Motor variability within a multi-effector system: experimental and analytical studies of multi-finger production of quick force pulses

Goodman, Simon R.; Shim, Jae Kun; Zatsiorsky, Vladimir M.; Latash, Mark L.

doi:10.1007/s00221-004-2147-z

Cited by 43 publications

(54 citation statements)

References 32 publications

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“…If the variance per dimension is larger along the UCM than in the orthogonal sub-space, a conclusion can be drawn that a multi-finger synergy stabilizes the total force across trials. We ran such analyses on the data sets and got results similar to those reported in earlier studies of steady-state and quick force pulse production (Goodman et al 2005;Olafsdottir et al 2005;Shim et al 2005).…”

Section: Experiment-1: Qfp-mentioning

confidence: 61%

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Do synergies decrease force variability? A study of single-finger and multi-finger force production

Shapkova

Goodman

et al. 2008

Exp Brain Res

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We tested a hypothesis that force production by multi-finger groups leads to lower indices of force variability as compared to similar single-finger tasks. Three experiments were performed with quick force production, steady-state force production under visual feedback, and steady-state force production without visual feedback. In all experiments, a range of force levels was used computed as percentages of the maximal voluntary contraction force for each involved finger combination. Force standard deviation increased linearly with force magnitude across all three experiments and all finger combinations. There were modest differences between multi-finger and single-finger tasks in the indices of force variability, significant only in the tasks with steady-state force production under visual feedback. When fingers acted in groups, each finger showed significantly higher force variability as compared to its single-finger task and as compared to the multi-finger group as a whole. Fingers that were not instructed to produce force also showed close to linear relations between force standard deviation and force magnitude. For these fingers, indices of force variability were much higher as compared to those computed for the forces produced by instructed fingers. We interpret the findings within a feed-forward scheme of multi-finger control with two inputs only one of which is related to the explicit task. The total force variability reflects variability in only the task-related component, while variability of the finger forces is also due to variability of the component that is not related to the task. The findings tentatively suggest that total force variability originates at an upper level of the control hierarchy in accordance to the Weber-Fechner law rather than reflects a "neural noise" at the segmental level.

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Section: Experiment-1: Qfp-mentioning

confidence: 61%

“…However, such actions are associated with a lack of negative co-variation among finger forces, i.e. lack of force-stabilizing synergy (Goodman et al 2005;Olafsdottir et al 2005). …”

Section: Introductionmentioning

confidence: 99%

Do synergies decrease force variability? A study of single-finger and multi-finger force production

Shapkova

Goodman

et al. 2008

Exp Brain Res

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“…There was no explicit template, and the rate of moment production was relatively high, comparable to those used in earlier studies with cyclic total force production (Scholz et al 2002). Note that an increase in the rate of production of a performance variable may by itself lead to apparently weaker synergies stabilizing that variable (Goodman et al 2005). Nevertheless, our primary hypothesis was that the subjects would be able to show multi-finger synergies stabilizing the total moment of force over the whole cycle.…”

Section: Introductionmentioning

confidence: 76%

“…Variance of the total moment of force peaked at a time when the rate of change of the total moment was maximal. This can be interpreted as a consequence of a timing error in specification of this performance variable (Goodman et al 2005). However, the index of moment stabilization, ΔV M showed a peak close to the time of peak PR moment and was minimal close to the time of peak SU moment, when the rate of moment change was close to zero.…”

Section: Variability and Synergiesmentioning

confidence: 98%

Finger synergies during multi-finger cyclic production of moment of force

2006

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We investigated multi-finger synergies stabilizing the total moment of force and the total force when the subjects produced a quick cyclic change in the total moment of force. The seated subjects performed the task with the fingers of the dominant arm while paced by the metronome at 1.33 Hz. They were required to produce a rhythmic, sine-like change in the total pronation-supination moment of force computed with respect to the midpoint between the middle and ring fingers. The framework of the uncontrolled manifold hypothesis was used to compute indices of stabilization of the total moment and of the total force across 20 cycles. Variance of the total moment showed a cyclic pattern with peaks close to the peak rate of the moment change. Variance of the total force was maximal close to peak moment into supination. Higher magnitudes of the moment directed against the required moment direction (antagonist moment) were produced by individual fingers during supination efforts as compared to pronation efforts. Indices of multi-finger synergies showed across-trials stabilization of the total moment over the whole cycle but not of the total force. These indices were smaller during supination efforts. We conclude that the central nervous system facilitates multi-finger synergies stabilizing the total rotational action across a variety of tasks. Synergies stabilizing the total force are not seen in tasks that do not explicitly require accurate force control. Pronation efforts are performed more efficiently and with better stabilization of the action.

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“…For single-joint actions, this has been formalized in a model by Goodman/Gutman and colleagues (Gutman and Gottlieb 1992;Gutman et al 1993). More recently, the model has been generalized for multi-finger force production tasks (Goodman et al 2005). The model has shown, in particular, that timing errors contribute significantly to the overall variability indices and that multi-finger synergies fail to reduce this component of variability ).…”

Section: Moment Of Force Of Its Variabilitymentioning

confidence: 99%

Accurate production of time-varying patterns of the moment of force in multi-finger tasks

2006

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We investigated the production of time profiles of the total moment of force produced in isometric conditions by the four fingers of a hand. We hypothesized that these tasks would be associated with multi-finger synergies stabilizing the time profile of the total moment across trials but not necessarily stabilizing the time profile of the total force produced by the fingers. We also expected the multifinger synergies to prevent an increase in the moment variability with its magnitude. Seated subjects pressed on force sensors with the four fingers of the right hand and produced two time profiles of the total moment of force, starting from a certain pronation effort, leading to a similar supination effort, and back to the initial pronation effort. One of the profiles was a sequence of straight lines (M-Ramp) while the other was a smooth curve (M-Sine). The subjects showed an increase in the total force during each task. This was accompanied by an increase in the force produced by the fingers opposing the required direction of the total moment-antagonist fingers. Variability of the total force and of the total moment showed complex, non-monotonic changes with the magnitude of the force and moment, respectively. In both tasks, the subjects showed patterns of co-variation of commands to fingers that stabilized the required moment profile over trials. The time profile of the total force was stabilized to a lesser degree or not stabilized at all. The share of fingers with larger moment arms (index finger for pronation efforts and little finger for supination efforts) was higher when the fingers acted to produce moments in a required direction but not necessarily when they acted as antagonists. The results demonstrate the existence of multi-finger synergies stabilizing the combined rotational action. They fit a hypothesis that stabilization of rotational actions may be a default strategy conditioned by everyday experience. The data also suggest that the mechanical advantage hypothesis is valid for sets of effectors that act in the required direction but not for sets of effectors that act as antagonists.

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Motor variability within a multi-effector system: experimental and analytical studies of multi-finger production of quick force pulses

Cited by 43 publications

References 32 publications

Do synergies decrease force variability? A study of single-finger and multi-finger force production

Do synergies decrease force variability? A study of single-finger and multi-finger force production

Finger synergies during multi-finger cyclic production of moment of force

Accurate production of time-varying patterns of the moment of force in multi-finger tasks

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