle (or finger) force sharing is an example. For such problems we obtain sufficient conditions for uniqueness and propose a method for determining the objective functions. To illustrate our method we analyze the problem of force sharing among the fingers in a grasping task. We estimate the objective function from the experimental data and show that it can predict the force-sharing pattern for a vast range of external forces and torques applied to the grasped object. The resulting objective function is quadratic with essentially non-zero linear terms.

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Prehension Synergies in the Grasps With Complex Friction Patterns: Local Versus Synergic Effects and the Template Control

Abstract: Niu X, Latash ML, Zatsiorsky VM. Prehension synergies in the grasps with complex friction patterns: local versus synergic effects and the template control.

Cited by 23 publications

References 32 publications

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

Adaptations to fatigue of a single digit violate the principle of superposition in a multi-finger static prehension task

Prehension synergies and control with referent hand configurations

An analytical approach to the problem of inverse optimization with additive objective functions: an application to human prehension

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