Figure 5
Elements that, according to model1 and model2, contribute to the half-sarcomere stiffness \(k_{hs}\). In the elastic model1, the myofilament compliance (\(C_{fil}\)) is in series with stiffnes \(k_{CB}\) the number of attached in-parallel myosin heads (cross-bridges, CB). The force generated by a single cross-bridge is assumed to be a constant, with an associated constant deflection (\(\Delta L_{CB}\)). Thus, the stiffness of the ensemble of cross-bridges only (\(k_{CB}\,=\,\frac{F_{CB}}{\Delta L_{CB}}\)) scales linearly with the number of attached myosin heads. In the non-linear, visco-elastic model2, the half-sarcomere stiffness \(k_{hs}\) is likewise determined by the number of in-parallel attached myosin heads, with each head’s driving non-linear force–length relation \(F_{CB}(L_{CB})\) depicted in the top right inset, and a collective of in-series passive stiffnesses denoted myofilament stiffness (\(k_{fil} = \frac{1}{C_{fil}}\)), see Eq. (8). We determined \(k_{CB} (F_{CB})\) (Eq. 7) under the assumption that \(L_{CB}\,=\,L_{CB,opt}\) = 7 nm, i.e., \(F_{CB}=F_{CB,max}\). See Supplementary Fig. S5 for \(F_{CB}(L_{CB}\)) as determined with original model parameters. Note that, to compare model1 and model2, we excluded the visco-elastic PDE from model2 (accordingly, PDE is marked in red). \(^\mathbf{* }\) The dashed line at the asterisk marks the end of the work-stroke.
