Voluntary performance.

Will, purpose, and volition have long been viewed as either causes of behavior or of no direct consequence to behavior. In this essay, volition affects a flexible direct coupling of participant to task, modulating the degrees of freedom for kinematics in action, a point of view first introduced in theories of motor coordination. The consequence is an explanation consistent with present knowledge about involuntary and voluntary sources of control in human performance, and also the changes of the body expressed in aging and dynamical disease. Specifically, this view explains how tradeoffs between sources of overly regular versus overly random dynamics change the structure of variability in repeated measurements of voluntary performance.