I don't think it will be profitable ... Fitness apps are not that popular. Why don't you do something more useful? For example, an application that would help in choosing the right diet. Finding the right food for an ordinary person is very difficult, so I think that such an application would become very popular. It can also be associated with fitness or bodybuilding. For example, you can add a list of specific supplements to help your body recover faster. Do not forget about calculating calories, as well as synchronizing with various fitness bracelets. Now there is a separate application for each of these points, but don't you think that if you combine all this into one large application, it would be much more convenient?