Changes in wrist muscle activity with forearm posture: implications for the study of sensorimotor transformations

Area of Science:

• Neuroscience
• Biomechanics
• Motor Control

Background:

• The primate wrist has been used to study how the central nervous system codes movement in different reference frames.
• Previous studies suggested a dissociation between extrinsic, muscle, and joint reference frames for wrist movements.

Purpose of the Study:

• To reassess the relationship between wrist muscle activity and forearm posture in humans.
• To investigate the dissociation between joint and muscle spaces during wrist movements.

Main Methods:

• Isometric force-aiming task in humans with precise control over musculoskeletal configuration.
• Extracellular cortical and spinal recordings from monkeys (for comparison).
• Intramuscular electrical stimulation to determine muscle-pulling directions.

Main Results:

• Preferred directions of wrist muscle activation closely followed forearm orientation (88%).
• This close alignment was consistent whether the hand was passively clamped or actively grasping.
• A small remaining discrepancy also occurred with direct muscle stimulation, suggesting it's not solely due to joint-muscle space dissociation.

Conclusions:

• The distinction between joint and muscle spaces in the wrist may be smaller than previously reported.
• Findings necessitate a re-evaluation of sensorimotor transformation studies using the wrist.
• Implications for understanding motor adaptation and control mechanisms.