God, this question is very strange! Personally, I think the hollow solid fitness ball is because of its weight. Hollow ones are lighter, women use them better, and men use them heavier and harder. For example, push-ups, there is an auxiliary action, that is, holding the fitness ball with both hands, you can practice the inside of the chest muscles. The solid ball can assist in doing sit-ups (not pressing the feet), but holding the ball. .