Most of the Medline articles I found on ALS were submitted by neurologists, or are in neurological journals. There are also some studies conducted by kinesiology departments, so it’s possible that a PhD in kinesiology would be sufficient to research ALS.
Kinesiologists and neurologists would conduct research into different aspects of ALS. You should probably talk with a few members of both fields to get an idea of what their research is like, and choose the field that interests you more. They should be able to help you decide if doing research would be worthwhile too.