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Some places actually have a physical therapy undergraduate major, which would of course be perfect, but if that option is not available, you could look into biology or physiology. Either of these majors could give you a good background in how the body works. It might also be helpful to look at current postings for graduate students in physical therapy and see what they are looking for (some grad school positions will say specifically that they want someone with a degree in X, Y, or Z). Hope this helps!