Electromyographic activity of the anterolateral abdominal wall muscles during rectal filling and evacuation

The role of the anterolateral abdominal wall muscles (AAWMs) at defecation has not received sufficient attention in the literature. We investigated the hypothesis that the AAWMs exhibit increased electromyographic (EMG) activity on rectal distension, which presumably assists in rectal evacuation.

The effect of rectal balloon distension on the AAWMs EMG and on anal and rectal pressure was examined in 23 healthy volunteers (37.2 +/- 9.4 SD years, 14 men, 9 women); this effect was tested before and after rectal and AAWMs anesthetization.

The rectal and anal pressures increased gradually upon incremental rectal balloon distension starting at 70 mL balloon distension until, at a mean of 113.6 +/- 5.6 mL, the balloon was expelled to the exterior. The AAWMs showed no EMG activity at rest or on rectal distension up to the time of balloon expulsion when they exhibited significant increase of EMG. This effect was abolished on individual rectal or AAWMs anesthetization but not with saline administration.

AAWMs appear to contract simultaneously with rectal contraction; this action seems to increase the intra-abdominal pressure and assist rectal evacuation. The AAWMs contraction upon rectal contraction appears to be mediated through a reflex, which we call the "recto-abdominal wall reflex". Further studies are required to investigate the role of this reflex in defecation disorders.