MULTI-MUSCLE COORDINATION IN POSTURAL TASKS
Open Access
- Author:
- Dos Santos, Alessander
- Graduate Program:
- Kinesiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 05, 2008
- Committee Members:
- Mark Latash, Committee Chair/Co-Chair
Vladimir M Zatsiorsky, Committee Member
Semyon Slobounov, Committee Member
David A. Rosenbaum, Committee Member - Keywords:
- emg
posture
muscles
coordination
human - Abstract:
- In order to stand upright, humans are required to overcome a series of challenges posed by the mechanical design of our body. The vertical orientation of the head-leg-trunk segments, high center of mass, large number of joints, and narrow base of support contribute to making the body mechanically unstable. We study multi-muscle synergies involved in the control of vertical posture. For most of the studies to be presented, we define synergies as task-specific organizations of elemental variables, which stabilize particular performance variables, in a sense of reducing their variability across repetitive trials. We use a computational method of identifying and analyzing muscle synergies, based on the uncontrolled manifold (UCM) hypothesis. UCM analysis is performed in the space of hypothetical elemental variables (M-modes) during repetitive or cyclic whole body tasks. As such, the analysis involves two steps. First, M-modes have to be identified over sets of trials at similar tasks with different parameters. Second, variance in the magnitude of the M-modes has to be analyzed with respect to particular performance variables over repetitive attempts at the same task. More specifically, we studied the multi-muscle coordination during the performance of cyclic whole-body voluntary movements (sway) in the anterior-posterior (AP) direction and production of the moment of force around the vertical axis of the body (MZ). Three studies were performed assuming the existence of at least two levels of control with synergies possible at each level. At the higher level, values or time profiles of specific mechanical variables (COP or MZ trajectory) are stabilized by covariation of magnitudes of M-modes. The hierarchically lower level is the level where M-modes are formed. This lower level ensures proportional involvement of muscles within a group; in other words, it defines the direction of an eigenvector in muscle activation space corresponding to a M-mode. The main findings of the studies are: (1) The UCM method allows to quantify multi-muscle synergies; (2) Postural muscles are united into a small set of M-modes similar across subjects and task parameters for simple sway tasks; (3) The number of significant M-modes and their composition change when the tasks are performed in more challenging condition; (4) Synergies stabilizing performance variables can be built on different sets of M-modes.