The mechanism of faecal incontinence following low anterior resection (LAR) has been speculative and the role of disordered neorectal dynamics difficult to quantify. Using a new methodology which quantifies rectal response to rapid and ramp inflation, in combination with anal physiology, we have evaluated 25 LAR-7 with major incontinence and 5 with minor incontinence. The three groups had comparable age, duration post surgery and anastomotic distance from the puborectalis. The resting anal canal pressure (RAP) did not related to the anastomotic distance (R2 = 0.09). With the anastomosis at and below 3 cms from the puborectalis, the rectoanal inhibitory reflex (RAIR) was a sustained drop in the mid anal canal pressure, in contrast to the normal pattern of recovery above this level. Major incontinence was characterised by a subnormal anal defence, hypersensitive neorectal dynamics and high amplitude contractile wave while minor incontinence was characterised by a hypernormal anal defence and a lesser degree of neorectal hypersensitivity. The mathematical viscoelastic rectal model, defined an increasing longitudinal smooth muscle tone and a decreasing functional collagen with increasing severity of incontinence as well as a high and low circular smooth muscle (CSM) tone with major and minor incontinence respectively. This correlated with previous in vitro studies on myenteric plexus denervation and localised damage to the inferior mesenteric plexus respectively. Based on the findings in this study, we conclude that major incontinence is secondary to neurotenesis of the inferior mesenteric ganglia and the hypogastric plexus, whereas minor incontinence represents a localised neurotenesis/neuropraxia of the inferior mesenteric plexus.