Direct electrical stimulation with paired pulses at varied intervals was used to study the propagation velocity and action potential amplitude recovery functions (VRF and ARF) of single muscle fibers. Following a subnormal period with slowed conduction, most of the muscle fibers tested in healthy subjects showed a period of supernormal propagation velocity starting at 3 to 12 ms, with a peak between about 5 and 15 ms, a mean increase of 7%, and an approximately logarithmic decay toward 1 second. The onset of supernormality was earlier in muscle fibers from patients with muscular dystrophy and significantly delayed in those from denervated muscles. Denervated muscle fibers also had a significantly longer refractory period.