Intermittent hypoxic resistance training: Is metabolic stress the key moderator? Brendan R. Scott a,⇑ , Katie M. Slattery a,b , Ben J. Dascombe a a Applied Sports Science and Exercise Testing Laboratory, Faculty of Science and Information Technology, University of Newcastle, Ourimbah, NSW 2258, Australia b New South Wales Institute of Sport, Sydney Olympic Park, NSW 2127, Australia article info Article history: Received 28 August 2014 Accepted 3 December 2014 Available online xxxx 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 environ- ment 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-sec- tional area and maximum strength by significantly greater amounts than the equivalent training in nor- moxia. 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 oxi- dative 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. Ó 2014 Elsevier Ltd. All rights reserved. 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 sug- gests 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 restric- tion (BFR) during resistance training has become increasingly pop- ular 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 tra- ditional 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 http://dx.doi.org/10.1016/j.mehy.2014.12.001 0306-9877/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author at: School of Environmental and Life Sciences, Faculty of Science and Information Technology, University of Newcastle, PO Box 127, Ourim- bah, NSW 2258, Australia. Tel.: +61 2 43484149; fax: +61 2 4348 4145. E-mail address: brendan.scott@uon.edu.au (B.R. Scott). Medical Hypotheses xxx (2014) xxx–xxx Contents lists available at ScienceDirect Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy Please cite this article in press as: Scott BR et al. Intermittent hypoxic resistance training: Is metabolic stress the key moderator? Med Hypotheses (2014), http://dx.doi.org/10.1016/j.mehy.2014.12.001