Who do you trust? It is a question that all of us must answer to some  degree in our lives on a daily basis. While many people will say their  parents, siblings, teachers or some other authority figure; many people  will also tell you it is their medical doctor. People’s behaviors are  heavily swayed by others who they see as an authority in the area that  they are seeking advice. In the case of exercise, research is showing  that medical doctors are playing an increasingly important role in  exercise behavior patterns of patients.
http://www.fitness-health-wellness.com/lifestyle-management/behavioral-strategies/articles/fit-doctors-more-likely-recommend-exercise