SPEED AND PRESSURE CHARACTERISTICS OF EXTERNAL ANAL-SPHINCTER CONTRACTIONS

The principle of isochronism reflects constant contraction time for va rying strengths of muscle contraction. This principle was studied for the innervation of the pelvic floor in humans using motor-evoked poten tials (MEPs) and evoked pressure curves (EPCs) from the external anal sphincter muscle (EAS). MEPs and EPCs were simultanesously recorded af ter transcranial magnetic stimulation of the motor cortex. Voluntary c ontractions were also studied. Contraction times of the EAS were signi ficantly longer in voluntary contractions (mean, 237 ms) than in EPCs (mean, 90 ms). Depending on either mode of contraction, contraction ti mes varied only slightly despite a wide range of contraction strengths . It is shown that the contractile behavior of the EAS is a function o f slow- and fast-twitch muscle fiber distribution and that the princip le of isochronism governs motor performance not only of limb muscles b ut also of the EAS. There exists a unique optimal working range of eac h muscle to meet its individual function. Disturbance of this principl e results in a less efficient contraction with either inappropriate ba sic tone or disturbed reflex activation in the EAS. Both are possible causes of incontinence.