Abstract
Anatol Feldman introduced his equilibrium point theory of movement control in the mid-1960's. The theory itself has evolved in a way which is as dynamic as the neurobiomechanical states it postulates. It continuously presents challenges to the scientific community to devise focused, quantitative experimental approaches to probing the nature of posture and movement. In this paper, we analyze several forms of equilibrium point theories and the evidence that has been used to evaluate them, in terms of whether the questions are framed precisely and whether the methods are appropriate for answering the questions. Specifically, we compare studies with mechanical, contacting perturbations against studies with non-contacting, inertial perturbations, and we address the relevance of studies with deafferented animals and humans, the relevance of studies in intact humans with the "do not intervene" instruction, and factors which must be considered in predicting movement dynamics from measurements made in isometric conditions. In addition, we point out that critical studies of the dynamics of reaching behavior in experimental animals may be confusing adjustments of grip force in controlling a manipulandum with adaptive recalibration of arm movement dynamics. We conclude that the exact contribution of equilibrium point control to movement regulation has yet to be firmly established. Nevertheless, the contribution of equilibrium point theories to progress in the field is paramount. It is both a pleasure and privilege to participate in honoring Anatol Feldman on the 40th anniversary of his introduction of the equilibrium point theory of motor control. Over the years, he has refined and extended his original views in a long series of creative and comprehensive experiments. His work has also inspired many other motor control scientists leading to innumerable experimental studies and observations that have greatly expanded our knowledge of motor function.