The importance of the Cl conductance in mammalian muscle is apparent from the problems that occur with muscle function when it is absent (3, 62, 445). Individuals with generalized (or Bekker’s) myotonia, where there is no muscle Cl conductance, are unable to maintain a 60% MVC, and force drops within a few seconds, associated with progressive then complete failure of the sarcolemma
AP in most fibers. This effect likely occurs because the high frequency of APs causes a rapid build-up of K in the T system, which depolarizes the fiber as a whole, and causes sarcolemma AP failure. Interestingly, the subjects are able to subsequently increase force output over the following 20–60 s to close to 60% MVC again (“warm-up” phenomenon), which appears to be due to the reduced rate of AP stimulation of the muscle occurring atthose later times