I had a very interesting conversation with a student that will hopefully become a professional opera singer. He told me that they are strongly discouraged against doing sit ups and crunches as they are told this makes the diaphragm tight. He asked me what he could do and if this was true or not.
The diaphragm does have a very strong anatomical attachment into the psoas major muscle. Exercises like sit ups and crunches will indeed make this muscle tight. It does most of the work in exercises such as these. This is why I generally discourage there use in patients/athletes with any lower back pain. The psoas main role is in lumbar stabilization. When it is tight it doesn't fulfill its job to the best of its ability.
Back to singing. He showed me how when he is braced he is able to use his diaphragm better and hit the bigger notes. It was rather impressive. We went over some drills where he is bracing his core hard, but learning to take deep inhales/exhales.
The easiest position was him in an incline push up position. Abs braced. Diaphragmatic breathing.
Very interesting when a unique subculture can be interwoven into anatomy and training.