Figure 3.

Schematic diagram of the cross-bridge model. Following Hai and Murphy (1988) the myosin is assumed to exist in four forms; unphosphorylated and unattached (M), phosphorylated and unattached (Mp), phosphorylated and attached (AMp), attached and unphosphorylated (AM). Cycling between the Mp and AMp states generates active force, while the AM state can maintain tension but cannot generate force (and is thus called the latch-state). Rates of phosphorylation are assumed to be sigmoidal functions of Ca²⁺ concentration (c). The rates of attachment and detachment are functions of the distance (x) to the actin-binding site, and thus the model is a system of partial differential equations of conservation type.