A comparison of single fiber and routine EMG in normal subjects and patients with inflammatory myopathy.

We performed electromyography on the vastus lateralis muscle in 36 normal subjects and in 18 patients with inflammatory myopathy using a specially constructed dual electrode. This permitted simultaneous recording of a motor unit from a single fiber (SF) and standard sized electrode surface. We then correlated parameters of the SF potential (jitter, blocking, fiber density and duration) and the motor unit potential (MUP) (number of phases, duration) for different sites within the same individual. The mean values for the above parameters were correlated between individuals, and the group means were compared for the control and the inflammatory myopathy group.