If you are planning to further your education after undergrad, there are many options. If you plan to go more of a motor learning/ biomechanics route, math or physics would be best. If you are more interested in physical therapy or athletic training, I’ve found students successfully double major in psychology or physiology and become well rounded clinicians. Business administration/ management or public health is a good option if your goal is to become a wellness director or work in policy change.
On the other hand, if you plan to enter the workforce after undergrad (or plan to have further your education part time while your employer pays for tuition), it may be a better use of your time to have vigorous internships and/or research opportunities than devoting time to extra classes for a double major. For example, internships will likely open the door for employment after college if you plan to have a career in cardiac rehab. Getting involved in faculty research as a data collector or transcriber will help most if your goal is clinical research.
However, I understand double majors are mandatory for some programs. If that is the case and you are are unsure of your career goals, I would suggest you stick with psychology or physiology if you’re leaning towards treating patients; public health or business if you’re leaning towards implementing wellness changes.