The case for an internal dynamics model versus equilibrium point control in human movement
Hinder, MR and Milner, TE, The case for an internal dynamics model versus equilibrium point control in human movement, Journal of Physiology, 549, (3) pp. 953-963. ISSN 0022-3751 (2003) [Refereed Article]
The equilibrium point hypothesis (EPH) was conceived as a means whereby the central nervous
system could control limb movements by a relatively simple shift in equilibrium position without
the need to explicitly compensate for task dynamics. Many recent studies have questioned this view
with results that suggest the formation of an internal dynamics model of the specific task. However,
supporters of the EPH have argued that these results are not incompatible with the EPH and that
there is no reason to abandon it. In this study, we have tested one of the fundamental predictions of
the EPH, namely, equifinality. Subjects learned to perform goal-directed wrist flexion movements
while a motor provided assistance in proportion to the instantaneous velocity. It was found that the
subjects stopped short of the target on the trials where the magnitude of the assistance was randomly
decreased, compared to the preceding control trials (P = 0.003), i.e. equifinality was not achieved.
This is contrary to the EPH, which predicts that final position should not be affected by external
loads that depend purely on velocity. However, such effects are entirely consistent with predictions
based on the formation of an internal dynamics model.