Factors influencing variability in load forces in a tripod grasp

Area of Science:

• Biomechanics
• Human motor control
• Robotics

Background:

• Precision grip is crucial for object manipulation.
• Variability in grip force can affect task performance and safety.
• Understanding factors influencing grip force is essential for human and robotic applications.

Purpose of the Study:

• To identify key factors contributing to load force variability during precision grip.
• To quantify the impact of surface orientation and center of mass location on grip forces.
• To investigate the role of digit pressure centers and tangential torques in grip force regulation.

Main Methods:

• Subjects performed a tripod grasp on objects with varied contact surface properties.
• Forces and torques from the thumb, index, and middle fingers were measured or computed.
• Analysis focused on the static hold phase of the precision grip.
• Variations in contact surface orientation and location relative to the center of mass were systematically altered.

Main Results:

• Load force variability was significantly influenced by object weight, digit pressure center location, tangential torque, and grip force.
• Deviations in load forces reached up to 20% or more due to parameter variations.
• For the thumb, vertical center of pressure location was the primary source of variability.
• For fingers, center of pressure location and tangential torque magnitude had comparable effects on variability.

Conclusions:

• Grip force variability arises from unpredictable interactions between object properties and digit placement.
• Online feedback regulation is critical for maintaining stable grip forces.
• Findings have implications for designing more adaptable prosthetic devices and robotic grippers.