Muscle modes during shifts of the center of pressure by standing persons: effect of instability and additional support.
Muscle synergies in postural tasks have recently been studied using the framework of the uncontrolled manifold (UCM) hypothesis. A set of three hypothetical control variables, named M-modes, derived from the activity of 11 postural muscles, were identified. It was shown that postural synergies composed of these three M-modes preserve a certain shift of the center of pressure (COP) when subjects perform postural tasks while standing on a stable surface. In the present study we investigated the effects of support surface instability and availability of a light touch or grasp of a stable external support on the M-modes and their co-variation. The study was performed in two sessions. In the first session subjects released a load behind the body under four conditions: standing on a stable surface with no support (ST), standing on an unstable surface with no support (UN), standing on an unstable surface with a light touch (UN,T) and standing on an unstable surface with grasp of a stable object (UN,G). In the second session subjects performed two tasks: an arm movement backward and voluntary sway forward (towards the toes) under three conditions--ST, UN and UN,T. Principal component analysis was used to identify M-modes from data in the first session, and a UCM analysis was performed to study M-mode synergies in postural stabilization from data in the second session. A 'menu' of five M-modes was found, which were named either reciprocal M-modes or co-contraction M-modes based on the agonist-antagonist relationship of muscles comprising each mode. For a given task, subjects chose any three of these five M-modes in a subject- and task-specific manner. The reciprocal and co-contraction M-modes occurred equally frequently whether subjects stood on a stable or unstable support surface or whether a light touch was available or not. However, the co-contraction M-modes predominated when grasp of an object was available. In this condition, when the arm could be used for stabilization, there were M-modes uniting hip and shoulder muscles. However, the identified M-mode synergies were not found to lead to a consistent shift in the COP in any of the stability conditions. Possible reasons for this finding are discussed.
Deptartment of Kinesiology, Rec. Hall 267L, The Pennsylvania State University, University Park, PA 16802, USA.
Vijaya
V
Krishnamoorthy
Mark L
ML
Latash
John P
JP
Scholz
Vladimir M
VM
Zatsiorsky
D000328:Adult; D004576:Electromyography / Q000706:statistics & numerical data; D005246:Feedback / Q000502:physiology; D005260:Female; D005385:Fingers / Q000294:innervation / Q000502:physiology; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000294:innervation / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation* / Q000502:physiology*; D004856:Postural Balance / Q000502:physiology*; D011312:Pressure; D012018:Reflex / Q000502:physiology; D014110:Touch / Q000502:physiology*; D016474:Weight-Bearing / Q000502:physiology