I've had IBS for more than 12 years now. My symptoms basically present as a very urgent need to the bathroom one or more times a day (usually more). I've always been able to make it to a bathroom somewhere when this has happened, so I've never had any kind of accident. I don't know if you would call extreme urgency 'involuntary defecation', but I could imagine a scenario in which it could become that if I didn't make it to the bathroom in time. I know that the degree of severity of IBS can range from a very mild annoyance to a life-ruining 'being on the toilet all the time' scenario. If you have this problem, you should start by talking to your doctor about it. He may prescribe you with an antispasmodic to help control your movements, or some other medication. You may also speak to a nutritionist and look in to changing your diet.
ibs and spastic bowl do what they wan when they want
im on to many med my system all messed
its diet is very important
some experimenting this food did this and so on till you have your avoid list
good luck if you need more ask or if embarrassing pm me