“Training intensity” is the percentage of maximum load that is being used for a given exercise.

Training intensity should not be confused with training exertion (see the “Exertion section).

For example, using a 75% load (e.g. if you can use 100kg maximum and are lifting 75kg), is a lower intensity than a 95% load (you’re using 95kg of your 100kg maximum). Another perspective would be that completing a set of 5 repetitions is a higher intensity than completing a set of 10 repetitions, given the exertion in both sets is identical (as the weight used in the 5 repetition set would be higher).

Training intensity is not of significant concern for hypertrophy but, plays a larger role in strength training

The truth is that you can increase muscle mass with any chosen training intensity, as long as the principle of progressive overload remains (see “Progression” section). However, a limit on how high a rep range can go before becoming more ineffective is apparent. The American College of Sports Medicine’s recommendations for hypertrophy are that novice trainees perform 8-12 repetitions per set [24].

fitness-barbell for strength training on floor

A meta-analysis confirms this, finding that high loads (>65% 1RM) had a greater effect size for hypertrophy than low loads (<60% 1RM). Based on this, it is advisable to avoid going above ~15 repetitions, especially for compound exercises, even though progress can still be made with very low intensities. Some recent EMG evidence suggests the biggest/fastest motor units are not recruited as effectively during very high rep sets, so less muscle fibers are recruited in comparison to higher intensities [23].

As long as intensities are not very low, studies show no differences in muscle growth seen between the various rep ranges

Campos found no difference in hypertrophy between 3-5, or 9-11 repetitions, although no hypertrophy at all was seen in the 20-28 repetition group, after an 8-week progressive resistance-training program in 32 untrained men [25]. Schoenfeld also found no significant differences in muscle growth between groups performing 2-4 or 8-12 repetitions [26]. However, in both studies the 3-5 rep group, and the 2-4 rep group, respectively, were seen to make the most improvements in strength and this should be noted for people more strength-training focused.

Rep ranges under 5 (80%+ intensity) achieve the largest changes in myofibrillar volume and density, and neuro-muscular adaptations, both large contributors to strength gain.

For hypertrophy, low-moderate intensities (high repetitions) are a more popular choice and is advised by many performance coaches as it is a time efficient method compared to using high intensities. It is clearly more time efficient to complete 3 sets of 10 repetitions compared to 10 sets of 3 repetitions.

The key point is that total training volume remains identical. As long as training volume is sufficient and progressive overload occurs, the combination of rep and set amounts is largely irrelevant.

Summary – Training intensity is irrelevant when repetitions per set are below ~15.

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23) Schoenfeld BJ, Contreras B, Willardson JM, Fontana F, Tiryaki-sonmez G. 2014. Muscle activation during low- versus high-load resistance training in well-trained men. Eur J Appl Physiol. 114(12):2491-7
24) American College of Sports Medicine. 2009. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 41(3):687-708
25) Campos GE, Luecke TJ, Wendeln HK, Toma K, Hagerman FC, Murray TF, Ragg KE, Ratamess NA, Kraemer WJ, Staron RS. 2002. Muscular adaptations in response to three different resistance-training regimens: specificity of repetition maximum training zones. Eur J Appl Physiol. 88:50-60.
26) Schoenfeld BJ, Ratamess NA, Peterson MD, Contreras B, Sonmez GT, Alvar BA. 2014. Effects of different volume-equated resistance training loading strategies on muscular adaptations in well-trained men. J Strength Cond Res. 28(10):2909-18.