Smooth muscle is found extensively in the human body, including in blood vessels, airways, the gastrointestinal tract, and the urinary bladder. Although the contractile proteins of smooth muscle are very similar to those of striated muscle, smooth muscle's contractile mechanism has not been studied as extensively as those for cardiac and skeletal muscle. Previous studies developed a lumped model of muscle contraction and applied it to cardiac muscle and to skeletal muscle. In this study, this model is used to quantitatively describe the contractile properties of canine smooth muscle, using data from the literature. Results show that a single equation relating muscle force to muscle length and time, and a single set of model parameters, is able to describe smooth muscle's passive and active isometric forces, isometric twitch contractions, isotonic contractions, and an inverse force-velocity relation. The latter arises from the model without assumption of a particular force-velocity curve embodied as a contractile element. This new constitutive relation may be used to describe smooth muscle within larger physiological models, for instance to describe blood vessel constriction or urinary bladder function.