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## Citation

Krishnamoorthy V, Latash ML, Scholz JP, Zatsiorsky VM. Muscle modes during shifts of the center of pressure by standing persons: effect of instability and additional support. Exp Brain Research. 2004 Aug;157(1):18-31. DOI

## 10 Word Summary

CoP and muscle synergies are related, but link is unexplained.

## Abstract

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 lsquomenursquo 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.

## Notes

• Synergy - task-specific group of elements used to stabilize particular performance variables.
• M-modes - basis for synergies (the control variables)
• UCM - uncontrolled manifold
• DoF with greatest variance are considered to be "uncontrolled"
• Thesis questions:
• Are similar M-modes used in conditions of instability as in stable conditions?
• Are the M-modes used in conditions of postural instability modified by a light finger touch to a stable surface or a hand grasp of stable support?
• Under the hand-grasp conditions do two groups of M-modes exist? Are muscles grouped into common M-modes across effectors?
• Is CoP shift stabilized by co-variations of M-modes under the various conditions, and are the synergies the same?
• On page 21 the integrals look like: $\int EMG_{LR,AM}=\int_{-100}^{t_0}EMG\:dt-2\int_{-500}^{-450}EMG_{bl}\:dt$
• ∫EMG indices for dorsal (ventral) muscles were divided by integrals of EMG over 100 ms in the middle of the control trial.
• M-modes
• Reciprocal
• Push-back
• Push-forward
• Co-contraction
• Ankle
• Knee
• Hip
• Appears that different M-modes were observed for different effectors.
• No consistent pattern was observed between variations in M-mode synergies and shifts in the CoP.
• Postural synergies are specific to the task since they change with changes in stability conditions during standing, and new M-modes emerge to account for changes in the postural responses.