Comparative study of the motor unit potentials of the external urethral sphincter, anal sphincter, and bulbocavernosus muscle in normal men

Motor unit potentials (MUP) of the perineal muscles and in particular of the external anal sphincter (EA sph.), of the external urethral sphincter (EU sph.) and of the bulbocavernosus muscle (BC) have been seldom analyzed. There is no uniformity of view about amplitude and duration of MUP of these muscles. According to different authors, duration varies from 2 to 10 msec, and amplitude varies from 50 μ V to 3 mV. In this paper, we studied EMG findings of EU sph., EA sph., and BC in 37 normal men (15 BC, 12 EU sph., and 10 EA sph.). In total we recorded 239 different MUP in BC, 208 in EU sph., and 134 in EA sph. There are no differences among the three groups in amplitude (mean values about 250 μ V). On the contrary, EU sph. showed a significantly shorter duration (3.73 msec) compared to BC (5.58 msec) and EA sph. (5.07 msec). Polyphasic potentials range from 2.3% to 5.9%. Gosling et al (Br J Urol 53:35–41, 1981) has shown that the striated muscle of EU sph. is made of fibers smaller than those of adjacent periurethral muscles. Possibly, motor units are smaller.