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Hierarchical control of static prehension: II. Multi-digit synergies

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Abstract

The purpose of this study was to explore the ability of the central nervous system (CNS) to organize synergies at two levels of a hypothetical control hierarchy involved in two-hand multi-finger prehension tasks with one or more persons participating in the task together. At the higher level of the hierarchy, the total force and moment of force produced on an object are distributed between the thumb and the virtual finger (an imagined finger with mechanical output equal to the involved fingers of the hand), while at the lower level the virtual finger action is distributed among the four fingers. We tested a hypothesis that the CNS is able to organize synergies at only one level of the hierarchy. The subjects held vertically one of the two handles, a narrow one and a wide one. They used the four fingers of the right hand opposed by the right hand thumb, the left hand thumb, the left hand index finger, the thumb of an experimenter, the index finger of an experimenter, or an inanimate object. Forces and moments of force produced by each digit were recorded. Indices of synergies stabilizing the mechanical output variables at each of the two levels were computed. Contrary to the expectations, force and moment of force stabilizing synergies were found at one or both levels of the hierarchy across all tasks. Unimanual tasks exhibited higher synergy indices compared to all tasks, while intrapersonal synergy indices were higher than those of interpersonal synergies. The results suggest that both feed-forward and feedback mechanisms may be used to create force and moment of force stabilizing synergies. We invoke the notion of chain effects and generalize it for relations among variance components related to stabilization of different mechanical variables. The reference configuration hypothesis offers a fruitful framework for analysis of prehension synergies.

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Acknowledgments

The study was in part supported by NIH grants AG-018751, NS-035032, and AR-048563. We would like to thank Elizaveta M. Latash for her assistance in data collection.

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Authors and Affiliations

  1. Department of Kinesiology, The Pennsylvania State University, Rec.Hall-267, University Park, PA, 16802, USA

    Stacey L. Gorniak, Vladimir M. Zatsiorsky & Mark L. Latash

Authors
  1. Stacey L. Gorniak
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  2. Vladimir M. Zatsiorsky
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  3. Mark L. Latash
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Corresponding author

Correspondence to Mark L. Latash.

Appendix

Appendix

Force and moment of force sharing at the VF-TH level

The equations of statics impose constraints on the forces and moments of force produced by the elements at both VF-TH and IF levels. However, the system is redundant at both levels (reviewed in Zatsiorsky and Latash 2008); hence, different magnitudes of the outputs of individual elements can be used at both levels. Only data for the virtual finger––opposing effector level are presented here; the data for the individual fingers within the VF can be found in the companion paper (Gorniak et al. 2009).

Figure 4 presents average data across subjects normal force, tangential force, and moment of total force data for the virtual finger and opposing effector across all tested conditions. Force and moment of force produced by the virtual finger are represented by white bars while opposing effector forces and moments of force are represented by gray bars. Data are presented in local frames of reference (sensor based frames of reference); note that moments produced by the virtual finger would pronate the object while moments exerted by the opposing effector would supinate the object.

Fig. 4
figure 4

The normal force (F n , panels a and b), tangential force (F t, panels c and d), and total moment of force (M TOT, panels e and f) exerted on the handle for the narrow (panels a, c, and e) and wide (panels b, d, and f) handle widths. The data averaged across all subjects with standard error bars are shown. White bars indicate forces and moments of force produced by the virtual finger (VF) while grey bars indicate forces and moments of force produced by the opposing effector (OE). The data averaged across all subjects with standard error bars are shown. Abbreviations are the same as in Fig. 1

Full size image

Panels a and b of Fig. 4 show the magnitudes of normal forces for both the narrow and wide handle widths, respectively. Note that each of the two opposing effectors produced normal forces that varied broadly across the conditions while, within each condition, the forces were nearly equal to each other in magnitude. The tangential forces produced by the two effectors (panels c and d of Fig. 4) were similar to each other in all conditions except the one-hand grasp, TR + VFR, for which VF produced significantly lower force as compared to the opposing effector. The total moment of force was not exactly zero, corresponding to a net supination moment of force as shown in panels e and f of Fig. 4. Statistical analysis revealed that total moment of force was smallest for the narrow handle and finger configurations involving two persons. This was confirmed with ANOVA-1 and ANOVA-2; main effects of Width (F 1,81 = 20.72, P < 0.001) and Finger Configuration (F 4,81 = 14.32, P < 0.001 and F 5,45 = 16.2, P < 0.001) were found, respectively. Pair-wise comparisons have shown that finger configurations TE + VFR and IE + VFR yielded the smallest overall total moment of force.

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Gorniak, S.L., Zatsiorsky, V.M. & Latash, M.L. Hierarchical control of static prehension: II. Multi-digit synergies. Exp Brain Res 194, 1–15 (2009). https://doi.org/10.1007/s00221-008-1663-7

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