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Our understanding of the relationship of training intensity (or load) and hypertrophy has changed significantly within the past ten years. While early recommendations suggested loads in excess of 70%-1RM were required to promote, or maximize, muscle growth, a growing amount of data suggests that when training is completed to failure, comparable muscle growth can occur across a range of training intensities (30-80%-1RM) (1-7).
Despite this, there is still resistance to the idea that low loads can be as effective for muscle growth as high loads (8,9). One argument is based on observed differences in surface electromyography (sEMG) amplitudes, that may differ between high and low-load conditions. Much of our understanding of the adaptation to strength training assumes that muscle fibers must be recruited to grow, and that lower sEMG amplitude with low-load training means fewer active motor units, and ultimately reduced growth. Both assumptions need to be held to scrutiny, however, today I’ll focus on the relationship of EMG to muscle growth.
My previous article published on Strengtheory (now Stronger by Science) used evidence from a variety of sources to show that even if sEMG amplitude is higher with high load training (8,10) the difference does not appear to result in a meaningful difference in the resultant muscle growth. This article was linked closely to two letters to the editor regarding the issue (11,12), work I contributed to and lead by Andrew Vigotsky.
At that time, both sides (myself included) were making indirect arguments as we lacked studies that considered both changes in sEMG and muscle hypertrophy following longer term training. There were studies that considered EMG differences between high and low-loads, but these were typically acute studies, completed in a single session (8,10). Then we had the studies that evaluated hypertrophic changes with varying intensities, but these neglected acute differences in sEMG amplitude. That is until now.
Considering changes in both EMG signal and hypertrophy with strength training
Given the observed limitations of the existing literature regarding sEMG amplitudes and the hypertrophic response to varying training loads, Jenkins et al (13) followed their 2015 study with a longer term intervention. Participants were randomized to train with either low (30%-1RM) or high loads (80%-1RM) for three sets of leg extension to concentric failure three times per week over six weeks. The authors considered changes in muscle thickness via ultrasound, one repetition maximum, maximum isometric voluntary contractions (MVIC) and evoked contractile properties after three and six weeks of training. Prior to this work, Jenkins et al (10) had established greater sEMG amplitude with 80%-1RM in the leg extension exercise across three sets to failure.
As expected, the low-load training group completed more repetitions (1751.7±140.8 vs 558.2±45.1) and had higher time under load (3219.0±200.9 s vs 1100.2±66 s) than high load training. Despite this, muscle growth was comparable between the groups (6.7% vs 6.0% relative to baseline for high and low-load respectively). While hypertrophy was comparable, muscular strength was differentially affected across intensities. The high load training group increased leg extension 1RM and maximal isometric voluntary contraction (knee extension) by by 27.7% and 28% respectively over six weeks (est. 26.7±2.0 kg vs 47.0±3.0 kg; 211±16.8 nm vs 271.1±16.6 nm). Conversely the low load group had increases of 9.5% and 13.4% (est. 34.9±2.9 kg vs 38.3±2.5 kg; 195.5±18.9 nm vs 221.9±22.2 nm).
Now the divergent effects of low or high intensity training on muscular strength were accompanied by differential neuromuscular responses. Only high-load training saw increases in voluntary activation and quadriceps sEMG amplitude during maximal actions, and reductions in voluntary activation and sEMG amplitude during submaximal actions. A detailed discussion of the contribution of these adaptations, including evoked twitch parameters is beyond the scope of this post. Rather, irrespective of neural and neuromuscular changes, this paper demonstrates directly that difference in EMG signal parameters, specifically sEMG amplitude between high and low-load training (10), do not correspond to a meaningful difference in the resultant muscular growth from strength training (13).
Conclusion
The practical implications of Jenkins et al (13) supports what I’ve said in my previous post. If your primary goal is hypertrophy, you can train across the training load/intensity spectrum (30-80%-1RM), assuming effort is high (concentric failure). If you compete in events that require high loading on specific lifts (i.e. powerlifting), or simply wish to maximize muscular strength, then there is likely some advantage to training with high load, irrespective of changes in muscle mass.
**Values preceded by est. were estimated through digitization of figures when values were not provided**
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