Organization of Muscle Modes and Synergies during Voluntary Body Sway

Alessander Danna-dos-Santos
Kinesiology Dept at The Penn State University

Kajetan Slomka
Department of Human Motor Behavior, Academy of Physical Education, Katowice, Poland

Vladimir Zatsiorsky
Kinesiology Dept at The Penn State University

Mark Latash
Kinesiology Dept at The Penn State University

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     Last modified: February 27, 2007

Abstract
We studied the coordination of muscle activity during voluntary body sway performed by human subjects at five different frequencies (0.125, 0.25, 0.50, 0.75, 1.00 Hz). Subjects stood on the force platform and performed cyclic anterior-posterior shifts of the center of pressure (COP) while being paced by the metronome. EMG of ten postural muscles were recorded (soleus, gastrocnemius lateralis, tibialis anterior, biceps femoris, semitendinosus, rectus femoris, vastus lateralis, vastus medialis, lumbar erector spinae, and rectus abdominis). A major question was: Does the makeup of muscle synergies and their ability to assure reproducible sway trajectory vary with the speed of the sway? Principal component analysis was used to identify three muscle groups (M-modes) within the space of integrated indices of muscle activity obtained from the EMG signals. M-mode vectors were similar across both subjects and sway frequencies (p>0.05). The first M-mode was formed of dorsal muscles. The second M-mode was formed of ventral muscles. The third M-mode typically was formed of only a couple of muscles and varied across the subjects There were also similar relations between changes in the magnitudes of all three M-modes and COP shifts (the Jacobians) across the sway frequencies (p>0.05). Variance in the M-mode space across sway cycles was partitioned into two components, one that did not affect the average value of COP shift (?good variance?) and the other that did (?bad variance?). An index (Delta-V) was computed reflecting the relative amount of the ?good variance?; this index has been interpreted as reflecting a multi-M-mode synergy stabilizing the COP trajectory. The average value of Delta-V was similar across all sway frequencies; Delta-V showed a within-a-cycle modulation at low but not at high sway frequencies. The modulation was mostly due to variations in the ?good variance?. We conclude that muscle modes and their mapping on COP shifts are robust across a wide range of rates of COP shifts. Multi-M-mode synergies stabilize COP shifts (assure its reproducibility) within a wide range of its speeds, but only during cyclic COP changes. Taken together with earlier studies that showed weak or absent multi-M-mode synergies during fast discrete COP shifts, the results suggest a basic difference between the neural control assuring stability of steady-state processes (postural or oscillatory) and transient processes (such as discrete actions). Current results provide the most comprehensive support for the notion of multi-M-mode synergies stabilizing time profiles of important performance variables in motor tasks involving large muscle groups.