Contractility of the urinary bladder.

Measurements have been made in vitro of the active mechanical properties of complete pig bladders, electrically stimulated to contract. The results are described with the aid of a model of the bladder wall consisting of a contractile element in series with an elastic element. For the contractile element the active force depends on the velocity of shortening. This relation is well described by a classical Hill equation, provided force is normalized by dividing it by the isometric force at the same bladder volume. The force-extension relation of the series elastic element is non-linear and can be described by an elastic modulus which depends monoexponentially on the extension. In the light of these findings the limitations of existing clinical methods of assessing bladder contractility, and the possibility of developing new methods, are discussed.