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Prehension stability: experiments with expanding and contracting handle
Vladimir M Zatsiorsky1, Fan Gao, Mark L Latash
1Department of Kinesiology, The Pennsylvania State University, University Park, PA 16802, USA. vxz1@psu.edu
Journal of Neurophysiology
|December 2, 2005
Summary
Human grasp adjusts digit forces to maintain stability during changing handle widths. Complex finger synergies, not just simple spring-like reactions, are key to controlling forces and moments for stable object manipulation.
Area of Science:
- Biomechanics
- Human Motor Control
- Robotics
Background:
- Understanding how humans maintain grip stability during dynamic changes is crucial for prosthetics and robotics.
- Previous models, like the "virtual springs theory," may not fully capture the complexity of human grasping adjustments.
Purpose of the Study:
- To investigate digit force and moment adjustments during externally imposed changes in grasp width.
- To analyze how individual finger and combined virtual finger (VF) forces adapt to maintain equilibrium.
Main Methods:
- Eight subjects held a motorized handle while its width expanded or contracted at varying speeds.
- External torques were applied, and forces/moments at digit tips were measured using six-component sensors.
- Analysis was conducted at both individual finger (IF) and virtual finger (VF) levels.
Main Results:
- Normal VF and thumb forces increased with handle expansion and decreased with contraction.
- VF tangential forces showed an inverse relationship, decreasing with expansion and increasing with contraction.
- Individual finger adjustments varied, contributing to overall force and moment control, with different strategies for supination versus pronation tasks.
Conclusions:
- Human grasping involves complex, synergistic adjustments of individual digit forces and moments.
- Both local stiffness-like reactions and task-specific synergies contribute to maintaining grip stability.
- The "virtual springs theory" is insufficient to explain the observed human grasping control mechanisms.