Good fitness hashtags should be not too generic, otherwise they will get lost in the noise. On the other hand, using very specific tags may cause an adverse effect as your post will not be shown to a vast audience and thus gain few likes.
A wise strategy is to put both popular and niche Insta fitness hashtags. The former will push your post to the top while the latter will make them more searchable. The best fitness hashtags for getting more followers are those that are relevant to your content, contextual and easy to remember.