Performance-Stabilizing Synergies in a Complex Motor Skill: Analysis Based on the Uncontrolled Manifold Hypothesis

Area of Science:

• Motor control and biomechanics
• Human movement analysis
• Robotics and control theory

Background:

• Motor control research often investigates how humans stabilize movements despite variability in task-relevant variables.
• The uncontrolled manifold (UM) hypothesis provides a framework for analyzing motor synergies and their role in stabilizing task-specific variables.

Purpose of the Study:

• To investigate the role of vision in stabilizing hand coordinates and velocity during a throwing task.
• To examine how stability indices (ΔV) change over movement duration and across different sensory conditions.

Main Methods:

• Ten participants performed a ball-throwing task under eyes-open (vision) and eyes-closed (no vision) conditions.
• Intertrial variance in joint configuration and velocity spaces was analyzed using the uncontrolled manifold hypothesis.
• Stability indices (ΔV) for hand coordinate and velocity were calculated.

Main Results:

• Both coordinate- and velocity-stabilizing synergies (ΔV > 0) were confirmed, indicating robust motor control.
• Intertrial variance was significantly larger in the no-vision condition compared to the vision condition.
• While coordinate stability remained constant, velocity stability decreased over the movement duration, particularly in the no-vision condition.

Conclusions:

• The findings support a hierarchical control model where different levels of motor control exhibit distinct stability properties.
• A trade-off exists between stability at different hierarchical levels, influenced by sensory feedback.
• Vision plays a crucial role in maintaining velocity stability during dynamic motor tasks.