The uncontrolled manifold concept: identifying control variables for a functional task

The degrees of freedom problem is often posed by asking which of the many p ossible degrees of freedom does the nervous system control? By implication, other degrees of freedom are not controlled. We give an operational meanin g to "controlled" and "uncontrolled" and describe a method of analysis thro ugh which hypotheses about controlled and uncontrolled degrees of freedom c an be tested. In this conception, control refers to stabilization, so that lack of control implies reduced stability. The method was used to analyze a n experiment on the sit-to-stand transition. By testing different hypothese s about the controlled variables, we systematically approximated the struct ure of control in joint space. We found that, for the task of sit-to-stand, the position of the center of mass in the sagittal plane was controlled. T he horizontal head position and the position of the hand were controlled le ss stably, while vertical head position appears to be no more controlled th an joint motions.