Let me clear things up before I continue.
I’m a big fan of scientific studies on fitness and a lover of science in general. I think science is vital for increasing our understanding of anything, fitness being no exception. It has helped us understand so much about both the theory and practice of all things fitness: conditioning, bodybuilding, powerlifting, calisthenics, nutrition, etc. But there’s only so much that science alone can tell us. You must pair it with your own experience to truly understand it all.
Science has limitations.
Arguably the biggest limitation is its lack of funding. This is also the root cause for most of the following issues.
For example, because of the huge number of variables that contribute to fitness, it is extremely difficult to perform a legitimately controlled experiment to study a single variable. While it is possible to measure things in a lab with a high degree of accuracy, you cannot keep the human subjects in a lab for the duration of a fitness study without a tremendous amount of resources. (Obviously you would not keep them in an actual lab, but in a controlled environment where these variables can be properly controlled and accounted for.)
Further, the longer a study is, the more expensive it becomes to maintain, and the more difficult it becomes to control.
Eight weeks and twelves weeks are typical in studies observing different hypertrophic methods. These do lead to good insights in most cases. In fact, it is through studies such as these that we have discovered that “6-35 rep sets performed close to failure” are similar in their ability to generate muscular gains. Yet often we need longer studies to be able to properly observe other variables that may be tested. For example, long-term effects of different protein levels, training volume, accumulation of fatigue, etc.
A third major barrier is the number of persons in a study.
The greater the number of subjects, the more data you have to work with. Thus, the more accurate the results will hypothetically be. However, testing a group of fifty requires far less support than controlling a test group of five hundred. The latter would obviously be much more effective in most studies, but that is simply too great a number for most studies to recruit with their financial restraints, much less handle.
The current way that we combat these limitations is by sheer number of studies. And the opinions of trusted experts dedicated to analyzing and aggregating the results of these studies. This allows us to find out exactly what we can and cannot know as a result of these studies. While we have learned a huge amount through this, it is also apparent to those keeping up with the literature that the gaps in our knowledge are expansive.
This is why it is so important that personal trainers not only be educated but experienced as well.
A trainer with a large amount of theory and education tucked under their belt will struggle to teach proper technique to someone with atypical body proportions. Despite their vast knowledge of cutting-edge fitness literature, they are not aware of how different each individual is. And also how to apply their knowledge in terms of general principles rather than absolute rules.
Conversely, an experienced trainer who received a single certification fifteen years ago may still believe that 8-12 reps is the only prime rep range for hypertrophy. And that you should only ever rest 30-60 seconds between sets for bodybuilding purposes. While their methods may continue to yield results, their techniques would be suboptimal to anyone keeping up with the science.
Good trainers must keep up with the research, but have enough experience to be able to fill in the myriad of gaps with what they have learned firsthand.
It is simply not enough to keep up with the fitness science and studies, contrary to what some keyboard warriors may have you believe. They must also practice and train to understand the limitations of the literature. And connect the dots of book knowledge with their experience into holistic pictures of practical training through their own trial and error.