Grasp stiffness as a function of grasp force and finger span

Area of Science:

• Biomechanics
• Motor Control
• Human Factors

Background:

• Understanding grasp stiffness is crucial for motor control research.
• Previous studies inferred stiffness from force-space characteristics, but direct measurements were lacking.

Purpose of the Study:

• To compare direct measurements of grasp stiffness with stiffness inferred from force-space characteristics.
• To investigate the relationship between grasp stiffness, initial force, and finger span.

Main Methods:

• Direct grasp stiffness was measured using step displacements in a do-not-intervene paradigm.
• Subjects performed a three-finger pinch on a motorized manipulandum, with one hand randomly perturbed.
• Stiffness was calculated from force and span data, examining effects of step amplitude, rise-time, and load stiffness.

Main Results:

• Grasp stiffness decreased with larger step amplitudes and slightly increased with longer rise-times.
• Grasp stiffness increased proportionally with initial force but was minimally affected by finger span.
• Directly measured stiffness was not equivalent to stiffness inferred from effort-matching data.

Conclusions:

• Direct perturbation measurements and inferred stiffness from effort-matching represent different components of motor behavior.
• The findings highlight the complexity of grasp stiffness and suggest distinct neural control mechanisms.