Is it true that having an athletic training certificate while pursuing a doctorate in physical therapy significantly increases my likelihood at being accepted into graduate school and, later, being hired for a job?

Colleen Hughes is right, simply being a certified athletic trainer does not guarantee acceptance into a physical therapy program. While all graduate programs are seeking candidates with diverse experiences, they definitely look for demonstrated potential for academic success (grades), and a commitment to the profession/ field (essay) you are applying. However, consider that in order to be eligible to take the athletic training board of certification exam requires successful completion of an approved entry-level athletic training program. This coursework, requires students have diverse healthcare related experiences. Similarly, passing the certification exam will provide you an opportunity to gain additional experience as a healthcare professional. In the end, it is your successful completion of these diverse healthcare experiences that add to the richness of your application. So, becoming an ATC may not guarantee acceptance, it should not hurt your chances.

Absolutely not! Your undergraduate degree is going to strengthen the type of therapist you become. Grades tend to be the trend for acceptance to most programs- but honestly, I think they look for diversity. My class had a film major and a psychology major. Most schools don't want all of the same type of student. Use your undergraduate as an opportunity to find what you are interested in and excel at it!