ABSTRACT
General background Important prerequisites for continence are intactness of the vesicourethral supportive structures and of the smooth and the striated muscles of the urethra, the latter being composed of the intramural striated sphincter and the paraurethral components of the pelvic floor muscles. Most striated muscles of the body are composed of three motor unit types, one with slowly contracting muscle fibers and two with fast contraction properties.1 The intramural urethral sphincter is special in being composed of slow fibers only, whereas the paraurethral striated muscles have varying numbers of all three types. The three motor unit types differ with respect to their maximal force development, fusion frequency – that is the activation frequency for a smooth sustained contraction – and resistance to fatigue. The slow units develop little force but are resistant to fatigue. Their fusion frequency is about 10 Hz. The fastest units can produce 10-20 times more contraction force but fatigue rapidly. Their fusion frequency is around 40-50 Hz. The intermediate fast units are somewhat weaker but considerably more fatigue-resistant. It follows that the intramural striated sphincter can generate a well-sustained but rather limited increase in urethral pressure. The main function of this muscle seems to be to accomplish urethral closure during bladder filling at rest, when there is little physical stress. In more provocative situations, when the intra-abdominal pressure suddenly increases, e.g. lifting, coughing, and running (when most women with stress urinary incontinence leak), the fast motor units of the paraurethral pelvic floor muscles provide a rapidly induced, strong closing force upon the urethra. This contraction is in fact governed by the central motor program during selfgenerated increases of the intra-abdominal pressure, thereby allowing these muscles to contract in advance of the pressure rise. They are also promptly reflexly engaged by pressure increases from the outside caused by a sudden push towards the abdominal wall, but in this situation the contraction lags behind the pressure increase. The pressures generated by the pelvic floor muscles upon the urethra clearly exceed the maximal detrusor or
intra-abdominal pressures in intact subjects. Thus, there is normally a reliable safety margin.
