Fig. 5.
Muscle synergies underlying isometric force generation were conserved across experimental protocols (spatial, load, and position protocols) and target force amplitudes (10–60% MLF). Data for subject S4 are shown. The 4 synergies are identified according to the mechanical action of the muscles activated within each synergy: elbow flexor (E Flex), elbow extensor (E Ext), shoulder adductor/flexor (S Add/Flex), and shoulder abductor/extensor (S Abd/Ext). Synergies extracted from spatial datasets (in the 1st column) were considered as control synergies, and their similarities with synergies extracted from the remaining datasets were assessed by calculating the scalar product (indicated by r values in each subpanel). When 4 synergies (not shown in this figure) were extracted for the position protocol, the synergies were similar to those in the spatial protocol (r values 0.999, 0.953, 0.995, and 0.988, respectively). When 5 synergies were extracted, the E Ext synergy appeared to split into 2 synergies dominated by TRIlong or TRIlat. In all cases, except for the 2nd synergy out of the 5 synergies in the position protocol, the scalar products between matched synergy pairs were higher than the level expected by chance (P < 0.05).