Just a grain of salt; as a paramedic, when i see abdominal breathing, it's noted as accessory muscle respirations (same wording as abdominal really) and as a bad thing that contributes to SOB (shortness of breath). Although, these people's accessory muscle respirations are involuntary, as opposed to what you're proposing. They are involuntary bc they're helping the body breath in a SOB situation. I am also an long term amateur athlete and in addition to paramedic work, i have never heard anything about intentional accessory muscle breathing to help. It is an interesting proposition though. Other coping methods to SOB are sitting with hands on knees (tripod position), another is pursed lip breathing. The last one we typically do naturally, and helps maintian air pressure in the lungs to keep them expanded. There are other methods im not thinking as well. As a footnote, I am not recommending to intentionally do the above methods, but it's interesting and worth looking up peer reviewed articles about!