Professional swimmers need to go through a series of tests and training in all aspects. Flexibility, endurance, explosiveness. Training is also extremely strict, generally including land training and underwater training, and the training is targeted.

Systematic training and auxiliary training, as well as the supplement and adjustment of diet and nutrition. Eat more high-protein food. Don't think that swimming doesn't need fat. Fat has buoyancy in water, which can keep the balance of the body in water. Maintaining proper fat or useful physical strength and endurance can be improved by swimming for a long distance. Running more is also a way. You can play dumbbells with your arms.