Greater Electromyographic Responses Do Not Imply Greater Motor Unit Recruitment and ‘Hypertrophic Potential’ Cannot Be Inferred

To the Editor: We read with interest the study by Looney et al. (13), investigating the effects of load on electromyographic (EMG) amplitude and rating of perceived exertion (RPE) during squats taken to muscular failure. There are numerous interesting takeaways from this study, including the similar RPE outcomes of different loads when sets are taken to failure; however, we demur with the authors' interpretation of the findings. In the title and the body of the article, the term motor unit (MU) recruitment is used synonymously with EMG amplitude. This is an incorrect assumption, but regrettably a common mistake in sports and exercise science. We find this mistake being made especially when dealing with fatiguing and dynamic conditions, such as those investigated by Looney et al. (13). In fact, Enoka and Duchateau (7) recently described how numerous studies have misinterpreted surface EMG signals by inferring specific MU recruitment. More than 2 decades previously, De Luca (4) stated, “To its detriment, electromyography is too easy to use and consequently too easy to abuse.” Looney et al. (13) state that MU firing rate decreases with fatigue (10,15) and consequently that the increase in EMG amplitude is caused by increased MU recruitment (19–21) and has applied that same logic to the subsequent interpretation of the findings, as the authors repeatedly state that the greater EMG amplitude observed in the heavier conditions is indicative of greater MU recruitment. Regrettably, the interpretation of EMG is not so straightforward. Moreover, different quadriceps muscles may use different neural strategies to maintain force generation during repeated concentric contractions (6), which makes the findings of Looney et al. (13) particularly difficult to interpret. Although EMG amplitude is influenced by MU recruitment, MU recruitment cannot be inferred from changes in surface EMG amplitude. The recruitment threshold of high threshold MUs is reduced during sustained, fatiguing contractions (1), and the subsequent recruitment of these MUs assists in the maintenance force production. However, MU cycling may momentarily derecruit fatigued MUs to reduce fatigue (22). This means that, in scenarios that require less force output, such as low-load conditions, there may be lower simultaneous MU recruitment compared with high-load conditions. Ultimately, a comparable complement of the MU population of a particular muscle may be recruited, but not simultaneously as in high-load conditions. This would explain the observation of reduced peak EMG amplitude in low-load training, as reported by Looney et al. (13). These factors, including the reduced recruitment threshold of high threshold MUs, in addition to MU cycling during fatiguing contractions, may also explain other recent work showing differences in peak amplitude measured during surface EMG for high-load and low-load conditions (12,16). Electromyographic amplitude during fatiguing conditions can be extraordinarily misleading, as EMG measures consist not only of multiple neural components (MU recruitment, rate coding, and possibly MU synchronization) but also of multiple peripheral constituents: muscle fiber propagation velocity and intracellular action potentials (5). Intracellular action potentials are of particular interest during fatiguing conditions, as the ensuing increase in length of intracellular action potentials may augment surface EMG signals, despite a decrease in intracellular action potential magnitude. These inherent limitations make it impossible to discern MU recruitment from increases in EMG amplitude during fatiguing, dynamic conditions (2,5,8,9). It may be true that greater loads induce greater MU recruitment, but to measure this, more advanced methods are needed, such as spike-triggered averaging (3) or initial wavelet analysis followed by principal component classification of major frequency properties and optimization to tune wavelets to these frequencies (11). In addition to our concerns regarding the confusion of EMG amplitude with MU recruitment, we note that inferring chronic adaptations from acute, mechanistic variables is very difficult. Looney et al. (13) suggest that their findings support the use of heavier loads for hypertrophy. Such a conclusion is unwarranted, as the literature does not currently differentiate between the long-term effects of heavy and light loads on increases in muscular size (18). Data from Mitchell et al. (14) also demonstrated comparable growth of type I and II fibers after 10 weeks of strength training at either low (30% 1 repetition maximum [1RM]) or high-loads (80% 1RM). If the differential EMG amplitude between high and low-load training observed by Looney et al. (13) and others (12,16) is representative of greater recruitment of presumably high threshold MUs, then one would expect a differential hypertrophic response between low and high threshold MUs, which is presently not supported. In fact, from an evidence-based perspective, Schoenfeld et al. (18), in their meta-analysis, showed no difference between studies that have used lighter or heavier loads to induce hypertrophy. A recent study by the same author confirmed that this was true even in well trained participants (17). Thus, longitudinal trials are clearly needed to elucidate these mechanisms, in addition to comparing individual loading with combined loading schemes. The findings of Looney et al. (13) provide more data that unequal EMG amplitudes are obtained during fatiguing contractions with low-load and high-load conditions and the novel finding that both conditions elicit similar RPE. What these data do not provide, however, is evidence that heavier load contractions recruit more MUs and that this can be inferred to result in greater hypertrophy. We hope that our letter helps put these findings into a clearer perspective.

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