Equifinality and its violations in multi-joint positional tasks
Open Access
- Author:
- Zhou, Tao
- Graduate Program:
- Kinesiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- August 03, 2015
- Committee Members:
- Mark Latash, Dissertation Advisor/Co-Advisor
Mark Latash, Committee Chair/Co-Chair
Jinger Gottschall, Committee Member
H Joseph Sommer Iii, Committee Member
Vladimir M Zatsiorsky, Committee Member - Keywords:
- Multi-joint movement
Equifinality
Referent configuration hypothesis
Back-coupling
Synergy
Uncontrolled manifold hypothesis - Abstract:
- According to one of the influential hypotheses in the field of motor control, the equilibrium-point (EP) hypothesis, human voluntary movements are expected to show equifinality defined as a property of a neuromotor system to come to an originally planned final position despite transient changes in external forces. Earlier studies of equifinality have been used to argue for and against the EP-hypothesis. The five studies in this dissertation explored equifinality and its violations in multi-joint position-holding tasks. In all the studies, seated subjects grasped a handle, and occupied an initial arm configuration against a bias force produced by a robot (HapticMaster). The robot applied a smooth, transient change in the force (perturbation). The subjects were instructed “not to intervene voluntarily” with hand deviations. The first study confirmed hand equifinality following transient changes in external force, while individual joint rotations showed relatively large deviations from initial values such that significantly more variance in the joint configuration space was compatible with unchanged hand position. In the second study, the application of a transient force change with a dwell time between the force application (that moved the hand away from the body) and removal led to violations of equifinality; the hand stopped short of the initial position after the perturbation force removal. The magnitude of the undershoot increased linearly with the peak hand displacement and exponentially with the duration of perturbation with the time constant of about 1 s. In contrast, when the hand was perturbed toward the body, no violation of equifinality was observed. The third study explored the change of apparent stiffness of the arm during dwell time. The apparent stiffness increased in the direction of force action over 1-2 s (with a transient peak at about 0.4 s), without changes orthogonal to the force direction. The fourth study explored the unintentional movements produced by multiple transient changes in the external force. With repetitive perturbations, the hand moved further away from the body with violations of equifinality after each perturbation. The last study investigated the structure of joint configuration variance for both unintentional and intentional movements to about the same hand locations. Most variance at the final steady states was compatible with unchanged values of both hand position and orientation following both intentional and unintentional movements. The main result of these studies is the demonstration of violations of equifinality (unintentional movements), which happen when a transient perturbation is interrupted by a dwell time. These results can be explained within a recent development of the EP-hypothesis as reflections an unintentional drift of the referent body configuration towards its actual configuration. We term this hypothetical mechanism RC-back-coupling. The results emphasize the importance of task-specific stability of movements and incorporate the notions of intentional and unintentional movements into a single motor control scheme.