Intensity or “load” in the context of research is defined as a percentage of 1 repetition max (RM) and represents the number of repetitions that can be performed with a given weight or resistance. Intensity has been shown to impact muscle hypertrophy and is a significantly important variable in a resistance training protocol.
Heavier loading is often recommended as way to maximize muscle hypertrophy with resistance training (1) as it is associated with full muscle fiber recruitment and activation of the type II fibers which are known to be more responsive to hypertrophy (3). The use of moderate and lower repetitions with heavier loads (≥90% 1RM) has been reported to be superior to higher repetition ranges with lighter loads (<65% 1RM) in initiating a hypertrophic response. (11,12) and the general opinion in the scientific literature is that loads of less than approximately 65% of 1RM are insufficient to promote substantial hypertrophy (13)
Noteworthy is the work of Fry 2004 which analyzed numerous studies on muscle growth of both type I and II fibers and found that there was a dose-dependent response between intensity and muscle growth. The data suggested that maximal hypertrophy occurs with loads from 80-95% 1RM. However, there appeared to be a range where most benefits were observed around 80-85 % maximal intensity or about 8-12 repetitions. This is in general agreement with a preponderance of the research literature.
However, pinpointing the mechanisms by which heavier training loads would provide a more preferential stimulus for greater muscle hypertrophy compared with a lighter load lifted to the point of muscular failure has not been fully elucidated and there is evidence that lower-load protocols may result in a large amount of muscle fiber recruitment and hypertrophy.
Some researchers have opined that muscles containing a greater percentage of type I fibers might achieve a better hypertrophic response to lighter load, high-repetition training and that type II fibers would respond best to higher loads and lower repetitions (14,15). And contrary to popular bodybuilding lore, type I fibers have been shown to hypertrophy considerably due to a progressive overload (16, 17).
Support for Lighter Load Training
Cameron et al. 
2012 demonstrated that a single bout of resistance exercise performed at 30% of 1RM to the point of momentary muscle failure was equally as effective in stimulating muscle protein synthesis rates (MPS) as loads lifted at 90% of 1RM (also taken to failure). Additionally, the 30%-1RM protocol resulted in a more prolonged muscle protein synthetic response with a greater elevation of MPS rates than the 90% of 1RM program 24 hours after exercise (2).
In the aforementioned study, the average area of both type I and II fibers increased equally with heavy and light relative loads, which is suggestive that both fiber types were recruited during training and to a roughly equal extent. It is suggestive that as the repetitions at lighter loads are repeated, the point of muscular failure may ultimately necessitate near maximal motor unit recruitment to sustain muscle tension (4). Therefore, it is possible that relatively lighter loads lifted to the point of failure may result in a similar muscle fiber activation compared with heavier loads lifted to failure (5, 6)
Behm et al. 2002 tested 14 resistanc- trained males (age ∼ 21 years) after performing 5, 10 and 20 RM dumbbell curls with a resultant time-under-tension of 35, 70 and 140 seconds for the 5, 10 and 20 RM, respectively. There was no significant difference in voluntary motor unit activation following 5 RM (95.5%), 10 RM (93.5%) and 20 RM (95.1%). The three different loads of 5, 10 and 20 RM as well as the different time-under-tension 35, 70 and 140 seconds, elicited similar activation of motor units (93.5–95.5%). Perhaps most importantly, the 5 RM protocol did not produce greater motor unit activation than the 10RM or 20 RM protocols. This study demonstrated the relationship between intensity of effort and not the amount of resistance or time under tension and voluntary motor unit activation.
Ogasawara and colleagues (2013) showed six weeks of high-load (75% 1 RM) resistance training resulted in significant skeletal muscle hypertrophy. But after 12 months of detraining the same subjects then performed low-load (30% 1 RM) resistance training to failure and found similar increases in skeletal muscle hypertrophy compared to that observed with high-load training. This is contrary to previous research [7, 8] and recommendations [1] that report higher-loads to be superior. However, it appears that in order for low-loads to increase muscle hypertrophy to levels similar to high-loads, exercise must be taken to muscular failure [9].
This study is in agreement with the results from Burd et al. 2010 who found similar increases in MPS independent of load when exercise was taken to voluntary muscular failure. This could potentially be related to the increase in time under tension when repetitions are taken to voluntary failure as this has been found to be an important variable in the hypertrophic response [2].
The “Ideal” Range
Although there are many opinions in the research literature, what constitutes an optimal stimulus for hypertrophic gains is not entirely clear. The prevailing belief is that a range of approximately 6–12 reps is best for inducing a hypertrophic response. (18, 19)
However, an exact repetition range remains a matter of debate as varying repetition ranges have shown to produce significant gains in muscle growth. Numerous studies suggest that if a maximal or near maximal effort were applied at the end of a set varying loads will elicit similar outcomes.
Practical application
In order to elicit hypertrophic responses selecting an ideal protocol may 
essentially come down to the personal preference of the individual. Although moderate to heavier loads are the predominant recommendation, there is evidence that lighter loads (as high as 20 RM or that limit time under tension not longer than 2-3 minutes) may be very effective. In the end, claims of the superiority of a specific load as the optimal way to maximize muscle hypertrophy with resistance training have not been unequivocally proven and this suggests a need for further research.
Based on the evidence, bodybuilders and strength athletes with a goal of maximizing muscle hypertrophy should not fear but embrace the concept of employing lighter loads (at least periodically) in order to stimulate continuous hypertrophic adaptations. Whether in the form of a scheduled periodization or randomly integrated, it appears that lighter loads can elicit significant motor unit recruitment as well as hypertrophic responses. A more experienced bodybuilder might benefit by employing lighter load training more “instinctively” and even vary repetition schemes based on different muscle groups, exercises and individual responses. Lighter load training may also provide a more safe and viable alternative to elicit muscular gains for those with musculoskeletal dysfunctions and/or other orthopedic considerations.
Train Smart and Good Luck!
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Other References:
Review Article; THE SIZE PRINCIPLE AND A CRITICAL ANALYSIS OF THE UNSUBSTANTIATED HEAVIER-IS-BETTER RECOMMENDATION FOR RESISTANCE TRAINING, Ralph N. Carpinelli Human Performance Laboratory, Adelphi University, Garden City, New York, USA J Exerc Sci Fit Vol 6 No 2 67–86 2008
Riki Ogasawara, Jeremy P. Loenneke, Robert S. Thiebaud, Takashi Abe : Low-Load Bench Press Training to Fatigue Results in Muscle Hypertrophy Similar to High-Load Bench Press Training; International Journal of Clinical Medicine, 2013, 4, 114-121
Cameron J. Mitchell, Tyler A. Churchward-Venne, Daniel W. D. West, Nicholas A. Burd, Leigh Breen, Steven K. Baker, and Stuart M. Phillips; Resistance exercise load does not determine training-mediated hypertrophic gains in young men; J Appl Physiol (1985). Jul 1, 2012; 113(1): 71-77
Andrew C. Fry, The Role of Resistance Exercise; Intensity on Muscle Fibre Adaptations; Sports Med 2004; 34 (10): 663-679
Schoenfeld, Brad J The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training; Journal of Strength & Conditioning Research: October 2010 - Volume 24 - Issue 10 - pp 2857-2872
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