Intermittent hypoxic resistance training: Is metabolic stress the key moderator?

Abstract

Traditionally, researchers and practitioners have manipulated acute resistance exercise variables to elicit the desired responses to training. However, recent research indicates that altering the muscular environment during resistance training, namely by implementing a hypoxic stimulus, can augment muscle hypertrophy and strength.Intermittent hypoxic resistance training (IHRT), whereby participants inspire hypoxic air during resistance training, has been previously demonstrated to increase muscle cross-sectional area and maximum strength by significantly greater amounts than the equivalent training in normoxia. However, some recent evidence has provided conflicting results, reporting that the use of systemic hypoxia during resistance training provided no added benefit. While the definitive mechanisms that may augment muscular responses to IHRT are not yet fully understood, an increased metabolic stress is thought to be important for moderating many downstream processes related to hypertrophy. It is likely that methodological differences between conflicting IHRT studies have resulted in different degrees of metabolic stress during training, particularly when considering the inter-set recovery intervals used. Given that the most fundamental physiological stresses resulting from hypoxia are disturbances to oxidative metabolism, it becomes apparent that resistance training may only benefit from additional hypoxia if the exercise is structured to elicit a strong metabolic response. We hypothesize that for IHRT to be more effective in producing muscular hypertrophy and increasing strength than the equivalent normoxic training, exercise should be performed with relatively brief inter-set recovery periods, with the aim of providing a potent metabolic stimulus to enhance anabolic responses.3

Heavy Resistance Training in Hypoxia Enhances 1RM Squat Performance

“…Since blood flow restriction is usually matched with low intensity resistance training, the changes in cross sectional area and strength may not be accompanied by a concomitant increase in connective tissue strength. This is due to decreased mechanical loading through low intensity resistance training used with blood flow restriction (Scott et al, 2015a), therefore the strength of muscles and connective tissue will adapt disproportionately. Increased tensile strength of the tendon might be expected to maintain the safety of the tendon under increasing loads (Buchanan and Marsh, 2002).…”

High-Intensity Exercise in Hypoxia: Is Increased Reliance on Anaerobic Metabolism Important?

“…In IHRT research, these contrasting findings may result from differences in the structure of exercise. If sufficient repetition volume during sets is not performed, the contraction time during which metabolites accumulate is decreased, and if inter-set rest periods are too long there is a greater chance for intramuscular metabolites to be removed into circulation and PCr resynthesis to occur (Scott et al, 2014, 2015a). Likewise, if adequate recovery is given between efforts during RSH training protocols, the replenishment of PCr stores may occur despite the hypoxic conditions (Girard et al, 2011).…”

“…This is supported by recent research that has demonstrated heightened integrated electromyography response during moderate-load IHRT compared to the same exercise in normoxia (Scott et al, 2016). If more motor units are recruited during training, a larger portion of the muscle will be stimulated to adapt (Scott et al, 2015a). Cell swelling is another potential mediator for muscle hypertrophy, resulting from metabolite accumulation within the cells and a resultant inflow of water to equilibrate the osmotic gradient (Loenneke et al, 2012).…”

“…If brief inter-set rest periods are implemented, it may be possible to attenuate the clearance of these metabolites, meaning that the next set would begin with already elevated metabolite concentrations within the muscles. Furthermore, brief rest periods would also not allow for PCr stores to be resynthesized to the same levels in hypoxia as in normoxia, whereas extended recovery periods may allow for similar PCr recover, irrespective of hypoxia (Scott et al, 2015a). Current evidence suggests that IHRT should be structured using light to moderate loads (20–70% 1-repetition maximum), which allow for substantial repetitions in each set (10–30 repetitions) and short recovery periods (30–60 s) (Scott et al, 2015a).…”

“…Although performing these high-intensity activities in hypoxia appears to provide some benefits for training adaptations, the mechanisms underpinning these responses are not fully understood. The beneficial responses to high-intensity exercise in hypoxia may result from a greater reliance on anaerobic metabolism, suggesting that increased metabolic stress may drive (or at least contribute to) these adaptations (Faiss et al, 2013b; Scott et al, 2015a). Considering the likely importance of metabolic stress on adaptation to IHRT and RSH strategies, the purpose of this paper is to briefly discuss the potential benefits of high-intensity training in hypoxia with reference to the role of anaerobic processes.…”