Electromyography of the pelvic floor muscles may be used to assess their innervation using the technique of motor unit analysis. Prolongation of motor unit duration and an increase in motor unit amplitude reflects denervation and reinnervation of these muscles. This principle may be used to detect and investigate nerve damage in a variety of disorders which affect the musculature of the pelvic floor, including multiple system atrophy. It has also been used to investigate the effects of childbirth on the pelvic floor and to investigate the relationship between damage to the innervation of the pelvic floor and stress incontinence. Finally, urethral sphincter electromyography has detected abnormal electrical activity in some women with obstructed voiding, which may reflect abnormal relaxation of the striated urethral sphincter.