What's the best rep range for maximizing muscle size? How about for increasing strength and power?
If you answered 8-15 reps for the former and 1-6 reps for the latter, most trainers would say you're on the right track. I would, too, with one caveat: that your time under tension (TUT for short) also falls in the appropriate range for your goal, either size or strength.
What Is Time Under Tension?
TUT is just another way to measure a set. Whereas most people define a set as how many reps are performed, TUT expresses it in terms of how long it takes to complete the set—in other words, how long your muscles were under the tension provided by the resistance you used.
For example, if you did a set of 8 reps, and each rep took you 4 seconds to complete (2 seconds on the positive, 2 seconds on the negative), your TUT for the set was 32 seconds. A 15-rep set with the same rep speed would have a TUT of 60 seconds, and so on.
So, which definition of a set is better, rep count or TUT? Counting reps is obviously more common, not to mention more convenient, as keeping a tally in your head during a set is easier than keeping an eye on a stopwatch. So, for sheer practicality, I say feel free to keep counting reps.
I also highly recommend monitoring your rep speed, which will determine your TUT for a given set. In my opinion, the best way to ensure you're maximizing your goal, be it size or strength, is to be in the sweet spot that hits both ideal reps and TUT.
The sweet spots for TUT appear to be 4-20 seconds for strength and power and 40-60 seconds for hypertrophy, aka muscle growth.
Monitoring TUT While Counting Reps
Now ask yourself, do your strength-building sets of 1-6 reps fall in the 4-20-second range? Chances are they do, as it only takes 1-2 reps to make a set last 4 seconds, and most superheavy sets are impossible to continue for longer than 20 seconds.
Where hypertrophy sets are concerned, it may be a different story. You may think you're training for size, but if you fly through your 8-rep sets in only 20-30 seconds (1-2 seconds up, 1-2 seconds down on every rep), your TUT could be falling well short of the muscle-building sweet spot.
Jim's Take-Home Advice on TUT
If you want to maximize muscle size, make sure your sets are falling in the 40-60-second TUT sweet spot, not just the 8-15-rep range. And if you're partial to the low end of that range (say, sets of 8-10), keep your reps fairly slow and deliberate. If it only takes you 20 seconds to do 10 reps of dumbbell presses, you're probably going too fast to pack on ample muscle mass.
Also, keep in mind that a set can fall outside of the 8-15-rep range but still be good for promoting hypertrophy—for example, a set of 5 super-slow reps, where rep speed is 5 seconds up, 5 seconds down (10 seconds per rep, 50 seconds TUT for the set). Likewise, a 20-rep set with faster reps (say, 3 seconds per rep for a TUT of 60 seconds) is perfectly acceptable, provided you control the negative on every rep.
As for strength, if you're putting your true 1-6-rep max on the bar, your rep speed will more or less take care of itself and probably fall in the range of 4-20 seconds. Make sure you're lowering the weight on the descent under control and pressing (or pulling) it with maximal force. Other than that, I don't advise slowing down or speeding up your reps.
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