It is certainly possible to rupture stitches by coughing within the first few weeks after a tummy tuck, but the majority of people who cough in this situation will not rupture sutures or have any adverse effects. Tenderness may be simply due to stress on the muscle from coughing. Some surgeons, such as myself, will place multiple sutures along the muscle repair, rather than only long "running" suture, so that even if one suture were to rupture, it should have no effect on the repair. In all likelihood, coughing will not have caused any problems to be concerned about.
