Equifinality and its violations in a redundant system: control with referent configurations in a multi-joint...

Area of Science:

• Motor control
• Human movement science
• Robotics

Background:

• Equifinality, the ability to reach the same outcome through different movement paths, is a key concept in motor control.
• The relationship between task-level equifinality and the variability of underlying joint movements (elemental variables) is not fully understood.

Purpose of the Study:

• To investigate whether task-level equifinality in arm movements is associated with violations of equifinality at the joint level.
• To test the hypothesis that consistent hand position and orientation are maintained despite variability in joint configurations during a perturbed reaching task.

Main Methods:

• Seated subjects performed a reaching task involving grasping a handle against a robot-generated bias force.
• A transient force perturbation was applied by the robot, and subjects were instructed not to intervene voluntarily.
• Analysis of across-trials variance in joint configuration space was used to assess hand position and orientation changes.

Main Results:

• Most variance in the difference between initial and final states was compatible with unchanged hand position and orientation.
• Across-trials variance in joint configurations did not necessarily lead to changes in hand position and orientation, suggesting task-level equifinality.

Conclusions:

• The findings provide direct support for theoretical schemes combining synergies and control with referent configurations.
• Task-level equifinality can be maintained even when the underlying elemental joint variables exhibit variability, highlighting the robustness of motor control strategies.