I am not sure. I know that Jack LaLane (not sure on the spelling) opened a fitness club type gym in the late 1930s or 1940s. He had a television show that was aimed at “housewives” in the sixties. My mother used to exercise along with the show when I was a kid. And there were lots of bodybuilding gyms throughout the country in the 50s and 60s. In the late 70s (1977 I think) the ACSM had a conference in Wisconsin at one of the University of Wisconsin campuses, I can’t remember now which one. I think it was La Crosse. They were certifying “exercise technicians” back then and I was too young to participate in the program.
The NSCA accredited the first nationally accredited personal trainer certification in 1996. Although, most give Jack LaLanne and Dr. Ken Cooper credit for starting the PT movement. Credit could also be given to martial arts instructors that have personally trained individuals for centuries, as well as the Greeks for the Olympics and yoga which dates back 2500 years ago,
One more think to keep in mind is that since the beginning of human history and the creation of military units there were always those who would train the soldiers to be the best shape they could be in. The training always consisted of a combination of physical and mental factors. So in an essence these “drill sergeants” were creators of physical conditioning and fitness training.