Speed and pressure characteristics of external anal sphincter contractions.

The principle of isochronism reflects constant contraction time for varying strengths of muscle contraction. This principle was studied for the innervation of the pelvic floor in humans using motor-evoked potentials (MEPs) and evoked pressure curves (EPCs) from the external anal sphincter muscle (EAS). MEPs and EPCs were simultaneously recorded after transcranial magnetic stimulation of the motor cortex. Voluntary contractions were also studied. Contraction times of the EAS were significantly longer in voluntary contractions (mean, 237 ms) than in EPCs (mean, 90 ms). Depending on either mode of contraction, contraction times varied only slightly despite a wide range of contraction strengths. It is shown that the contractile behavior of the EAS is a function of slow- and fast-twitch muscle fiber distribution and that the principle of isochronism governs motor performance not only of limb muscles but also of the EAS. There exists a unique optimal working range of each muscle to meet its individual function. Disturbance of this principle results in a less efficient contraction with either inappropriate basic tone or disturbed reflex activation in the EAS. Both are possible causes of incontinence.