The time-under-tension figures are the results of scientific research on substrate utilization curves, motor unit recruitment, and exercise protocol comparisons, to name a few, plus the practical experience of myself and other strength coaches.
Keep in mind that I have also mentioned many times before that empirical and experimental evidence have shown that hypertrophy can occur with sets of time-under-tension that are below 20 seconds. There are plenty of massive powerlifters, e.g., Roger Estep, and weightlifters like Arakelov and Rigert who have developed extraordinary hypertrophy levels using sets of 3 reps or less. That is why I advocated low-rep training in my "Maximal Weights" article. Over the years, I have built my arms up to over 19 inches in girth with sets averaging only 3 reps.
Conversely, training for hypertrophy with sets that are between 40-70 seconds long in duration will also increase maximal strength, as there is a correlation between size increases and strength increases, but not necessarily a correlation between strength and cross-section. In other words, Bob with a 14-inch arm may curl and press more than his training partner Bill, who has a 16 inch-arm. Of course, if you increase Bob's cross-section to the point where he has a 16-inch arm, you can be sure his maximal strength will further increase.
Please don't write me for references, though. I am a strength coach, not a librarian. I used to forward the references for such inquiries but it started to take too much time. Strangely enough, I normally get these questions in March and November, which is about the same time university students in exercise physiology are scrambling to do term papers and are too lazy to do their own research.
I do not mean to give the impression that my training beliefs are accepted by everybody, though. For instance, frequent Internet-poster and alleged exercise physiologist Mel Siff recently had yet another post on the Internet questioning the validity of my TUT figures. Instead of coming up with logical and practical solutions for the reader, he simply—as is customary for him—attempted to "answer" a question by asking a series of questions.
In answer to that particular posting, I recently received a communication from top Australian strength coach Ian King. King wrote:
"On the subject of answering questions with questions, I respect the power of a question. Many leading authors quote 'questions are answers.' However the practitioner needs more than questions to survive—he needs answers, or at least paradigms with which to guide their actions…
"In the interim, Mel perhaps could provide his interpretation of TUT, it's relationship with specific adaptations, and guidelines for the practitioner. For in reality what Charles has presented in his TUT guidelines are nothing more than paradigms. Paradigms that I support and use daily.
"In relation to finding the 'right answer,' I refer to Jacob Bronowski in 'Ascent of Man,' who wrote '...there is no right absolute knowledge, and those who claim it—whether they are scientists or dogmatists—open the door to tragedy. All information is imperfect. We have to treat it with humility....' On the subject of humility, perhaps Mel could use some...."