Somatosensory control of precision grip during unpredictable pulling loads. II. Changes in load force rate

Area of Science:

• Neuroscience
• Human Motor Control
• Somatosensory System

Background:

• Previous research established that automatic grip force adjustments are graded by load amplitude.
• The somatosensory system plays a crucial role in regulating precision grip.
• Understanding the dynamics of grip force modulation is key to human motor control.

Purpose of the Study:

• To investigate the human capacity to respond to imposed load forces applied at various rates.
• To determine how grip force adjustments are scaled and timed in response to different loading rates.
• To explore the role of afferent information in updating grip force responses.

Main Methods:

• Subjects performed a precision grip task while restraining an object subjected to unpredictable pulling forces.
• Grip force, load force, and object position were recorded.
• Trapezoidal load force profiles at rates of 2, 4, 8, and 32 N/s were applied.

Main Results:

• Response latency increased with decreasing load force rate (80-174 ms).
• The amplitude of the grip force 'catch-up' response was scaled by the load force rate.
• The time course of the catch-up response was consistent across load rates, suggesting a unified motor expression.
• Afferent information appears to be continuously utilized for updating responses with very short latencies (45-50 ms).

Conclusions:

• The grip force response is triggered after a minimum latency once a load threshold is exceeded.
• The 'catch-up' response amplitude is modulated by load rate, facilitating rapid force reconciliation to prevent slips.
• Continuous sensory feedback is used to update motor commands for grip force, even with rapid load changes.