Virtually all world class athletes in sports which require significant physical performance have good posture all the time, not just when they are competing in their sport. From sprints to long distance running to power lifting, the best in the world have head-up, shoulders back, chest out posture, all the time.
The reason for this is that the muscle development and flexibility which produce proper posture also produce the the most muscular strength and endurance, the best force development for throwing, hitting, kicking, etc., the most efficient movement capabilities in general, and the best cardiovascular and pulmonary functions.
Anyone who has posture which is the least bit round-shouldered, shrug-shouldered, slouchy, or forward head, will never reach their physical potential. Correct posture which is the result of correct flexibility and muscle strength balances, as developed by the PowerPosture™ program, is absolutely necessarily for any athlete who wants to be the best that she or he can be, at any age!