Light weights promote hypertrophy (and strength) and here’s the evidence… We showed, a while ago now, that so long as weights are lifted to fatigue MPS is as robustly (or perhaps even more so) stimulated as lifting heavy (heavier) weights (2). Then we did a training study (yes unilateral, but a good proof of principle trial… or so I thought?) (3). We even wrote a review (1). Then Brad Schoenfeld helped us out with a nice proof-of-principle trial in well trained men (4) … well trained people are supposedly (so a number of social media folks tell me) less malleable than untrained men! Despite this evidence, sometimes you have to go to an evidence-based answer and Brad Schoenfeld performed a meta-analysis and here’s what he and his co-authors said (5)
“The purpose of this paper therefore was to conduct a meta-analysis of randomised controlled trials to compare the effects of low-load (</=60% 1 repetition maximum [RM]) versus high-load (>/=65% 1 RM) training in enhancing post-exercise muscular adaptations… There was a trend for strength outcomes to be greater with high loads compared to low loads (difference = 1.07 +/- 0.60; CI: -0.18, 2.32; p = 0.09). The mean ES for low loads was 1.23 +/- 0.43 (CI: 0.32, 2.13). The mean ES for high loads was 2.30 +/- 0.43 (CI: 1.41, 3.19). There was a trend for hypertrophy outcomes to be greater with high loads compared to low loads (difference = 0.43 +/- 0.24; CI: -0.05, 0.92; p = 0.076). The mean ES for low loads was 0.39 +/- 0.17 (CI: 0.05, 0.73). The mean ES for high loads was 0.82 +/- 0.17 (CI: 0.49, 1.16). In conclusion, training with loads </=50% 1 RM was found to promote substantial increases in muscle strength and hypertrophy in untrained individuals, but a trend was noted for superiority of heavy loading with respect to these outcome measures with null findings likely attributed to a relatively small number of studies on the topic.”
So there you have it. Light loads work, perhaps heavy loads are superior, but the studies done are few and many (most?) are not appropriately powered to show differences (that means the effect isn’t that big). Also, many only test participants' strength pre- and post-training so why would I ever expect a person lifting light weights to get stronger than someone lifting heavy weights if I do that? Specificity dictates that neurological adaptations will always favour a heavy lifting group in terms of strength. That's a neurological outcome, not a muscular outcome!
Given that the rate of participation in RT is less than 10% in North America (that's self-report so it's likely closer to 5% in reality) it seems odd to me that people are often vehemently opposed to lifting lighter weights (I get a few emails asking me why I promote this approach saying that lifting heavy is the ONLY way to go). Clearly, both approaches are sufficient, but neither is necessary for the development of strength and hypertrophy. Unless one is absolutely bent on the development of maximal strength or hypertrophy the approach works. Even then I'd say that even very occasional heavy sessions or weeks and you'd be good. If RT is a lifelong pursuit then after regular RT for 30 years let's see folks get under the bar and lift heavy on a regular basis (sorry millenials it just ain't happening). Of course there always exceptions, from whom I am sure I'll hear! I still think there is work to be done here and given this evidence I think it’s time folks consider that programs of RT need to incorporate both heavy and light load training. And… there’s no ‘risk’ (at least from my perspective) with promoting lifting of lighter loads… for some it might be actually be a better way to lift weights (or keep them lifting weights). One email from an acquaintance contained the statement that "lifting light weights is for girls and people who are afraid to hypertrophy..." well chalk one up for the ladies! Paradigms change folks... tune up your knowledge, don't tune out just because it doesn't fit your belief system!
P.S. Then we published this paper: https://www.ncbi.nlm.nih.gov/pubmed/27174923... not a fluke, not a bad design, simply confirming lots of other evidence!
1. Burd NA, Mitchell CJ, Churchward-Venne TA and Phillips SM. Bigger weights may not beget bigger muscles: evidence from acute muscle protein synthetic responses after resistance exercise. Appl Physiol Nutr Metab 37: 551-554, 2012.
2. Burd NA, West DW, Staples AW, Atherton PJ, Baker JM, Moore DR, Holwerda AM, Parise G, Rennie MJ, Baker SK and Phillips SM. Low-Load High Volume Resistance Exercise Stimulates Muscle Protein Synthesis More Than High-Load Low Volume Resistance Exercise in Young Men. PLoS ONE 5: e12033, 2010.
3. Mitchell CJ, Churchward-Venne TA, West DD, Burd NA, Breen L, Baker SK and Phillips SM. Resistance exercise load does not determine training-mediated hypertrophic gains in young men. J Appl Physiol 113: 71-77, 2012.
4. Schoenfeld BJ, Peterson MD, Ogborn D, Contreras B and Sonmez GT. Effects of Low- vs. High-Load Resistance Training on Muscle Strength and Hypertrophy in Well-Trained Men. J Strength Cond Res 29: 2954-2963, 2015.
5. Schoenfeld BJ, Wilson JM, Lowery RP and Krieger JW. Muscular adaptations in low- versus high-load resistance training: A meta-analysis. Eur J Sport Sci 1-10, 2014.