Your pelvic floor muscles are the group of muscles at the base of your pelvis involved in urination, defecation, and sexual response. When they relax, they elongate towards the floor and the sphincters open. To help them relax, you can breathe in very deeply, all the way into your pelvis, so that the pressure of your breath helps them lengthen. If you do this when you go to have a bowel movement, it may enhance your emptying.
Additionally, consider elevating your feet on a stool when sitting on a toilet to defecate. Putting the body into a deep squat position helps to further elongate the pelvic floor muscles, enhancing emptying and reducing bloat.