Experimental Brain Research

, Volume 180, Issue 1, pp 163–179 | Cite as

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

  • J. P. Scholz
  • G. Schöner
  • W. L. Hsu
  • J. J. Jeka
  • F. Horak
  • V. Martin
Research Article


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 (CMAP) 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 CMAP 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 CMAP 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 CMAP position rather than leading to a different CMAP 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 CMAP position, indicating that joint motion was damped in directions leading to a change away from the pre-perturbation CMAP 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 (CMAP 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).


Posture Motor control Reflexes Synergy Quiet stance 



We would like to recognize Dr. Sue Park for collection of the data used in this report. Support for this research was partially provided by NIH grant NS-50880 awarded to John Scholz, NIH grants 2RO1NS35070 and 1RO1NS046065 (NSF/NIH Collaborative Research in Computational Neuroscience Program) awarded to John Jeka, and NIH grants R29DC0231201A1 and NIH grant R01AG06457 to Fay Horak.


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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • J. P. Scholz
    • 1
    • 2
  • G. Schöner
    • 3
  • W. L. Hsu
    • 2
  • J. J. Jeka
    • 4
  • F. Horak
    • 5
  • V. Martin
    • 3
  1. 1.Department of Physical TherapyUniversity of DelawareNewarkUSA
  2. 2.Biomechanics and Movement Science ProgramUniversity of DelawareNewarkUSA
  3. 3.Institute für NeuroinformatikRuhr UniversityBochumGermany
  4. 4.Department of KinesiologyUniversity of MarylandCollege ParkUSA
  5. 5.Neurological Sciences Institute of Oregon Health and Science UniversityPortlandUSA

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