Intermittent hypoxic resistance training: Is metabolic stress the key moderator?
Introduction
Resistance exercise is known to have a potent affect on skeletal muscle morphology and functional adaptations. Traditionally, researchers and practitioners have focused on manipulating acute resistance exercise variables to elicit the desired training response. These variables include the muscle action, loading and volume, exercise selection and order, inter-set rest periods, repetition velocity and training frequency [1]. However, recent evidence suggests that methods to alter the intramuscular environment during resistance exercise can be beneficial for stimulating hypertrophy and increases in muscular strength. The use of blood flow restriction (BFR) during resistance training has become increasingly popular for this purpose. This technique involves the application of a restrictive cuff, tourniquet or elastic wraps around the top of a limb, with the aim to somewhat maintain arterial inflow while occluding venous return from the exercising limb [2].
This technique creates a localized hypoxic environment in the limb during exercise, which is proposed to impact on downstream mechanisms that promote muscular development [3]. The novel aspect of training with BFR is that substantial improvements in muscular hypertrophy and strength are possible even when using low-loads (20–40% of concentric 1-repetition maximum [1RM]) for both clinical [4] and athletic [5], [6] populations. However, while the muscles of the trunk may benefit to some degree from BFR exercise [7], the trunk muscles are unable to be trained under the same conditions as the limbs. Furthermore, due to the low-loads employed during BFR exercise, motor unit recruitment (as estimated by surface electromyography) is lower than during traditional high-load exercise [8], [9], therefore limiting the potential for neuromuscular adaptations.
Another method to manipulate the intramuscular environment during resistance exercise that is not affected by these limitations is the addition of systemic hypoxia during training. Research has demonstrated that hypertrophic and strength responses can be enhanced by breathing hypoxic air during low-load (20% 1RM) [6], [10] and moderate-load (70% 1RM) [11] resistance training. However, some more recent evidence has provided conflicting results, reporting no additional benefit for muscular development following resistance training in systemic hypoxia [12], [13]. While scientific understanding of intermittent hypoxic resistance training (IHRT) is in its infancy, it appears that these conflicting results may be a result of differences in the research methodologies employed.
In particular, the inter-set rest periods used by researchers has varied greatly (30–120 s). Inter-set rest periods are often overlooked in the design of resistance training programs, particularly in recreational training settings. However, the rest period is a primary determinant of the overall intensity of a training session, particularly when the level of available oxygen is altered as it will directly impact on the metabolic stress induced by exercise [14]. As the metabolic response to resistance exercise is a proposed key moderator of subsequent adaptive responses [15], it stands to reason that inter-set rest periods should be carefully programmed during IHRT. In this paper, we hypothesize that due to the disturbances in energetic metabolism induced by hypoxia, the inter-set rest periods employed during IHRT are of primary importance to enhanced hypertrophic responses.
Section snippets
Conflicting results of IHRT studies
To date, six separate investigations have assessed the efficacy of IHRT for increased muscle hypertrophy and strength, compared with the equivalent training in normoxia. These studies are summarized in Table 1. Two papers have used low-load resistance training (20–30% 1RM), with Manimmanakorn et al. [6], [10] employing very brief inter-set rest periods (30 s), while Friedmann et al. [16] used longer rest intervals (60 s). Interestingly, Manimmanakorn et al. [6], [10] reported that IHRT elicited
Effects of inter-set rest periods on energetic metabolism
Inter-set rest periods are most often manipulated in response to the intensity of exercise being performed. For example, maximal strength and power training (1–6 repetitions per set using heavy loads) generally utilize long rest intervals (180–480 s) to allow for sufficient neuromuscular recovery, and replenishment of adenosine triphosphate (ATP) and phosphocreatine (PCr) stores [14], [18]. However, training focused on hypertrophic responses (8–12 repetitions per set using moderate loads)
Hypoxia-mediated challenges for energetic metabolism
The importance of oxygen availability for PCr resynthesis was first established by Harris et al. [20], who implemented a pneumatic cuff around the thigh (240 mmHg) for 6 min following a bout of isometric knee extension. As a result of the ischemic condition, PCr resynthesis was completely suppressed. Furthermore, when the cuff was deflated for 25 s following 90 s of ischemic recovery and then reinflated, PCr stores recovered to levels that would be expected following 25 s of free flow recovery.
Anabolic effects of metabolic stress
The hypertrophic responses to resistance training are thought to be largely related to the metabolic stress induced by exercise, which is typically estimated via the accumulation of metabolites such as lactate, hydrogen ions and inorganic phosphate, and by changes in pH levels. Recently, it has been proposed that increased levels of metabolic stress can impact on several downstream mechanisms to facilitate muscular hypertrophy [3], [15]. For example, it is possible that metabolic stress can
Considerations for training programs
From current evidence, it appears that relatively short inter-set rest periods are required during IHRT to take advantage of hypoxia-mediated disturbances to oxidative metabolism, and subsequently enhance hypertrophic responses. Furthermore, it is likely that, as with traditional resistance training, the inter-set rest period is related to the intensity of the loads lifted. Low-load IHRT appears beneficial when using very brief rest periods (30 s), whereas moderate-load IHRT is most effective
Conclusions
In conclusion, it is important when designing IHRT research studies that we consider the fundamental physiological stressors imposed by hypoxia (disturbances to oxidative metabolic processes). We hypothesize that the added benefits for muscular growth and strength will only be facilitated by IHRT programs that make use of short inter-set rest periods, with the aim to enhance the metabolic responses to exercise. More specifically, it appears that low-load IHRT requires rest intervals of ∼30 s,
Conflict of interest statement
None of the authors report any conflicts of interest.
Acknowledgments
This paper was not supported by funding from an outside source.
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