Here is an excerpt from a conversation I had with an online client, regarding his DR.
"I need to know the limitations I have with this condition as far as what ab exercises to do and not to do."
Answer:The exercises you cannot do encompass any in which you are unable to keep your transverse muscle (deep abdominal muscle) engaged completely- as if to your spine. This includes crunches and seated V- type exercises. It is physiologically impossible to keep that transverse engaged in a supine position when your shoulder blades leave the floor. Any exercise that involves a forceful downward motion is prohibited, as such a forceful downward pressure can enlarge an existing DR, or cause a split if there isn't one.
Now, prone positions such as planks (my favorite!), push ups etc. are perfectly fine AS LONG AS you are strong enough to keep your transverse engaged in this position. My team of trainers does NOT encourage these exercises until AFTER a DR is closed. Other exercises that will enlarge a DR are "slicing" movements such as golf and tennis, as you will want to avoid all activities that place stress on the midline, that stretch or overly expand the abdominal wall; movements where the upper body twists and the arm on that side reaches backward, such as during a tennis serve or golf swing, any exercises that cause your abdominal wall to bulge out upon exertion, such as rising from a supine position without rolling onto your side first and using the arms to help push you up.