Confusion about reps vs time under tension: aren't these two things contradictory?

Question

So I was reading about hypertrophy, and heard that the optimal "time under tension" is about 45-60 seconds.

Elsewhere, I read that the optimal rep range for hypertrophy is 8-12 sets.

These two things sound contradictory to me. If I had to do 12 reps of, say, bicep curls, it would take me roughly 1-2 seconds per rep. That means that 12 reps would be 24 seconds. How does that correspond to 45-60 seconds of time under tension?

Rather, if I take 2 seconds per curl, I should be doing 30 reps in order to reach a time under tension of 60 seconds... and yet, I'm hearing nobody recommend 30 reps, they all usually recommend 12 at most.

What's going on here? Are my curls just too fast? Surely not, I think 2 seconds is a decent amount of time for a curl.

Answer 1

The claim that there's an optimal time under tension for hypertrophy appears to have come from Charles Poliquin, who wrote in his 1997 book ("The Poliquin Principles") that 20-70 seconds of total time under tension per set is desirable for bodybuilders. No citation was given for this claim.

The idea that 8-12 reps is the optimal rep range for hypertrophy is an even older dogma that probably can't be attributed to any single author.

Both of these ideas are wrong.

Research has reliably shown than rep range does not matter for hypertrophy, as long as sets are done sufficiently close to failure and the reps are not so ridiculously high (more than 30 reps per set) that it starts turning into cardio^{1, 2}. Sufficiently close to failure means that if you can lift 100kg for 15 reps, or 140kg for 5 reps, those two will have similar effects on hypertrophy, but lifting 100kg for 5 reps cannot be compared to lifting 140kg for 5 reps. Stronger By Science has a good write-up summarising this research.

Research has also shown that while time under tension does increase hypertrophy³, this is counteracted by the fact that you can lift less weight (or do fewer reps) with a deliberately slowed tempo than you could when lifting at a normal speed. The overall result is that lifting light weights for low reps with a slow tempo, lifting light weights for high reps with a fast tempo, and lifting heavy weights for low reps with a fast tempo all produce similar levels of hypertrophy⁴, again provided that you're training similarly close to failure in all cases.

Probably the only important thing to take away from this is that slow tempos or higher reps can act as a substitute for heavier weights for hypertrophy training, which may be useful to people who have limited access to equipment due to Covid-19 restrictions.

Answer 2

In my opinion, two seconds is fast for a bicep curl.

We can split the bicep curl into four parts: concentric, top-hold, eccentric, and bottom-hold. When you look at timing for a bicep curl you can find these four numbers prescribed in seconds. For example, if I said to perform a 1-0-1-0 bicep curl, that's one second up, no pause at the top, one second down, no pause at the bottom, and the completes one rep. This is what it sounds like you're doing, about two seconds for one rep.

I think a solid bicep curl looks more like a 2-1-2-1, in other words, two seconds up, one-second pause at the top, two seconds down, and a one-second pause at the bottom. The totals six seconds per rep putting a 10 rep set at 60 s time-under-tension.