FIG. 7.

Different activation hypotheses produce the same muscle force dimensionality, but different force covariance maps. A: in the synergistic activation hypothesis, the nervous system forms groups of muscles called synergies. To achieve force targets, the target force commands are used to coordinate the synergies, rather than the muscles directly. B: in the flexible activation hypothesis, there is a target-dependent function that chooses an appropriate activation for each muscle given the target. C and D: given a particular choice for unknown parameters, directional tuning curves (average muscle force as a function of target direction) are calculated for each muscle using the synergistic (C) and flexible (D) hypotheses. The action directions (measured experimentally) are shown in the endpoint force space and the directional tuning curve in endpoint force space is shown for each muscle offset from the origin for clarity. E and F: the average force in the 7 muscles as a function of target is subjected to principal components analysis and the resulting variance fraction accounted for as a function of number of principal components used is shown for the synergistic (E) and flexible (F) hypotheses. Notice that a large percentage of the total variance in average muscle force across targets is accounted for by the first few principal components, regardless of whether there are muscle synergies. G and H: the predicted force covariance maps are shown for the synergistic (G) and flexible (H) hypothesis. Notice that under the synergistic hypothesis, muscles are coupled, so producing any target force involves the cooperation of multiple muscles, generating non-target-directed covariance for all targets in the plane. However, the flexible activation hypothesis is capable of generating target-directed ellipses when performing a target with a single muscle is appropriate and non-target-directed ellipses when muscle cooperation is appropriate.