Standardization of anal sphincter electromyography: uniformity of the muscle

The different parts of the external anal sphincter (EAS) are usually regarded as one muscle with common EMG characteristics. This assumption was addressed by comparing the number of continuously firing motor units (MUs) during relaxation, as well as the parameters of motor unit potentials (MUPs) and interference pattern (IP) in the subcutaneous and the deeper parts of EAS. MUPs and IPs were analyzed in 44 subjects (2008 MUPs and 3014 IPs) without uroneurological or proctological disorders, and the number of continuously active MUs in 34 of these subjects was recorded (221 positions). No significant difference was found in IP and most MUP parameters between the two parts of the EAS muscle, but the number of continuously firing MUs was lower in the deeper part. As far as MUP and IP characteristics are concerned, the whole EAS can be considered as one muscle, but some differences in patterns of activation of MUs may exist in different regions.