I think you may be actually trying to discuss why core training has become the latest and greatest focus when training clients for fitness, rather than arguing what core training actually is, am I right?
Once upon a time a way to measure “fitness” was to count out how many sit ups and push ups a client could do, but now, have you noticed it is becoming common to test how long someone can hold a “plank” for, which is then used as a marker?
It is no longer enough to define “fitness” on how far someone can run, or how big their muscles are, but how close the person is to moving like a human being should move, how close to normal their posture is, how free of pain they are, and the condition and function of someone’s core plays a massive role in all these factors.
Trainers are now educating themselves in how the core functions, which is a smart move to make while client interest in the topic continues to grow, and the understanding the client has on how their body functions and moves improves.
More now than ever, clients are becoming aware that there are more functional ways to train the body other than by one muscle group at a time (especially on fixed plane machines) and “bodybuilding, and personal trainers are cottoning on to this and using core training as the latest tool for marketing and getting clients, while getting better than ever results from session.
Slowly but surely, the culture of the industry is shifting away from sessions that “smash and bash” the client until they are wiped out- the “go hard or go home” mentality, towards increasing energy and leaving the client invigorated and uplifted to go about the rest of their day without pain or fatigue.
In a few years, I believe that the volume of clients wanting traditional body building style sessions will be minimal, compared to how many want to train their body up on two feet in a functional and dynamic way, in line with natural human movement.
A trainer who invests time in understanding core function now, will be making the right investment for their career for the long run, which will be reflected in the results they get with clients of all fitness levels.
All the best with your research on this topic!