Motor equivalent control of the center of mass in response to support surface perturbations

Abstract

To claim that the center of mass (CM) of the body is a controlled variable of the postural system is difficult to verify experimentally. In this report, a new variant of the method of the uncontrolled manifold (UCM) hypothesis was used to evaluate CM control in response to an abrupt surface perturbation during stance. Subjects stood upright on a support surface that was displaced in the posterior direction. Support surface translations between 0.03 and 0.12 m, each lasting for 275 ms, were presented randomly. The UCM corresponding to all possible combinations of joints that are equivalent with respect to producing the average pre-perturbation anterior-posterior position of the center of mass (CM(AP)) were linearly estimated for each trial. At each point in time thereafter, the difference between the current joint configuration and the average pre-perturbation joint configuration was computed. This joint difference vector was then projected onto the pre-perturbation UCM as a measure of motor equivalence, and onto its complementary subspace, which represents joint combinations that lead to a different CM(AP) position. A similar analysis was performed related to control of the trunk's spatial orientation. The extent to which the joint velocity vector acted to stabilize the CM(AP) position was also examined. Excursions of the hip and ankle joints both increased linearly with perturbation magnitude. The configuration of joints at each instance during the perturbation differed from the mean configuration prior to the perturbation, as evidenced by the joint difference vector. Most of this joint difference vector was consistent, however, with the average pre-perturbation CM(AP) position rather than leading to a different CM(AP )position. This was not the case, however, when performing this analysis with respect to the UCM corresponding to the control of the pre-perturbation trunk orientation. The projection of the instantaneous joint velocity vector also was found to lie primarily in the UCM corresponding to the pre-perturbation CM(AP) position, indicating that joint motion was damped in directions leading to a change away from the pre-perturbation CM(AP) position. These results provide quantitative support for the argument that the CM position is a planned variable of the postural system and that its control is achieved through selective, motor equivalent changes in the joint configuration in response to support surface perturbations. The results suggest that the nervous system accomplishes postural control by a control strategy that considers all DOFs. This strategy presumably resists combinations of DOFs that affect the stability of important task-relevant variables (CM(AP) position) while, to a large extent, freeing from control combinations of those DOFs that have no effect on the task-relevant variables (Schöner in Ecol Psychol 8:291-314, 1995).

Control and Estimation of Posture During Quiet Stance Depends on Multijoint Coordination

“…Results of a recent reanalysis of support surface perturbations (Park et al 2004) also is consistent with this finding. Differences in the configuration of the joints between pre-and postperturbation phases were largely consistent with a motor equivalent response; i.e., different joint combinations between the phases achieved an equivalent CM position (Scholz et al 2007).…”

“…Finally, two recent studies support the hypothesis that many joints along the kinematic chain are coordinated to achieve postural stability (Krishnamoorthy et al 2005;Scholz et al 2007). Krishnamoorthy et al (2005) studied joint coordination in subjects who stood as quietly as possible on a narrow beam both with and without vision.…”

“…Stabilization of the CM may be the indirect result of the control of other variables, for example, the position of the joints relative to their baseline position. Only a few studies have employed experimental methods that test this assumption directly (Krishnamoorthy et al 2005;Scholz et al 2007). That the CM may be a controlled variable is consistent, however, with recent evidence that neurophysiological mechanisms exist that encode higher level task variables (Bosco et al 2003;Poppele et al 2002).…”

“…The reflective marker coordinates at each data sample were used to calculate joint angles at the ankle, knee, hip, lumbo-sacral junction, cervical spine (C 7 -T 1 junction), and atlanto-occipital joint in the sagittal plane. The atlanto-occipital joint was included based on previous work that revealed substantial motion of this joint independent of that of the C 7 -T 1 articulation (Park et al 2004;Scholz et al 2007). The angle between adjacent limb segments was calculated using a link-segment model.…”

The coordination of joint movements during sit-to-stand motion in old adults: the uncontrolled manifold analysis

“…Scholz et al 6 ) used UCM analysis to reveal that although coordination of each segment angle stabilized the COM in the sagittal plane during STS, the head and hand postions were not stabilized. Scholz et al 7 ) quantitatively showed that COM is a controlled variable of the postural system, and that its control is achieved through selective, motor equivalent changes in the joint configuration in response to support surface perturbations. The UCM analysis also revealed that the central nervous system (CNS) organizes co-variation of elemental variables to stabilize the important performance variable in a task-specific manner 8) .…”