Krishnamoorthy - 2004 - Muscle modes during shifts of the center of pressure ... instability ...

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?

$\int EMG_{LR,AM}=\int_{-100}^{t_0}EMG\:dt-2\int_{-500}^{-450}EMG_{bl}\:dt$

On page 21 the integrals look like:
∫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.
Posturalsynergies are specific to the task since they change with changesin stability conditions during standing, and new M-modes emerge to account for changes in the postural responses.