A thin filament-regulated latchbridge model of smooth muscle contraction is proposed to integrate thin filament-based inhibition of actomyosin ATPase activity with myosin phosphorylation in the regulation of smooth muscle mechanics. The model included two latchbridge cycles, one of which was identical to the four-state model as proposed by Hai and Murphy (9), while the ultra-slow crossbridge cycle has lower crossbridge cycling rates. The model fitted phorbol ester-induced slow contractions at constant myosin phosphorylation, and predicted steeper dependence of force on myosin phosphorylation in phorbol ester-stimulated smooth muscle. By shifting crossbridges between the two latchbridge cycles, the model predicts that a smooth muscle cell can either maintain force at extremely low energy cost, or changing its contractile state rapidly, if necessary. Depending on the fraction of crossbridges engaged in the ultra-slow latchbridge cycle, the model predicted biphasic kinetics of smooth muscle mechanics, and variable steady-state dependencies of force and shortening velocity on myosin phosphorylation. These results suggest that thin filament-based regulatory proteins may function as tuners of actomyosin ATPase activity, thus allowing a smooth muscle cell to have two discrete crossbridge cycles with different crossbridge cycling rates.