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To improve forced expiration, strengthening the transversus abdominis is beneficial because this muscle plays a crucial role in stabilizing the core and facilitating the expulsion of air from the lungs. The transversus abdominis is the deepest layer of abdominal muscles and acts to compress the abdominal cavity. During forced expiration, such as in activities like coughing or vigorous exercise, increased intra-abdominal pressure generated by this muscle helps push the diaphragm upwards more effectively, allowing for a more forceful expulsion of air.

While the scalenes, external intercostals, and diaphragm are important for inspiration and have roles in breathing mechanics, they don't directly contribute to the forceful expiration process to the same extent as the transversus abdominis. Strengthening the core muscles, particularly the transversus abdominis, can enhance the effectiveness of forced expiration by improving overall respiratory mechanics and support.