Hello! It's great that you are thinking ahead about college! I'm glad to pass along some advice for you.
It is a practical idea to combine your interests in sports medicine and biomedical engineering. Biomedical engineers can certainly specialize in applying their skills to sports medicine - analyzing an athlete's motions to help them perform better, designing sports and training equipment, etc. As a matter of fact, you will probably learn how to do fundamental motion analysis as an undergraduate student in a biomechanics laboratory, and you could probably work on sports equipment or rehabilitation aids as a design project in an undergraduate biomedical engineering program.
Getting a B.S. degree in biomedical engineering is a great start toward your goal. At that point, you could go and work for a company that develops and manufactures sports equipment or sports medicine equipment, or you could continue on for advanced education. I would bet that by the time you are a college senior, you'll have a more refined idea of exactly what kind of career you'd like to have (what sports you'd like to specialize in; whether you want to help people generally achieve higher performance levels or instead help them with rehab after an injury; etc.), and that will guide you toward what kind of advanced learning to pursue. It might be a Master's degree or a Ph.D. with a focus on biomechanics, or ergonomics, or kinesiology. I've known students who earned Bachelor's degrees in biomedical engineering and then went on to study in these areas.
For your planned career, sports medicine is a great start. Advanced coursework in math is always helpful in setting the foundations for any engineering major. The more biology you understand, the better. And finally, engineering students who can write clearly and professionally have a real advantage (in college, when looking for jobs, and in their careers!) over students who have focused on math and science and neglected to work on their communication skills. So I always encourage aspiring engineers to take classes that involve a lot of writing!
Best of luck! Thanks for asking.