An abdominal strain occurs when the rectus abdominus is torn or ruptured. The abdominal muscles allow movement, assist with breathing, and provide support to the spine. They consist of the internal and external obliques, rectus abdominus, and the transversus abdominus. The transversus abdominus muscle is triangular and flat, and the rectus abdominus is long and flat. If the abdominal muscles become weak or tight, they can cause poor posture and back pain.