MUSCLE SYNERGIES DURING STANDING

Author:

Krishnamoorthy, Vijaya

Graduate Program:

Kinesiology

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

July 18, 2003

Committee Members:

Mark Latash, Committee Chair/Co-Chair
Vladimir M Zatsiorsky, Committee Member
Dr Toby Mordkoff, Committee Member
Robert L Sainburg, Committee Member
Dr John P Scholz, Committee Member

Keywords:

muscle synergies
M-mode
posture

Abstract:

The main aim of this dissertation is to define the notion of a postural muscle synergy and to develop a method of identifying and analyzing muscle synergies during different tasks in standing persons. The term muscle synergy has been widely used in the motor control literature without a clear operational definition. With the introduction of the uncontrolled manifold (UCM) hypothesis, it has become possible to offer an operational definition: synergies are task-specific groups of control variables, which stabilize particular performance variables. This approach provides a framework for the identification and analysis of synergies. The hypothesis assumes that the controller (the central nervous system, CNS) acts in the state space of control variables (CVs) and selects in this space a manifold (UCM) corresponding to a value of a performance variable, which needs to be stabilized. The CNS selectively restricts variability of CVs in directions that do not span the UCM as compared to those within the UCM. If several attempts at a task are analyzed, variance per degree of freedom within the manifold is expected to be higher than orthogonal to such a manifold. The UCM approach has been applied to kinematic and kinetic variables and has been successful in identifying important performance variables that are selectively stabilized in several tasks. This dissertation is the first attempt to extend the use of this approach to a more physiological variable, muscle activation recorded by electromyography (EMG). This dissertation has two main parts. In the first part, we develop a method of using the UCM approach to identify muscle synergies during different postural tasks in stable standing. Our next goal has been to investigate the effects of standing on a surface with decreased area of support (unstable conditions) and of additional finger touch or hand grasp on the organization of muscle synergies. In order to use the UCM approach to identify muscle synergies, the first step is to identify CVs manipulated by the controller. We have assumed that the CNS does not manipulate levels of activation of individual muscles, but unites muscles into groups within which the activity of the muscles is scaled in parallel with the help of CVs. In the first study, we identified three such CVs, which we called muscle modes or M-modes and have shown that these M-modes are very similar across subjects and several postural tasks performed on a stable support surface. In the next study, we related changes in these three M-modes to shifts of an important performance variable in postural control, the center of pressure (COP). UCM analysis revealed that the three M-modes co-varied to stabilize a particular magnitude of the COP shift. An additional finding has been that there are separate synergies for COP shifts to the front and back. In the second part of this dissertation, we focus on the effects of changes in the support surface stability and availability of additional touch or grasp on the formation of and interaction among M-modes. We first performed an experiment to investigate the effects of different types of touch on postural sway. It is known that even a mechanically inefficient finger touch is enough to stabilize posture. Results of our study show that both the availability of a stable reference point as well as modulation of contact forces play important roles in reducing postural sway. Further, it is not finger touch alone that has a stabilizing effect. Even touch to the side of the head or neck can reduce sway, possibly because of the high sensitivity of the receptors of the head and neck to shear forces. These results demonstrate the amazing versatility of the system for postural stabilization, which can use information from different sensory sources related to different mechanical interactions with external objects. Support surface instability and availability of a finger touch or grasp have an effect on anticipatory changes in the activity of postural muscles prior to self-induced perturbations. We conducted an experiment to reveal the effects of these factors on the formation and co-variation of M-modes. We first compared M-modes during a postural task under four stability conditions: stable support surface with no touch, unstable support surface with no touch, unstable support surface with light touch, and unstable support surface with grasp to an external stable object. Across all these conditions, there was a “menu” of five modes from which three were chosen in any given condition in a subject specific way. Among the five modes, two modes showed reciprocal patterns of muscle activity that spanned all three major leg/trunk joints and resulted in displacement of the center of mass either forward or backwards (“push-forward” and “push-back” modes). The remaining three modes corresponded to joint-specific co-contraction patterns. Of the four conditions studied, the co-contraction patterns predominated over the reciprocal patterns during standing in unstable conditions with a hand grasp as compared to the other three conditions. UCM analysis failed to identify M-mode synergies in the three conditions studied: stable, unstable, and unstable with touch. This was possibly because there are no pre-existing synergies for the unstable conditions as these are not tasks that are commonly performed in everyday life. In this dissertation we have shown that the UCM method can be used to identify muscle synergies using a physiological variable, that is, muscle activity measured by EMG. When subjects performed various postural tasks under stable standing conditions, we found that M-modes were robust across tasks and subjects. Further analysis, under different stability conditions revealed that the organization of M-modes and their co-variation to preserve a particular magnitude of COP shift is task and subject specific. The UCM method can be applied in the future to study abnormal synergies in postural disorders. The method can also be used to study the evolution of synergies across the lifespan as well as during the learning of novel tasks.