How do I go from Math BS/MS to Exercise Physiology PhD? I'm graduating with a BS in Math in April 2014, and I'm starting a Math MS program in September 2014. I realized (a little late) that my true passion is in Exercise Physiology. How do I get from a Math MS program to an Exercise Physiology PhD program?

The only relevant courses I've had are freshmen-level general biology, junior-level chronic disease prevention, and senior-level plant physiology.
The way to get into any PhD program is to apply to it.  If you are interested in staying at your current institution, you can seek out faculty in Exercise Physiology (assuming there are any; I must admit that I know nothing about this particular program, including how common it is) and ask them whether your coursework so far would make you a reasonable candidate for the program, and if not, how much more you need to do.

One obvious question: are you interested in using your mathematical training in a significant way in your future career in Exercise Physiology?  If so, then continuing on to get the MS is math could be an advantage: I imagine that relatively few Exercise Physiology PhD students have that much mathematical training, but like in most academic fields there is always room for people who do have that training and want to use it.  

If you are having a real change of heart, I have to ask: why begin a master's program in math if you have decided you don't want to use mathematics later in life?  Why aren't you trying to take courses in Exercise Physiology?  Also, I am a mathematician, so I can say that getting an MS in math is not a triviality: it most typically takes two years, and if your heart's not really in it, it could drag for longer than that and really not be fun.