Postural control during upper body locomotor-like movements: similar synergies based on dissimilar muscle modes.

We studied the organization of leg and trunk muscles into groups (M-modes) and co-variation of M-mode involvement (M-mode synergies) during whole-body tasks associated with large variations of the moment of force about the vertical body axis. Our major questions were: (1) can muscle activation patterns during such tasks be described with a few M-modes common across tasks and subjects? (2) do these modes form the basis for synergies stabilizing M(z) time pattern? (3) will this organization differ between an explicit body-rotation task and a task associated with locomotor-like alternating arm movements? Healthy subjects stood barefoot on the force platform and performed two motor tasks while paced by the metronome at 0.7, 1.

Do synergies decrease force variability? A study of single-finger and multi-finger force production.

We tested a hypothesis that force production by multi-finger groups leads to lower indices of force variability as compared to similar single-finger tasks. Three experiments were performed with quick force production, steady-state force production under visual feedback, and steady-state force production without visual feedback. In all experiments, a range of force levels was used computed as percentages of the maximal voluntary contraction force for each involved finger combination.