Static prehension of a horizontally oriented object in three dimensions.

Area of Science:

• Biomechanics
• Human motor control
• Robotics

Background:

• Understanding how humans grasp and manipulate objects is crucial for fields like biomechanics and robotics.
• Previous research has explored force sharing among digits during grasping, but the interplay between hierarchical control and individual digit contributions remains incompletely understood.

Purpose of the Study:

• To investigate the distribution of forces and moments across digits during static prehension of a horizontally oriented object.
• To examine the presence of mechanically unnecessary forces and the trade-offs in multi-digit synergies at different levels of a proposed control hierarchy.

Main Methods:

• Subjects performed static prehension of a force/torque-instrumented handle under varying loads and torques.
• Force and moment data were collected using a six-component sensor system.
• Analysis focused on force sharing patterns between the thumb and virtual finger (upper level) and among individual fingers (lower level).

Main Results:

• The thumb generated mechanically unnecessary forces even without external torque.
• Tangential forces dominated the moment of force production when external torque was applied.
• Covaried adjustments of individual digit forces stabilized force and moment variables, with an exception for normal force at the lower hierarchy level.
• A trade-off was observed between synergy indices at the two hierarchical levels for force variables, but not for moment of force variables.

Conclusions:

• Digit forces are influenced by factors beyond task mechanics, including sensory receptor loading.
• A universal rule for force sharing across different tasks and digits is unlikely.
• The degree of redundancy at higher control levels appears significant in modulating force and moment control.