Jp. Scholz et G. Schoner, The uncontrolled manifold concept: identifying control variables for a functional task, EXP BRAIN R, 126(3), 1999, pp. 289-306
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.