Compensating for intersegmental dynamics across the shoulder, elbow, and wrist joints during feedforward and feedback...

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• Arm movement involves complex interactions between limb segments, known as intersegmental dynamics.
• These dynamics mean torques at one joint affect motion at others, requiring multi-joint torques for single-joint movements.

Purpose of the Study:

• To investigate if the nervous system accounts for intersegmental limb dynamics during self-initiated reaching and in response to external perturbations.
• To examine how muscle activity across shoulder, elbow, and wrist joints compensates for interaction torques.

Main Methods:

• Experiments involved self-initiated planar reaching tasks with varying initial arm configurations and movement speeds.
• External mechanical perturbations were applied to assess reflexive responses.
• Electromyography was used to measure muscle activity in shoulder, elbow, and wrist.

Main Results:

• Shoulder muscle activity preceded elbow movements and scaled to compensate for interaction torques from forearm rotation.
• Elbow muscle activity preceded wrist movements, compensating for interaction torques from hand rotation.
• Nervous system predicted interaction torques during both self-initiated movements and reactive control, adjusting muscle activity accordingly.

Conclusions:

• The nervous system robustly predicts and accounts for intersegmental limb dynamics.
• This predictive capability is consistent across different joints (shoulder, elbow, wrist) and control strategies (feedforward and feedback).