Iron Game

Training Science - Light Weights Can Kick Start Muscle Gains

“Lift heavy to grow!”
This mantra has been preached since the early days of bodybuilding. Current research-based resistance training guidelines are consistent with the heavier-is-better-mentality, professing that loads below ~65 percent of one-repetition maximum (1RM) primarily build muscle endurance with little effect on increasing muscle size.[SUP]1 [/SUP]The contention rests on the premise that heavy loads are required to recruit the full spectrum of motor units (MUs) in a given motor pool.[SUP]1,2[/SUP] If true, this would substantiate the need to go heavy, since maximal muscle development is dependent on recruiting as many MUs as possible, and keeping them stimulated for a sufficient period of time.

Recently, some in the field have challenged the allegation that heavy lifting is necessary to build muscle. Proponents claim that as long as training is carried out to muscular failure, light-load training will recruit the full spectrum of motor units (and thus muscle fibers), allowing for gains similar to that of using heavy loads.[SUP]3,4[/SUP] Indeed, there is evidence that fatiguing muscular contractions result in a corresponding increase in electromyographic (EMG) amplitude, presumably driven by an increased recruitment of high-threshold MUs to maintain force output.[SUP]5[/SUP]Whether EMG findings actually translate into greater hypertrophy remains speculative, however; the true answer can only be determined by training studies that measure changes in muscle size over time.

Conflicting Study Results
A number of longitudinal studies have been carried out to compare the effects of low- versus high-load training on muscular adaptations. The results of these studies have been conflicting; some show a hypertrophic superiority to heavier lifting[SUP]6-8[/SUP] while others show no differences, regardless of the magnitude of load.[SUP]9-13[/SUP] A big issue when trying to draw conclusions on the topic is a problem inherent to virtually all long-term resistance-training studies— they are very costly and time-consuming to carry out. This invariably leads to small sample sizes and a corresponding lack of statistical power to detect a significant difference between conditions (a so-called a type II error). In other words, there might actually be differences favoring one type of training protocol over the other, but statistical measures would not be sensitive enough to observe these differences.

In order to achieve clarity on the topic, my lab recently carried out a meta-analysis encompassing the current body of literature.[SUP]14[/SUP] A meta-analysis converts the results of all relevant studies into a something called an “effect size,” which is basically a measure of the magnitude of the effect in a given outcome (in this case, strength and hypertrophy). By standardizing results in this manner, the data can be pooled together and then compared, essentially as one big study. The upshot is that statistical power is heightened, providing a greater ability to detect significant differences between conditions if they do in fact exist.

To be included in our analysis, studies had to be randomized, controlled trials, involving both light- (<60% 1RM) and heavy-load (>65% 1RM) training that spanned at least six weeks, and directly measured muscle hypertrophy and/or strength. Ultimately, a total of 10 studies met our inclusion criteria— nine of which investigated strength as an outcome, and eight that investigated hypertrophy.
The results might surprise you.

No Significant Differences
Contrary to prevailing theory, no significant differences were seen between low- versus high-load training in either strength or hypertrophy, although a trend for greater increases was noted in both conditions. Based purely on statistical probability (i.e., the odds that results are due to chance), this implies that from a strength- or muscle-building standpoint, it really doesn’t matter whether you use heavy or light loads, provided training is carried out to muscle failure. While these findings might seem counterintuitive, several things need to be taken into account when attempting to draw practical conclusions.

First and foremost, statistical trends were noted for greater results in both strength and hypertrophy. Since only 10 studies met inclusion criteria, the trends imply that the statistical power of the meta-analysis may have been insufficient to detect a true difference for the heavy-load condition (the “type II” error previously mentioned). Other statistical measures suggest this was in fact the case. In particular, the pooled effect size for strength was markedly higher in the heavy- versus light-load condition (2.30 versus 1.23, respectively), indicating a large positive difference favoring high-load training. Moreover, all nine studies that investigated strength as an outcome favored high-load training, and six of them showed a moderate-to-strong difference in magnitude of effect. Considering the totality of the evidence, it can be concluded with confidence that maximal strength gains require heavier loads.

Differences in effect sizes were also seen for hypertrophy outcome studies, with an advantage seen for high versus low loads (0.82 versus 0.39). While the effect size differential wasn’t as great as that seen for strength, it nevertheless distinctly favored the use of heavier weights. Combined with the fact that increasing strength would allow for greater mechanical tension during hypertrophy training[SUP]15[/SUP], it would appear that at least some heavy-load training is needed if the goal is maximal hypertrophy.

Advantage of Heavier Loads
In summary, results of the meta-analysis provide compelling evidence that low-load training can be an effective means to increase muscle size and strength— and the popular belief that lifting light weights can’t build muscle clearly is misguided. That said, there does appear to be an advantage to using heavier loads for maximizing muscular adaptations. It is interesting to speculate that combining high and low loads might be the ideal approach from a hypertrophy standpoint. Recent work from my lab shows significantly greater mean and peak muscle activation when performing the leg press at 75 percent of 1RM compared to 30 percent of 1RM.[SUP]16[/SUP] One caveat: The light-load set lasted three to four times longer than the heavy-load set, indicating that fibers activated during light-load training received considerably greater time under load. Given the fact that type I fibers have a higher fatigue threshold, the greater time under load provided by low-load training would conceivably stimulate these fibers to a greater degree than the use of heavy weights. Hence, it can be hypothesized that performance of heavier load sets will best target the type II fibers, while incorporating lighter-load training will provide an optimal hypertrophic stimulus to the endurance-oriented type I fibers.

A primary limitation of the current literature is that the relevant studies have been carried out exclusively in untrained individuals— and as I’m sure you know, newbies don’t respond to intense training in the same manner as hardcore lifters. The good news is that I’m in the process of completing a study on the topic with subjects who are highly experienced in resistance training. I’ll look forward to sharing the results and practical implications of this study with the readers of MD in the near future. Stay tuned!

Brad Schoenfeld, Ph.D., CSCS, FNSCA, is widely regarded as one of the leading authorities on training for muscle development and fat loss. He has published over 60 peer-reviewed studies on various exercise- and nutrition-related topics. He is also the author of the best-selling book, The M.A.X. Muscle Plan and runs a popular website and blog at

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