Figure 2.

Phenomenological length dependence of sarcomere mechanics, as implemented in the original Land model. (a) The overall force magnitude is multiplied, ad hoc, by the phenomenological function $h\left(\lambda ;{\beta }_0\right)$, where $\lambda =\text{SL}/{\text{SL}}_0$ is the extension factor (${\text{SL}}_0$ is the resting sarcomere length) (39). In the range $0.87<\lambda <1.2$, $h\left(\lambda \right)$ varies linearly with gradient ${\beta }_0$. (b) The calcium sensitivity ${\left[{\text{Ca}}^{2+}\right]}_{50}$ (Eq. 4) is also assumed to vary linearly with a gradient ${\beta }_1$, for $\lambda <1.2$. (c) The effect of ${\beta }_0$ and ${\beta }_1$ manifests itself in the characteristic features of the F-pCa relationship (i.e., the maximum force and the pCa of the half-maximum point). This work seeks to reproduce these effects by means of the OFF-state dynamics instead of relying on ${\beta }_0$ and ${\beta }_1$. To see this figure in color, go online.