Pattern of electrical activity and force in normal and pathological muscle: S-index of turns and amplitude

Cenkovich et al. (1982) found that a rotated hyperbolic curve fitted the relationship between the logarithm of cumulative amplitude (log A) and the logarithm of turns (log T). They found the intercept of the hyperbola (S-index) to be independent of force and suggested to use the S-index without measurement of the force as a diagnostic criterion. We have applied this modification of the method to our material previously published. In controls we found a slight increase in S-index with increasing force. The S-index was often increased in patients with myopathy and decreased in patients with neurogenic disorders, mostly at low to moderate force. The S-index was not better in discriminating patients from controls than the ratio of turns to mean amplitude.