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The case for an internal dynamics model versus equilibrium point control in human movement.
Mark R Hinder1, Theodore E Milner
1School of Kinesiology, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6.
The Journal of Physiology
|April 30, 2003
Summary
The equilibrium point hypothesis (EPH) was challenged by a study on wrist movements. Findings suggest the brain forms internal models rather than relying solely on equilibrium shifts for motor control.
Area of Science:
- Neuroscience
- Motor Control
- Biomechanics
Background:
- The equilibrium point hypothesis (EPH) proposes simple central nervous system control of limb movements via equilibrium position shifts.
- Recent studies challenge EPH, suggesting internal dynamics models are formed for specific tasks.
- EPH proponents argue findings are compatible with the hypothesis, advocating against its abandonment.
Purpose of the Study:
- To test the fundamental prediction of equifinality within the equilibrium point hypothesis.
- To investigate whether motor control adapts to velocity-dependent forces, challenging EPH.
Main Methods:
- Subjects performed goal-directed wrist flexion movements.
- A motor provided assistance proportional to instantaneous velocity during learning.
- Random decreases in assistance magnitude were introduced to test for equifinality.
Main Results:
- Equifinality was not achieved; subjects stopped short of the target when assistance decreased.
- This outcome contradicts the EPH prediction that final position is unaffected by velocity-dependent loads.
- Results align with the formation of an internal dynamics model of the task.
Conclusions:
- The study's findings contradict the equilibrium point hypothesis.
- Evidence supports the formation of internal dynamics models for motor control, especially with changing task dynamics.
- Motor learning involves adapting to and predicting external forces based on internal models.