Good day! Many depends on what exactly application do you want to create, it future functionality, design, mobile OS and other. As you know there are hundreds of different fitness apps now, so if you want to win competition you should implement some new idea. Also such app must meet all modern design trends and requirements. For this purpose you will definitely need professional help from specialists with good experience in this niche. For example check services from
https://mangosoft.tech/healthcare/ they have many projects in healthcare! Hope this will help you!