Is there an optimum number of reps per set for promoting muscle growth?
While I don’t believe in a definite number, there is certainly a RANGE of reps which promotes the anabolic adaptations we desire.
Before I continue, please see the following 2 articles for a MUCH deeper look at ‘rep range’ and ‘time under load’.
From years of personal experience, and from watching others, I formed the opinion that selecting a weight that forces you to reach positive failure between 8 and 12 reps works best for increases in size.
The National Strength and Conditioning Association  also agrees with this relationship between number of repetitions performed and physiological responses. They propose that:
(1) Bouts of anaerobic training (2 to 4 repetitions) are best for
improving muscle power,
(2) Bouts of anaerobic training (5 to 6 repetitions) are best for
building muscle strength,
(3) Bouts of anaerobic training, 8 to 12 reps per set, are best
for increasing muscle hypertrophy/growth
Note: I have had a lot of feedback from people who need clarification on how to use the rep range. It’s quite simple,
“The weight you are lifting on each and every set should be light enough to allow 8 good reps, but heavy enough to prevent you getting more than 12.”
We know quite conclusively from this review paper  that the OPTIMAL range for building muscle is ~30-60 total reps per bodypart per workout.
Both the THT Volume and THT 2.0 workouts will have you training with 5 to 6 sets (per bodypart) of 8-12 reps-to-failure, which brings you in at 40 reps at the lowest, and 72 reps at the highest. That’s pretty much perfect.
This also ties in neatly with what we know about our energy systems. As people that desire larger muscles, we must train with an intensity that literally FORCES the termination of a set before 90 seconds is up. In fact, for the most part, it should be way before 90 seconds is up.
If you are using a weight that you can contract against for over 90 seconds, your aerobic system will be generating a large portion of the energy requirements. This is not desirable. We want our ANaerobic system to be doing the work. (Go here to learn more about the body’s 3 energy systems and more on why creatine supplementation makes sense).
Most THT sets will come in at 24 seconds at least, and 70 seconds at most (70secs of a THT HIT cycle set, 10 reps at 3-1-3 cadence). Again, this is the ideal scenario for all us MuscleHackers.
So that’s how we put the ‘Hypertrophy’ in ‘Targeted Hypertrophy Training‘.
There is a time in this cycle to work in a lower rep range, which I’ll cover a little later.
ref:  National Strength and Conditioning Association http://www.nsca-lift.org/
 “The influence of frequency, intensity, volume and mode of strength training on whole muscle cross-sectional area in humans.” Wernbom M, Augustsson J, Thomeé R. Sports Med. 2007;37(3):225-64.
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