Share this article on:

Restrictive Breathing Mask Reduces Repetitions to Failure During a Session of Lower-Body Resistance Exercise

Andre, Thomas L.1; Gann, Joshua J.2; Hwang, Paul S.3; Ziperman, Emily3; Magnussen, Marshall J.4; Willoughby, Darryn S.3

The Journal of Strength & Conditioning Research: August 2018 - Volume 32 - Issue 8 - p 2103–2108
doi: 10.1519/JSC.0000000000002648
Original Research

Andre, TL, Gann, JJ, Hwang, PS, Ziperman, E, Magnussen, MJ, and Willoughby, DS. Restrictive breathing mask reduces repetitions to failure during a session of lower-body resistance exercise. J Strength Cond Res 32(8): 2103–2108, 2018—The purpose of this study was to determine the effect of restrictive breathing mask (RBM) on muscle performance, hemodynamic, and perceived stress variables during a session of lower-body resistance exercise. In a crossover design, 10 participants performed 2 separate testing sessions, RBM and no mask, consisting of squat, leg press, and leg extension. The paired-samples t-test was used for session rating of perceived exertion (S-RPE), perceived stress before and after, heart rate (HR), pulse oximetry, and a 2 × 4 (session [mask, no mask] × time [squat exercise, leg press exercise, leg extension exercise, total resistance exercise session]) factorial analysis of variance with repeated measures (p ≤ 0.05). A significant decrease was found in total repetitions during the RBM condition (p < 0.01). A majority of the decrease in repetitions to failure occurred in the squat (p < 0.05) and in the leg press (p < 0.01), whereas no difference was observed in leg extension (p = 0.214). A significant increase was observed in S-RPE during the RBM session (p < 0.01). A significant increase was found in prestress (p < 0.01) and poststress (p = 0.01) in the RBM session. No significant difference existed for HR between exercise sessions (p = 0.08). A significant decrease existed in pulse oximetry during the RBM session (p < 0.01). The use of an RBM had a negative effect on the number of repetitions completed during an acute session of lower-body resistance training.

1Department of Health, Exercise Science, and Recreation Management, University of Mississippi, Oxford, Mississippi;

2Department of Kinesiology, University of Louisiana Monroe, Monroe, Louisiana;

Departments of 3Health and Human Performance, Baylor University, Waco, Texas; and

4Educational Administration, Baylor University, Waco, Texas

Address correspondence to Dr. Darryn S. Willoughby, Darryn_Willoughby@baylor.edu.

Back to Top | Article Outline

Introduction

A popular exercise training device has been developed recently, which has been promoted by companies as an ergogenic aid, known as a restrictive breathing mask (RBM). The RBM is to be worn in an attempt to simulate the effects that high altitude (hypoxia) has physiologically by limiting the level of oxygen intake with restrictive breathing. It has grown in popularity among professional and recreational athletes, but the literature on the products' effect on exercise and performance is limited.

During 8 minutes of an acute walk/run aerobic paradigm using an RBM, no significant differences occurred between trials for heart rate (HR), blood pressure (BP), arterial saturation percentage (SpO2%), blood lactate, and rating of perceived exertion (RPE) (17). In an anaerobic setting, no improvement in Wingate performance measures or body composition was observed in reserve officers training corps cadets after 6 weeks of mandatory physical training using a training mask (vs. a control group) (21). However, the testing modality may not have been appropriate given that standard military physical training does not include anaerobic cycling. The use of an RBM during 6 weeks of high-intensity interval training has shown a trend to improve predicted V[Combining Dot Above]O2max and forced inspiratory vital capacity (5). An early study suggested that an RBM could potentially induce a hypoxic breathing environment during treadmill exercise demonstrating an SpO2% of 92 (similar to an elevation of 3,000 m) during steady-state treadmill exercise (11). The authors attributed this to rebreathing of expired CO2 trapped in the dead-space of the mask.

Investigation into the effect of an RBM during resistance exercise has yet to be established; however, this product is often used by cross-training athletes. Resistance exercise has been recognized as a stimulus for muscle protein synthesis (MPS) and governed by the intracellular signaling pathway, PI3K/Akt-mTOR. The cumulative effects of increased MPS that occur over time can lead to muscular hypertrophic adaptations (6,14,20). The maintenance of skeletal muscle mass is the product of a net balance between MPS and degradation. Any disruption to this net balance can promote muscle atrophy because of issues such as muscle disuse, nutritional deficiencies, and hypoxia. Subsequently, any decrease in mechanical stimuli (i.e., reduction in repetitions and/or training volume) could negatively affect muscle hypertrophy over time. Given the increase in RPE when using an RBM during aerobic exercise (11), it is worthwhile to examine in an anaerobic paradigm.

It is plausible that the RBM could cause a reduction in the training volume given the increased perceived difficulty of exercise or a potential hypoxic environment. It is important to understand the possible effect of the RBM on resistance training given the product's apparent popularity and claims as a training aid. Therefore, the purpose of this study was to examine the effect of an RBM on muscle performance, hemodynamic variables, and perceived stress in response to a session of lower-body resistance exercise in resistance-trained males.

Back to Top | Article Outline

Methods

Experimental Approach to the Problem

This crossover designed study included 3 visits involving a baseline body composition and strength testing visit and 2 performance testing visits of lower-body resistance exercise, one wearing the RBM and one not wearing the RBM. During visits 2 and 3, muscle performance tests (total reps), perceptual measures of stress, HR, BP, and blood oxygen saturation were measured. Visit 2 for each participant was 72 hours after visit 1. A period of 7 days separated visits 2 and 3 with both sessions being completed at the time of day. Participants were instructed to not exercise 48 hours before testing sessions.

Back to Top | Article Outline

Subjects

Ten apparently healthy, resistance-trained men (18–35 years, at least 3 days of resistance training per week for the past 12 months) were recruited for the study. Enrollment was open to men of all ethnicities. Additional information on participant demographics can be found in Table 1. Participants interested in participating in the study were initially interviewed through e-mail or telephone to determine whether they qualified to participate in the study. Only participants considered as low risk for cardiovascular disease and with no contraindications to exercise as outlined by the American College of Sports Medicine (ACSM) and who had not consumed any nutritional supplements (excluding multivitamins) such as creatine monohydrate, nitric oxide, hydroxy-beta-methylbutyrate (HMB), various androstenedione derivatives, or pharmacologic agents such as anabolic steroids 3 months before the study were allowed to participate. After being informed of the procedures, all eligible participants signed university-approved informed consent documents, and approval was granted by the Baylor University Review Board for Human Subjects. Additionally, all experimental procedures involved in the study conformed to the ethical consideration of the Helsinki Code.

Table 1

Table 1

Back to Top | Article Outline

Procedures

Body Composition Assessment (Visit 1)

Body composition assessment was performed based on our previous studies (25,26,27). Total body mass and height were determined using a calibrated electronic scale and stadiometer with a precision of ±0.02 (Detecto, Webb City, MO, USA). Percent body fat, fat mass, and fat-free mass were determined using DEXA (Hologic Discovery Series W; Hologic Inc., Waltham, MA, USA). Quality control calibration procedures were performed on a spine phantom (Hologic X-CALIBER Model DPA/QDR-1 anthropometric spine phantom) and a density step calibration phantom before testing.

Back to Top | Article Outline

Determination of 12-Repetition Maximum (Visit 1)

Based on the procedures of Kraft et al. (15), a 12-repetition maximum (12RM) was determined for squat (Legend Fitness, Inc., Knoxville, TN, USA), leg press (Nebula Fitness, Inc., Versailles, OH, USA), and knee extension (Cybex International, Inc., Medway, MA, USA). Participants performed a warm-up set of 12 repetitions at 50% of their estimated 12RM (∼70 of 1RM). After a 3-minute recovery period, participants performed 1 set to volitional fatigue at their estimated 12RM. For both squat and leg press, a repetition was accepted as good at 90° knee flexion. A participant's 12RM was accepted if the participant achieved volitional fatigue within 11–13 reps. If participants completed ≤10 or ≥14 repetitions, resistance was adjusted and participants performed 1 set to volitional fatigue after a 5-minute recovery. This process was repeated for each of the 3 exercises. A 3-minute recovery period was given between exercises.

Back to Top | Article Outline

Resistance Exercise Sessions (Visits 2 and 3)

In a randomized crossover design, participants completed 2 lower-body resistance exercise sessions in the morning (with RBM or no RBM). One hour before arriving at the laboratory for the resistance training session for visits 2 and 3, participants were instructed to drink a 500-mL bottle of water and consume an Atkins bar (carbohydrate = 2 g, protein = 10 g, kilocalories = 150), which they were previously provided. This was done to encourage participants to arrive at the laboratory in a catabolic and/or dehydrated state. At each resistance exercise session, participants reported to the laboratory and then relaxed in a supine position for 15 minutes, after which their baseline HR, BP, stress, and SpO2% were measured. Participants then completed a lower-body resistance exercise session with the following exercises: barbell back squat, angled leg press, and knee extension with their previously established 12RM from visit 1. For the RBM trial, the RBM was worn before the warm-up set. For both trials, participants performed 1 warm-up set at 50% of their previously determined 12RM for each exercise. After a 3-minute rest, participants completed the first set of that exercise. Each exercise consisted of 4 sets of reps to failure with 2-minute rest between exercises and sets. Immediately after the completion of each set, the participant's SpO2% and HR were assessed. Oxygen saturation and HR were measured using a handheld pulse oximeter (Nellcor NPB-40; Nellcor Puritan Bennett, Inc., Pleasanton, CA, USA). On completion of the resistance exercise session, participants completed the postsession perceived stress of the exercise session and, for the RBM trial, were then permitted to remove the mask.

Ten minutes after the resistance exercise session, the participant was asked to rate their RPE for the exercise session (S-RPE) using the 10-point category ratings of a perceived exertion scale (25). Before exercise, the RPE scale was verbally anchored by indicating to participants that RPE “1” corresponds to feelings of exertion during seated rest, whereas RPE “10” corresponds to feelings at maximal exertion.

The participants' diets were not standardized, but participants were instructed to duplicate their diet for the 2 testing sessions. Participants were required to record their dietary intake for 1 day before each of the 2 testing sessions and through the 24-hour postexercise time point (48-hour total). They were provided a copy of their initial dietary intake form completed for the first testing session and were instructed to duplicate the diet before the second testing session.

Back to Top | Article Outline

Restrictive Breathing Mask (Visit 2 or 3)

Training Mask 2.0 (Training Mask, Inc., Cadillac, MI, USA) is constructed of neoprene and uses valves to increase resistance to inhalation and exhalation. The product has several resistance levels to simulate how it would feel to breathe at 3,000, 6,000, 9,000, 12,000, 15,000, and 18,000 ft above sea level. The resistance level on the mask for this study was set at 12,000 ft (3,657.6 m), similar to previous research (17).

Back to Top | Article Outline

Statistical Analyses

Statistical analyses were performed by using a 2 × 4 (session [mask, no mask] × time [squat exercise, leg press exercise, leg extension exercise, total resistance exercise session]) factorial analysis of variance (ANOVA) with repeated measures. The paired samples t-test for average session HR, average session oxygen saturation, SRPE, and pre-/post-stress scales. Significant within-session and within-time differences were determined using Fisher's least significant difference post hoc test. If within-group assumption of sphericity was violated in Mauchly's test of sphericity, the Greenhouse-Geisser correction factor was used to evaluate observed within-group F-ratios to avoid a type I error. Interaction effects were investigated using separate repeated-measures ANOVA for each session and time point. The effect size was measured using Cohen's d. The sample size calculations for a 2-tailed study design yielded a minimum sample size of 10 for each session to attain a statistical power of 0.80 in the performance variable of total repetitions (mean difference of 12 ± 12 reps). Statistical procedures were performed using SPSS 21.0 software (Chicago, IL, USA), and a probability level of ≤0.05 was adopted throughout.

Back to Top | Article Outline

Results

A statistically significant interaction was found between session and exercise (p < 0.01), the main effect of session (p < 0.01), and main effect of exercise (p < 0.01) for repetitions. There was a significant reduction in total session reps for the RBM trial (p < 0.01; d = 1.03) Much of the reduction in training intensity occurred during the squat (p < 0.05, d = 1.03) and leg press (p < 0.01; d = 1.16), whereas no statistical difference occurred in leg extension (p = 0.21; d = 0.02) (Table 2). The average HR for each session was slightly elevated during the RBM session, but no statistical difference was observed (p = 0.08; d = 0.23). Significant increases were found in prestress (p < 0.01; d = 1.11) and poststress (p = 0.01; d = 1.47) scales for the RBM. Furthermore, there was an increase in SRPE for the RBM (p < 0.01; d = 1.33) and a significant reduction in oxygen saturation for RBM (p < 0.01; d = 0.97) (Table 3).

Table 2

Table 2

Table 3

Table 3

Back to Top | Article Outline

Discussion

This study was the first to investigate the effect of an RBM on a single lower-body resistance exercise session. Based on a previous study using endurance exercise (11), our assumption was that using the RBM with resistance exercise could potentially cause mild hypoxia (SpO2% of 92) by the rebreathing of exhaled CO2 collected in the dead space of the mask. However, in the current investigation, mild hypoxia did not occur with lower-body resistance exercise, which is in agreement with previous research that used short-duration treadmill exercise (17). In the current study, the RBM diminished total repetitions and, therefore, may potentially blunt known hypertrophic adaptation responses to resistance exercise with chronic use because of a decrease in mechanical stimulus (i.e., reduction in training intensity and time under tension).

Compared with the no mask condition, use of the RBM negatively affected performance as 12 fewer total repetitions occurred (Table 2). The resulting diminished performance (reduced volume) provides a plausible rationale that the consistent use of an RBM with resistance exercise could negatively affect muscle strength and hypertrophy expected over time with resistance training, given the reduction in training intensity. In regard to MPS and hypertrophy, increases in the phosphorylation of the mTOR substrate p70S6K after resistance training have been closely associated with exercise intensity in both rat and human muscle (3,16,24). A reduction in time under tension could subsequently result in a diminished mTOR response because there is a minimum intensity threshold required for maximal muscle hypertrophy (19). In support of this, a dose-response (1 set vs. 3 sets) with resistance exercise has been observed with a diminished MPS response at 5 hours after exercise compared with 3 sets (7). Thus, the reduction in the total volume lifted during the RBM could negatively affect MPS over time.

It is conceivable that we found no difference in leg extension reps because the exercise intensity was lower, i.e., single joint exercise compared with multijoint for squat and leg press (22). One study to date examined the potential training effect of using an RBM in an anaerobic condition over 6 weeks in Reserve Officer Training Corps observing no differences in V[Combining Dot Above]O2peak or Wingate performance (21). However, the authors did note that given the high fitness level and consistent training habits of the participants, they may have physiologically been near their genetic ceiling, potentially explaining why no differences existed.

Although the oxygen saturation data in this study exhibited a significant decrease during the RBM session, physiologically, this is not significant because mild hypoxia is considered ∼94%. Resistance training has an established hypertrophic response (6,14,20), but this response has been observed to be blunted when exposed to hypoxic conditions (10). Most hypoxic resistance training studies have typically used local hypoxia induced by blood flow restriction training and have demonstrated an anabolic response (1,2,4,23,24,28). Conversely, the limited hypoxia studies that have used systematic normobaric hypoxia (∼15% oxygen; ∼SpO2 90%) included untrained male participants and used a lower-intensity resistance training stimulus with only a single exercise examining hormonal responses as opposed to performance variables (12,13). Previous research using a modified RBM demonstrated an SpO2% of 90 during 20 minutes of steady-state treadmill exercise (11). Comparatively, additional research demonstrated no change in SpO2% while using an RBM during 10 minutes of treadmill exercise (17). At an elevation of ∼2700m a SpO2 of ∼89% would be expected, similar to the results of the previously mentioned study (11). These differences in results may be explained by the type of exercise (aerobic vs. anaerobic), duration of exercise, modified mask, and the rest times between sets allowed for participants to recover vs. steady-state exercise. The inclusion of rest periods between sets and exercises and the use of 2 common lower-body exercises in which an individual is seated potentially affected the SpO2% numbers. Potentially, shorter rest periods in an anaerobic paradigm combined with multijoint body weight exercises (squat, pushups, etc.) could increase the rate of rebreathing expired CO2 that was trapped in the dead space of the mask. This could subsequently lower the SpO2% to induce hypoxia. Although studies have used a training mask during chronic exercise training, none has examined oxygen saturation (5,18,21), but it seems unlikely that chronic use during training would affect SpO2%.

This study is in agreement with previous research observing no difference in HR while wearing an RBM during varied exercise modalities (11,17). A similar HR response despite the associated reduction in repetitions for the RBM session suggests that participants experienced similar physiological intensities despite a decrease in training volume and performance. Potentially, an increase in intrathoracic pressure moderately increased HR to maintain the cardiac output, despite a reduced training volume because of the increased resistive pressure of breathing with the RBM (11).

Session rating of perceived exertion was elevated during the RBM session, equating to the session being perceived as more difficult despite completing fewer repetitions during the session and physiologically (through HR response) being the same (Table 3). It is unclear whether the perceived increased difficulty of wearing the mask during exercise caused the significant reduction in repetitions to failure as a deterrent to complete additional repetitions or whether there was a physiological inference blunting performance. There are limited studies to date examining the effect of an RBM on RPE, but one previous study found similar results of increased RPE during exercise with increased physiological variables between sessions (11), whereas another found no effect (17). Participants in the first study (11) observed a reduction in both V[Combining Dot Above]O2 and V[Combining Dot Above]CO2 during the RBM trials despite the intensity being clamped for all sessions, which potentially explains the increase in RPE that was observed. However, in the current investigation, no difference in HR or SpO2% was observed suggesting that other factors may have affected performance, including perceptual.

Elevated levels in perceived stress were observed in the RBM session compared with the no-mask session in both the pre and post time points. The Perceived Stress Scale (10-item) was used to assess participants' day-to-day stress levels to establish whether a stress response was potentially due to the RBM or whether the participants were quantified as high-stress individuals (8,9). None of the participants in the present study qualified as high-stress on the scale (>27), and only 3 participants qualified as moderate stress (14–26). Overall, the mean score was 10.5 ± 6.2 for the current investigation. It can be interpreted that the RBM sufficiently induced an anticipatory stressful event in the participants, and subsequently the participants experienced what was perceived as a stressful event. Participants gauged their perceived stress before and after exercise with a significant difference being found for both pre- and post-exercise between sessions. These results are in agreement with the only other study to date examining stress with an RBM, and it used the Beck Anxiety Inventory during aerobic exercise (11). Potentially, this increased stress negatively affected participant's repetitions to failure with a desire to discontinue exercise although the participants were quantified as low-stress individuals. This is the first study to date to examine the perceived and anticipated stressfulness associated with an RBM during resistance exercise because of the increased difficulty to breathe. Given the perceived increase in stress for acute exercise, it would not be recommended for recreational use. However, further investigation is needed to quantify whether the stress response would be blunted with consistent use of an RBM during resistance training.

Back to Top | Article Outline

Practical Applications

In the current setting, the RBM sufficiently reduced the total repetitions during a single session of lower-body resistance exercise while increasing the perceived difficulty of the exercise session. Moreover, at the device setting used in this study, the RBM failed to significantly induce a hypoxic environment during resistance exercise. Therefore, use of an RBM during resistance exercise sessions is not recommended given the reduction in the training volume with associated increased RPE and stress response. This reduction in training volume could affect strength and hypertrophic adaptations over time while simultaneously exposing individuals to unnecessary emotional and/or physiological stress.

Back to Top | Article Outline

References

1. Abe T, Kearns CF, Sato Y. Muscle size and strength are increased following walk training with restricted venous blood flow from the leg muscle, Kaatsu-walk training. J Appl Physiol 100: 1460–1466, 2006.
2. Abe T, Fujita S, Nakajima T, Sakamaki M, Ozaki H, Ogasawara R. Effects of low-intensity cycle training with restricted leg blood flow on thigh muscle volume and VO2max in young men. J Sports Sci Med 9: 452–458, 2010.
3. Baar K, Esser K. Phosphorylation of p70(S6k) correlates with increased skeletal muscle mass following resistance exercise. Am J Physiol 276: C120–C127, 1999.
4. Beekley MD, Sato Y, Abe T. KAATSU-walk training increases serum bone-specific alkaline phosphatase in young men. Int J KAATSU Training Res 2: 77–81, 2005.
5. Biggs NC, England BS, Turcotte NJ, Cook MR, Williams AL. Effects of simulated altitude on maximal oxygen uptake and inspiratory fitness. Int J Exerc Sci 10: 127–136, 2017.
6. Bodine SC, Stitt TN, Gonzalez M, Kline WO, Stover GL, Bauerlein R, et al. Akt/mTOR pathway is a crucial regulator of skeletal muscle hypertrophy and can prevent muscle atrophy in vivo. Nat Cell Biol 3: 1014–1019, 2001.
7. Burd NA, Holwerda AM, Selby KC, West D, Staples AW, Cain NE, et al. Resistance exercise volume affects myofibrillar protein synthesis and anabolic signaling molecule phosphorylation in young men. J Physiol 588: 3119–3130, 2010.
8. Cohen S, Kamarck T, Mermelstein R. A global measure of perceived stress. J Health Soc Behav 24: 386–396, 1983.
9. Cohen S, Janicki-Deverts D. Who's stressed? Distributions of psychological stress in the United States in probability samples from 1983, 2006, and 2009. J Appl Soc Psych 42: 1320–1334, 2012.
10. Etheridge T, Atherton PJ, Wilkinson D, Selby A, Rankin D, Webborn N, et al. Effects of hypoxia on muscle protein synthesis and anabolic signaling at rest and in response to acute resistance exercise. Am J Physiol Endocrinol Metab 301: E697–E702, 2011.
11. Granados J, Gillum TL, Castillo W, Christmas KM, Kuennen MR. Functional respiratory muscle training during endurance exercise causes modest hypoxemia but overall is well tolerated. J Strength Cond Res 30: 755–762, 2016.
12. Ho JY, Huang TY, Chien YC, Chen YC, Liu SY. Effects of acute exposure to mild simulated hypoxia on hormonal responses to low-intensity resistance exercise in untrained men. Res Sports Med 22: 240–252, 2014.
13. Kon M, Ohiwa N, Honda A, Matsubayashi T, Ikeda T, Akimoto T, et al. Effects of systemic hypoxia on human muscular adaptations to resistance exercise training. Physiol Rep 2: e12033, 2014.
14. Koopman R, Zorenc AG, Gransier RJ, Cameron-Smith D, van Loon LC. Increase in S6K1 phosphorylation in human skeletal muscle following resistance exercise occurs mainly in type II muscle fibers. Am J Physiol Endocrinol Metab 290: 1245–1252, 2006.
15. Kraft JA, Green JM, Bishop PA, Richardson MT, Neggers YH, Leeper JD. Impact of dehydration on a full body resistance exercise protocol. Eur J Appl Physiol 109: 259–267, 2010.
16. Kumar V, Selby A, Rankin D, Patel R, Atherton P, Hildebrandt W, et al. Age-related differences in the dose-response relationship of muscle protein synthesis to resistance exercise in young and old men. J Physiol 587: 211–217, 2009.
17. Maspero M, Smith JD. Effect of an acute bout of exercise using an altitude training mask simulating 12,000 ft on physiological and perceptual variables. Int J Exerx Sci 2, 2016. Article 90. Available at: http://digitalcommons.wku.edu/ijesab/vol2/iss8/90. Accessed February 1, 2017.
18. Porcari JP, Probst L, Forrester K, Doberstein S, Foster C, Cress ML, et al. Effect of wearing the elevation training mask on aerobic capacity, lung function, and hematological variables. J Sports Sci Med 15: 379–386, 2016.
19. Ratamess NA, Alvar BA, Evetoch TK, Housh TJ, Kibler WB, Kraemer WJ, et al. American College of Sports Medicine position stand. Progression models in resistance training for healthy adults. Med Sci Sports Exerc 41: 687, 2009.
20. Sandri M. Signaling in muscle atrophy and hypertrophy. Physiology (Bethesda) 23: 160–170, 2008.
21. Sellers J, Monaghan TP, Jacobson BH, Schnaiter JA, Hester GM, Pope ZK. Efficacy of a ventilatory training mask to improve anaerobic capacity in reserve officers' training corps cadets. J Strength Cond Res 30: 1155–1160, 2016.
22. Simão R, Farinatti PT, Polito MD, Viveiros L, Fleck SJ. Influence of exercise order on the number of repetitions performed and perceived exertion during resistance exercise in women. J Strength Cond Res 21: 23–28, 2007.
23. Takarada Y, Takazawa H, Sato Y, Takebayahi S, Tanaka T, Ishii N. Effects of resistance exercise combined with moderate vascular occlusion on muscular function in humans. J Appl Pyshiol 88: 2097–2106, 2000.
24. Terzis G, Georgiadis G, Stratakos G, Vogiatzis I, Kavouras S, Manta P, et al. Resistance exercise-induced increase in muscle mass correlates with p70S6 kinase phosphorylation in human subjects. Eur J Appl Physiol 102: 145–152, 2008.
25. Utter AC, Robertson RJ, Green JM, Suminski RR, McAnulty SR, Nieman DC. Validation of the Adult OMNI Scale of perceived exertion for walking/running exercise. Med Sci Sports Exerc 36: 1776–1780, 2004.
26. Willoughby DS, Leutholtz B. D-aspartic acid supplementation combined with 28 days of heavy resistance training has no effect on body composition, muscle strength, and serum hormones associated with the hypothalamo-pituitary-gonadal axis in resistance-trained men. Nutr Res 33: 803–810, 2013.
27. Willoughby DS, Spillane M, Schwarz N. Heavy resistance training and supplementation with the alleged testosterone booster nmda has no effect on body composition, muscle performance, and serum hormones associated with the hypothalamo-pituitary-gonadal axis in resistance-trained males. J Sports Sci Med 13: 192–199, 2014.
28. Yasuda T, Abe T, Sato Y, Midorikawa T, Kearns CF, Inoue K, et al. Muscle fiber cross-sectional area is increased after two weeks of twice daily KAATSU-resistance training. Int J KAATSU Train Res 1: 65–70, 2005.
Keywords:

elevation; heart rate; hypoxia; RPE

Copyright © 2018 by the National Strength & Conditioning Association.