Lifting Intensity & Muscle Hypertrophy
The question is a good one: how much weight is best for muscle growth? Many bodybuilding magazines purport a middle level resistance, not incredibly heavy to a point a person can only lift a weight one or two times, but heavy enough to tire before reaching twelve and more repetitions. This belief has been long standing, but is it accurate? In this article, we will investigate the science behind the optimal weight lifted for muscle growth, as well as potential physiological explanations.
What is Intensity?
First, we have to understand the term “intensity” in the terms we want to define. In some cases, people consider intensity to be based around rest time between sets, and while that may be true on some level, we are defining intensity as resistance lifted per repetition; in short, intensity is how much weight is lifted per repetition or in a set of repetitions.
Intensity for Muscle Hypertrophy
So, are the bros right? Do you need to lift a particular weight that will land you in that infamous 8 to 12 repetition range?
In short, no – they were mostly wrong. That’s not to say the “old” technique didn’t, or doesn’t, work – it does, it just isn’t the only way to achieve muscle growth. In the last few years, there have been a couple studies that have looked at the range of intensities and their impact on muscle growth. Having equated for volume, they found that high intensity (>90%1RM) and the old moderate intensity (60-70%1RM) training styles led to the same amount of muscle growth . So, based on this, following a powerlifting (1-3 repetitions) or “bodybuilding” style (10 repetitions) training regimen does not matter, as long as the same volume is reached per individual.
However, is this also true of lower intensities?
Looking at our moderate group, but comparing it to a lighter load group performing double the repetitions (20 repetitions) – presumably, around 40%1RM, there were interesting results. Again, differences between the two groups were statistically insignificant, implying lower intensity elicits the same muscular growth as moderate intensity, if volume is equated .
So, while there are currently no studies (to my knowledge) that investigate low intensity to high intensity, using some inference, we might assume the intensity plays, certainly a critical, yet nonspecific role in muscle hypertrophy. Keep in mind, that in any of the three conditions, training should still lead to near failure fatigue – so, training at 40%1RM for 22 repetitions will not garner muscle growth if muscular fatigue does not set in, and if it does not, then it is a good indication this is no longer your 40%1RM.
The take away is, barring an equal want to gain strength, choosing any repetition scheme that suits you, and leads to adequate muscular fatigue, is acceptable for synthesizing muscle.
Understanding the Physiology
Understandably, this is all rather confusing, especially for those of us that have followed the strict lines of repetition ranges being used for various goals. While this is certainly not inaccurate for other aims, but for muscle growth it seems the lines have been shattered. What gives, though? Any explanation?
There are a few thoughts that have been presented.
The first was that between these conditions, the moderate and lower intensity sessions led to more metabolites accumulating, which have some evidence showing their impact in muscular hypertrophy beyond strict mechanical damage by leading to greater signaling . However, apparently, metabolites were about the same between conditions, so this is unlikely to be the reason.
The most plausible, although certainly unconfirmed, explanation is the same level of physical damage between conditions, which seems counterintuitive, but it may not be. One might imagine that a higher intensity group would see the highest levels of physical damage due to the increased weight lifted, even if for a shorter amount of time, but the nature of lifting between these conditions may offer the answer. In higher intensity lifting, relying on elastic rebound in movement, and trying to move a weight at the fastest velocity (power) does not allow for significant time under tension (aka, period of time in which the muscle fibers are contracting). However, a more moderate or light workload would certainly offer more TUT, so a repetition would be of higher “quality” by comparison, as the muscle would be in the lowering and rising phase of each repetition for longer (3 seconds vs 1.5 seconds, as an example).
So, while this is not confirmed in much capacity, it seems the small body of evidence we do have is pointed toward the latter explanation, and mechanistically, it makes sense.
All things considered, you can build muscle using a variety of different weight ranges, as long as the volume at the end of the day is the same and muscular fatigue is achieved. The reason for this likely lies in a balance between time under tension is lower intensity lifting compared to power output from higher intensity lifting.
Writer: Nicolas Verhoeven
 Schoenfeld, B., Ratames, N., Peterson, M., Contreras, B., Sonmez, G., & Alvar, B. (2014). EFFECTS OF DIFFERENT VOLUME-EQUATED RESISTANCE TRAINING LOADING STRATEGIES ON MUSCULAR ADAPTATIONS IN WELL-TRAINED MEN, 28(10), 2909–2918. https://doi.org/10.1519/JSC.0000000000000480
 Lopes, C. R., Aoki, M. S., Crisp, A. H., De Mattos, R. S., Lins, M. A., Da Mota, G. R., … Marchetti, P. H. (2017). The Effect of Different Resistance Training Load Schemes on Strength and Body Composition in Trained Men. Journal of Human Kinetics, 58(1), 177–186.
 Schoenfeld, B. (2010). THE MECHANISMS OF MUSCLE HYPERTROPHY AND THEIR APPLICATION TO RESISTANCE TRAINING. Journal of Strength and Conditioning Research, 24(10), 2857–2872.
 Schiaffino, S., Dyar, K. A., Ciciliot, S., Blaauw, B., & Sandri, M. (2013). Mechanisms
regulating skeletal muscle growth and atrophy. FEBS Journal, 280(17), 4294–4314.