Simplified scheme of the two-state cross-bridge model proposed by Brenner. Force-generating and non–force-generating states enclose all possible sub-states that are eligible to be populated by strong actin-interacting cross-bridges and by weak or non–actin-interacting cross-bridges, respectively. An equilibrium is established between the sub-states corresponding to each of the two groups of states. f_app represents the apparent rate constant of cross-bridges to enter force-generating states, which is modulated by the fast, dynamic, Ca²⁺-dependent equilibrium between turned-on and turned-off forms of the regulated actin units (therefore, f_app depends on Ca²⁺ concentration; Brenner and Chalovich, 1999; Stehle and Iorga, 2010). g_app represents the apparent rate constant of cross-bridges to leave force-generating states, which is independent of [Ca²⁺] (Brenner, 1988; Stehle and Iorga, 2010). If P_i concentration is negligible and f⁻_app can therefore be ignored, the rate constant of force redevelopment is k_tr = f_app + g_app, and the rate constant of the first, isometric MF relaxation phase is slow k_rel = g_app. Active isometric force (or tension) is P = n ⋅ F₀ ⋅ f_app / (f_app + g_app), where n is the maximum number of cross-bridges that are able to participate in turnover in a half-sarcomere at a given temperature, F₀ is the mean force produced by a single actin–myosin cross-bridge in the force-generating states, and f_app / (f_app + g_app) is the steady-state fraction of turning over cross-bridges in the force-generating states (Brenner, 1988).