The premise is incorrect. The internal anal sphincter is in a state of chronic contraction and is not under voluntary control. When defecation occurs the sphincter relaxes allowing the passage of the rectal contents. Even when there is a colostomy there is still mucus produced which needs to be expelled on a regular basis. There is no disuse occurring.
The answer is hypertrophy, but if the injury is disabling, then it will lead to muscle atrophy due to disuse. Note that body builders lift weight until causing minor injury.