The balance between tension and expansion forces on edge retraction at constant velocity. When the tension force decreases by forceΔF proportional to edge travel distance x, the elastic energy U of the membrane changes by ΔU (<0) in a linear approximation. At that time, the expansion force that maintains the constant velocity motion of the edge also decreases. The expansion force is proportional to the number of actin filaments since these filaments bind the substrate via clutch molecules. The downstream signals of Rho GTPases change the chemical potential W (amount of ATP) accumulated in actin filaments by ΔW (<0). Let wavg be the average value of GTPase activity and let wout be the energy released by actin depolymerization, equal to wavg. The imbalance wout > wavg + ΔG (ΔG <0) results in a reduced number of actin filaments.
