A bladder contractility constant

Muscle contractility can be characterized by two related properties: force and velocity. The initial velocity of a tetanic contraction is inversely related to preload. This was demonstrated experimentally by Hill and quantified in his well-known empiric equation. Subsequent investigators argued that a theoretical maximum contractile element velocity (V max) could be predicted from the rate of change of isometric force. V max has been applied clinically in heart studies, prompting others to use similar methods to evaluate bladder contractility. These attempts have so far been unsuccessful. The present study shows for whole canine bladders that the time to reach maximum isometric force from the moment of onset of active contraction is a constant independent of muscle length, preload, and maximum force. This can be expressed as a frequency constant (omega) whose calculation appears similar to that for V max. In contrast to V max, omega is obtained only from the active component of pressure.