Contraction of gluteal maximus muscle on increase of intra-abdominal pressure: role in the fecal continence mechanism.

The gluteus maximus muscle (GMM) appears to contract with increased intra-abdominal pressure (IAP). The hypothesis that GMM contraction with increased IAP was investigated. The study comprised 32 healthy volunteers. IAP was measured by intravesical catheter. The response of electromyography of the GMM and external anal sphincter to sudden momentary and slow sustained straining was registered. The procedure was repeated after individual urinary bladder and GMM anesthetization. Sudden straining increased electromyographic activity of the external anal sphincter and GMM. Slow, sustained straining raised electromyographic activity of the gluteus maximus and external sphincter at differing degrees depending on straining intensity. The anesthetized gluteus maximus or urinary bladder did not respond to straining. The suggested GMM contraction on straining seems mediated through a reflex that is called "straining-gluteal reflex." This reflex appears to assist anal closure through extended and laterally rotated femur induced by gluteus contraction.