Enslaving in a serial chain: interactions between grip force and hand force in isometric tasks

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Extrinsic hand muscles perform dual roles: generating grip force and wrist torque.
• Understanding the interplay between grip force and wrist torque is crucial for motor control research.

Purpose of the Study:

• To investigate the interactions between grip force and wrist torque during isometric force production.
• To test the hypothesis of 'enslaving,' where intentional changes in one variable unintentionally affect the other.

Main Methods:

• Young, healthy subjects performed isometric force production tasks involving grip and wrist actions.
• Analysis focused on unintentional changes in grip force during voluntary wrist torque modulation and vice versa.
• Quantified force covariation and synergy indices under different task conditions.

Main Results:

• Intentional grip force changes had minimal impact on wrist torque.
• Voluntary wrist torque changes consistently altered grip force, with greater effects during force decrease.
• These 'enslaving' effects intensified with higher initial grip force.
• When aiming for a combined force, strong negative covariation (synergy) between grip and wrist forces was observed, stabilizing total force.

Conclusions:

• 'Enslaving' effects suggest muscle coordination is influenced by common daily tasks.
• Observed synergistic effects support the 'referent configuration hypothesis' for stabilizing task-relevant forces.