Yes it does happen but I think less often these days.  \n\nIn the extreme exertion of a marathon the body loses fluids, electrolytes (salts) and tends to overheat.  If a runner gets to a state of extreme dehydration and exhaustion the muscles that retain the 'poo' simply let go involuntarily.\n\nWith more knowlegeable runners, proper diet and hydration I think that this phenomena is now mostly behind us.