Unpredictable elbow joint perturbation during reaching results in multijoint motor equivalence

Area of Science:

• Motor control
• Biomechanics
• Neuroscience

Background:

• Motor equivalence describes the reorganization of movement components to maintain performance variables despite perturbations.
• A quantitative method was developed to assess motor equivalence by analyzing joint configuration changes versus performance variable changes under perturbation.

Purpose of the Study:

• To investigate the presence and magnitude of motor equivalence throughout the entire reaching movement trajectory.
• To examine how different target types (spherical, cylindrical) influence motor equivalence.
• To determine the effect of varying perturbation stiffness (low, high) on motor equivalence.

Main Methods:

• Subjects performed reaching movements to different targets with unpredictable elbow perturbations (varying stiffness).
• A modified uncontrolled manifold method was used to analyze joint deviation vectors and their impact on hand position and orientation.
• Arm view was occluded, and perturbation conditions were randomized to prevent adaptation or prediction.

Main Results:

• Motor equivalence was evident from the start of the reach and increased with perturbation strength after 40% of the movement.
• Hand orientation exhibited stronger stabilization through motor equivalence than 3D hand position, irrespective of target type.
• The magnitude of motor equivalence was not significantly affected by target shape.

Conclusions:

• Motor equivalence is a dynamic process present throughout reaching, adapting to perturbations.
• Neural control models supporting flexible joint coordination and resistance to deviations affecting performance variables are consistent with findings.
• Synergic control frameworks with hierarchical referent configurations align with the observed motor equivalence patterns.