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Something many people don't realize is that deep proper breathing in itself is a basic primary abdominal/core exercise. When we inhale our diaphragm stretches down towards the abdominal cavity, the pelvic floor muscles relax a bit, and the abdominal and back muscles stretch to allow the ribcage to expand and the lungs to fill with air (I always remind my pilates and fitness clients that their lungs are under our ribs and not in our abdomens). When we exhale the pelvic floor muscles tighten to push up, the diaphragm pulls up under the ribs, and the abdominal/back muscles contract to pull the ribs in and allow the air to be fully expelled.
If you are not breathing fully ever in your day you will suffer from tight (but not necessarily strong) abs, back muscles, and neck/shoulder muscles. You will find it more difficult to relax fully and may have problems with many exercises that involve moving your ribcage, shoulders, and thoracic spine. In severe cases it may affect your pelvic floor function and ability to hold your urine.