Paper
Higher effort, rather than higher load, for resistance exercise‐induced activation of muscle fibres
Published Sep 1, 2019 · J. Grgic, B. Schoenfeld
The Journal of Physiology
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Abstract
Resistance exercise prescription is based on the manipulation of training variables such as muscle action type, type of resistance, exercise selection and their order, rest intervals, repetition speed, training frequency, volume and load. Of these variables, load has received particular attention in the literature. For increases in muscle size, the current American College of Sports Medicine position stand for resistance training recommends using higher loads (i.e. 70–85% of one repetition maximum [1RM]) for 8–12 repetitions per set. However, these recommendations have been challenged in a recent meta-analysis by Schoenfeld et al. (2017) that compared the effects of low-load resistance training (operationally defined as less than 60% 1RM with studies generally using even lower loads such as 30% 1RM) versus high-load resistance training (operationally defined as loads greater than 60% 1RM with studies generally using even higher loads such as 80% 1RM) on muscle hypertrophy. In that meta-analysis, Schoenfeld et al. (2017) reported that both lowand high-load resistance training produce similar increases in muscle size provided the training sets are performed to muscle failure. Despite this evidence, one aspect that should not be neglected is that these results are specific to whole-muscle hypertrophy obtained using measurement tools such as B-mode ultrasound, magnetic resonance imaging and computerized tomography. Schoenfeld et al. (2017) could not explore the effects of lowversus high-load resistance training on hypertrophy of type I and type II muscle fibres, due to the paucity of studies conducted on this topic. At the time of the review, the authors found only four relevant studies, and the findings across these studies are equivocal. Two studies reported divergent effects of low-load and high-load resistance training on type I and type II muscle fibre hypertrophy (Campos et al. 2002; Schuenke et al. 2012). Specifically, these studies presented data indicating that high-load resistance training elicits greater hypertrophy of both type I and type II muscle fibres than low-load training. In contrast to these findings, two studies have reported similar increases in fibre size of both types regardless of the load used in training (Mitchell et al. 2012; Morton et al. 2016). Due to the considerable physiological differences between type I and type II muscle fibres, we have hypothesized that low-load resistance training may be more effective in producing type I muscle fibre hypertrophy, while high-load may be more effective in producing type II muscle fibre hypertrophy (Grgic et al. 2018). A recent study published in The Journal of Physiology is a timely addition to the literature and provides intriguing information on this topic. Briefly, Morton et al. (2019) demonstrated that both low-load (30% 1RM) and high-load (80% 1RM) lower-body resistance exercise performed to muscle failure results in different effects on time-under-load, training volume and surface electromyography amplitude. Despite these differences, the depletion of muscle glycogen and anabolic signalling were similar in both muscle fibre types regardless of the load used in the exercise session. Therefore, it seems that low-load and high-load resistance training produces similar activation and anabolic signalling in both type I and type II muscle fibres, provided that sets are performed to muscle failure. Based on the results of this study, it would be tempting to conclude that both low-load and high-load resistance training may, over the long term, produce similar hypertrophic effects on type I and type II muscle fibre hypertrophy. However, we should not disregard that the study by Morton et al. (2019) had an acute design. Even though these findings might hypothetically suggest that hypertrophy of type I and type II muscle fibres would also be the same when training with low and high loads, there is still a need for future long-term studies that would directly answer this question. The need for future long-term studies is especially important to emphasize if we consider the recent evidence showing that blood flow restriction training performed with low loads ( 30% of 1RM) produces significant type I muscle fibre hypertrophy (Bjørnsen et al. 2019). In contrast, a group from the same study that trained with high-loads did not exhibit significant pre-to-post intervention increases in the size of type I muscle fibres (Bjørnsen et al. 2019). Even though traditional low-load training is not the same as low-load blood flow restriction training (i.e. the latter may alter physiological responses to resistance training, such as augmenting tissue hypoxia), it has been referred to as a ‘milder form of low-load blood flow restrictive exercise’ (Burd et al. 2013) and these results, therefore, suggest a possible loadand fibre type-specific response. While understanding the possible logistical challenges of this idea, it would be interesting for future studies to examine the time course of muscle fibre growth when training with low and high loads. This aspect may be intriguing to explore if we consider that the two previously mentioned studies that reported divergent effects of lowand high-load resistance training on muscle fibre hypertrophy utilized a shorter duration training protocol (6–8 weeks, equating to a total of 17 and 20 resistance exercise sessions; Campos et al. 2002; Schuenke et al. 2012). The studies that reported similar increases in both fibre types with lowand high-load strategies used longer duration interventions (10–12 weeks, equating to a total of 30 and 36 resistance exercise sessions; Mitchell et al. 2012; Morton et al. 2016). While this idea is a speculative one – because the data are from different cohorts – it might be that the time course of type I and type II muscle fibre hypertrophy differs with varying loads. The study (Bjørnsen et al. 2019) that reported type I muscle fibre hypertrophy with low-load blood flow restriction training but not with high-load utilized very short training blocks (i.e. 2 × 5 days of training). Hence, there
Both low-load and high-load resistance training can effectively increase muscle size, but high-load training may be more effective for type II muscle fiber hypertrophy.
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