Mechanical manifestations of activation in cardiac muscle.

W hen a muscle is stimulated electrically, its ability to contract rises and then falls. It is conceptually easy to imagine, therefore, that something called the "active state" might rise and fall with a well-defined time course. Although the concept is simple, the attempts to measure this time course have produced such conflicting results that the whole concept has fallen into disrepute.1 These conflicting results generally are attributed to the fact that the perturbations used to make the measurements frequently altered the active state. Another good reason is that many different parame- ters were being measured as a reflection of the active state, and some of them may have been inappropri- ate. As a result, the terms "active state" and "activation" became imprecise and vague, even though they have remained in general use and embody a potentially useful concept.