Effects of object compliance on three-digit grasping

Area of Science:

• Biomechanics
• Human motor control
• Robotics

Background:

• Grasping rigid objects is well-understood, but compliant objects introduce dynamic challenges.
• Object compliance alters contact surface geometry and force requirements during manipulation.
• Understanding these dynamics is crucial for developing advanced robotic grasping and human motor control strategies.

Purpose of the Study:

• To characterize fingertip force and muscle activation patterns when grasping and lifting compliant objects.
• To investigate how object compliance affects the coordination of grip and load forces.
• To compare motor control strategies for rigid versus compliant object manipulation.

Main Methods:

• Subjects grasped and lifted a 200-g object with three fingers (thumb, index, ring).
• A spring was introduced under contact surfaces to simulate compliance.
• Fingertip forces and surface electromyography (sEMG) from ten hand and one arm muscle were recorded.

Main Results:

• Grip force and muscle activity patterns were similar for compliant and rigid objects during loading and lifting.
• The amplitude of grip force and muscle activity increased with object compliance.
• Tangential (load) forces showed non-monotonic increases with compliance, decoupling from grip forces.

Conclusions:

• Human motor control adapts to object compliance by modulating grip force amplitude.
• Compliance introduces complexities in force control, leading to a decoupling of grip and load forces.
• Findings inform the design of robotic systems capable of handling deformable objects.