I wouldn't say "the only thing...", but rather, I would view this principle as one of many different ways of stimulating muscle growth. Or, increasing one's neuro-efficiency when the goal is to attain maximum power...performing 1 to 2 reps (with the maximum being 3).
Getting back to the cadence of the rep, I would say why not look at this (time under tension) in a slightly different way. From a personal point of view, I can not imagine myself slowing a rep down or performing it under some strict time period. I would much rather perform my time under tension through rep quantity instead of the constant cadence of the rep. A case in point here would be the high repetitions squat routines, where 20-25 rep sets are performed. Here my muscles are under constant tension due to the high rep range instead of the slowness of the way the reps are performed. My two cents...
Yes Fadi, (good to see you are back) I did state, in previous post, " if one was wanting to perform rep's 3 and below"
if one was wanting to spend longer under tension/load, that whatever protocol was used, it is the cabence of the rep that should remain the same, as opposed to the weight been thrown towards the end as fatigue sets in.
Spending time under load longer will have a greater effect on the system, in terms of health and strength.
Both people over time using the barbell curl as an example and all things being equal, they both want to curl and achieve their goal after a protracted period with an arditrary figure 10 rep's for 100kg, they both start out at 10 X 50kg and both reach that goal who is stronger?
The one who spent time doing a constant 2/*1/4 cadence?
Or the one that spent time doing a 2/2 rep cadence?
both options are progressive
* with the barbell curl, the one second contraction will not be possible due to the fact that there is no or minimal resistance so one would need to stop short of the full ROM to maintain tension.
One doesn't need to be anal about the time spent under load, you just need to be aware that it plays a significant role, and that training to be become stronger is a whole lot different to training to be better at a sport played.