Exercise and Sport Sciences Reviews: 2021 Paper of the Year : Exercise and Sport Sciences Reviews

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Exercise and Sport Sciences Reviews: 2021 Paper of the Year

Enoka, Roger M.

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Exercise and Sport Sciences Reviews: October 2022 - Volume 50 - Issue 4 - p 173-174
doi: 10.1249/JES.0000000000000292
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STRENGTH TRAINING: IN SEARCH OF OPTIMAL STRATEGIES TO MAXIMIZE NEUROMUSCULAR PERFORMANCE

In their Perspective for Progress article, Duchateau and colleagues (1) compare the relative influence of mechanical and metabolic stress on the gains achieved by individuals who participate in several weeks of strength training. The comparison examined the improvements elicited by classical heavy-load training (mechanical stress), low-load exercise with blood-flow restriction (metabolic stress), and a novel protocol that involves moderate-to-heavy loads (mechanical stress) with brief rest intervals (metabolic stress).

Conventional wisdom asserts that moderate-to-high loads are needed to induce muscle hypertrophy and to increase muscle strength, but that the magnitude of the load depends on the experience of the individual (2). However, comparable gains in muscle size and strength have been reported when a training program includes low-load exercises (<50% of maximum) in which blood flow to the active muscles is partially occluded, but the findings vary across studies and appear to depend on the characteristics of study participants.

Despite the similar outcomes for the two training modalities, the mechanisms underlying the resulting muscle hypertrophy differ. The increase in muscle size after high-load training is primarily attributed to mechanotransduction in which a mechanical stress is translated into chemical signals that eventually induce increases in net protein synthesis and the number of myonuclei. The result is an increase in the quantity of contractile proteins. In contrast, the gains achieved with the blood flow-restriction protocol result from a shift toward anaerobic metabolism due to a reduction in the availability of O2 and the accumulation of metabolites that engage systemic hormones and the possible augmentation of muscle activation. However, the specific pathways responsible for muscle hypertrophy after blood flow-restriction training remain uncertain.

We have known for several decades that some of the training-induced increases in muscle strength are attributable to neural adaptations. One of the key adaptations involves an increase in the level of voluntary activation, which reflects the capacity of supraspinal and spinal networks to activate populations of motor units during voluntary contractions. Some evidence suggests that increases in voluntary activation are responsible for up to one-third of the gains in muscle strength after conventional strength-training protocols. As summarized by Duchateau et al. (1), however, there are mixed findings on whether or not blood flow-restriction training elicits any functionally significant neural adaptations. If not, this is a significant concern because one rationale for this protocol is its accessibility for individuals who cannot tolerate training with heavy loads. Whether or not there are any neural adaptations with this training modality, clinical trials are needed to evaluate the functional benefits that it can produce.

A significant feature of the Perspective article (1) is the description of a novel strength-training program that Duchateau and colleagues have been studying in recent years. The approach is based on manipulating the rest interval between repetitive sets of an exercise to elicit a metabolic stress in addition to the mechanical stress associated with lifting moderate-to-heavy loads. The protocol, which was introduced by a French coach named Emmanuel Légeard, builds on differences in the training programs and outcome goals observed in body builders and powerlifters/weightlifters. Training regimens for body builders tend to comprise many repetitions with brief rest intervals that maximize muscle hypertrophy, whereas powerlifters/weightlifters favor fewer repetitions in each set with longer rest intervals to target increases in muscle strength.

The new approach, called the 3/7 method, requires the practitioner to perform five sets of an exercise with an incremental increase in repetitions across sets: each bout begins with three repetitions in the first set and ends with seven repetitions in the last set. The load is 70% of one-repetition maximum, and there is a 15-s rest between sets. When compared with the outcomes elicited by more traditional strength-training programs in which the training volumes were matched, the 3/7 method produced greater increases in muscle strength but there were no differences between the two methods in peak power production. These encouraging results need to be extended to identify the optimal combination of training variables (e.g., rest interval, workout volume) for different groups of muscles and training devices. Also, to expand on the favorable benefit-to-cost ratio of the 3/7 method, its relative merit needs to be compared with classic strength-training programs for improvements on a broader range of clinical tests (e.g., maximal walking speed, walking endurance, balance, chair-rise test).

In their section on Perspectives for Progress, Duchateau and colleagues identify several key gaps in knowledge that need to be addressed in order to advance the field. For example, it is not clear how an acute increase in muscle protein synthesis is translated into muscle hypertrophy after multiple training sessions. Also, the potential indirect role of anabolic hormones in promoting satellite cell incorporation in long-term adaptations needs to be clarified. As already noted, little is known about the mechanisms that increase muscle mass and strength after blood flow-restriction training and the benefits that can be realized with this training modality by healthy adults, including elite athletes.

The key points of the seminal article by Duchateau et al. include an insightful examination of contemporary approaches to strength training, a summary of initial work on a novel strength-training method, and the identification of significant issues that remain unresolved.

References

1. Duchateau J, Stragier S, Baudry S, Carpentier A. Strength training: in search of optimal strategies to maximize neuromuscular performance. Exerc. Sport Sci. Rev. 2021; 49(1):2–14.
2. American College of Sports Medicine. American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med. Sci. Sports Exerc. 2009; 41(3):687–708.
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