High-Intensity Interval Training (HIIT) in Hypoxia Improves Maximal Aerobic Capacity More Than HIIT in Normoxia: A Systematic Review, Meta-Analysis, and Meta-Regression

he present study aimed to determine the effect of high intensity interval training (HIIT) in hypoxia on maximal oxygen uptake (VO 2max) compared with HIIT in normoxia with a Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA)-accordant meta-analysis and meta-regression. Studies which measured VO 2maxfollowing a minimum of 2 weeks intervention featuring HIIT in hypoxia versus HIIT in normoxia were included. From 119 originally identi ed titles, nine studies were included (n= 194 participants). Meta-analysis was conducted on change in (D) VO 2maxusing standardised mean difference (SMD) and a random effects model. Meta-regression examined the relationship between the extent of environmental hypoxia (fractional inspired oxygen [FiO 2]) andDVO 2maxand intervention duration andDVO 2max. The overall SMD forDVO 2max following HIIT in hypoxia was 1.14 (95% CI = 0.56–1.72;p< 0.001). Meta-regressions identi ed no signi cant relationship between FiO 2(coef cient estimate = 0.074,p= 0.852) or intervention duration (coef cient estimate = 0.071,p= 0.423)

examined the relationship between the extent of environmental hypoxia (fractional inspired oxygen [FiO 2]) andDVO 2maxand intervention duration andDVO 2max. The overall SMD forDVO 2max following HIIT in hypoxia was 1.14 (95% CI = 0.56–1.72;p< 0.001). Meta-regressions identi ed no signi cant relationship between FiO 2(coef cient estimate = 0.074,p= 0.852) or intervention duration (coef cient estimate = 0.071,p= 0.423) andDVO 2max. In conclusion, HIIT in hypoxia improved VO 2maxcompared to HIIT in normoxia. Neither extent of hypoxia, nor training duration modi ed this effect, however the range in FiO 2was small, which limits interpretation of this meta-regression. Moreover, training duration is not the only training variable known to in uenceDVO 2max, and does not appropriately capture total training stress or load. This meta-analysis provides pooled evidence that HIIT in hypoxia may be more ef cacious at improving VO 2maxthan HIIT in normoxia. The application of these data suggest adding a hypoxic stimuli to a period of HIIT may be more effective at improving VO 2maxthan HIIT alone. Therefore, coaches and athletes with access to altitude (either natural or simulated) should consider implementing HIIT in hypoxia, rather than HIIT in normoxia where possible, assuming no negative side effects. Keywords:altitude; sprint; training; endurance; VO 2max 1. Introduction 1.1. Rationale A combination of reduced barometric pressure (PB), or a reduced effective inspired fraction of oxygen (FiO2), leads to reduced inspired partial pressure of oxygen (PaO2) which ultimately results in the physiological state of hypoxia [1,2]. In human research concerning hypoxia, hypoxia can be examined through two means, rstly hypobaric hypoxia (PB < 760 mmHg; FiO2= 20.9%) which generally re ects the state found on earth at altitude, and normobaric hypoxia (PB = 760 mmHg; FiO2< 20%) which can be considered simulated altitude [3]. Chronic exposure to natural altitude stimulates renal production of erythropoietin (EPO), driving increased haemoglobin mass (Hb mass) and red blood cell (RBC) count [4,5]. This increases oxygen carrying capacity of the blood, with well reported associations between increased RBC and Hb mass and improved Int. J. Environ. Res. Public Health2022,19, 14261.

[3]. Chronic exposure to natural altitude stimulates renal production of erythropoietin (EPO), driving increased haemoglobin mass (Hb mass) and red blood cell (RBC) count [4,5]. This increases oxygen carrying capacity of the blood, with well reported associations between increased RBC and Hb mass and improved Int. J. Environ. Res. Public Health2022,19, 14261.

Int. J. Environ. Res. Public Health2022,19, 14261 2 of 15 maximal oxygen uptake (VO2max) [6,7]. Therefore, athletes have been recommended to spend prolonged periods of time at moderate (2000–3000 m), to high altitude (>3000 m), to stimulate erythropoiesis [8]. Thus, `altitude training camps' are widely used by professional and recreational athletes alike [9], and the early 1990s saw the popularisation of the `live- high, train-low' (LHTL) paradigm by Levine and Stray- Gundersen [6]. In a pooled analysis of six previous experiments, studies adopting the LHTL and LHTH paradigms have shown improved test performance at sea-level, with a ~3% increase in VO2maxfollowing altitude training compared to control (i.e., normoxic) participants [10]. More recently, hypoxic training methods have been further developed into live high train high (LHTH), intermittent hypoxic exposure at rest, and live low train high (LLTH) [11–13]. In the last decade, high-intensity interval training (HIIT) has become in vogue, evi- denced by the American College of Sports Medicine (ACSM) reporting it as its number one tness trend in 2014, and number two in 2020 [14]. HIIT can be de ned as repeated bouts of high intensity effort followed by varied recovery times. Sprint-interval training (SIT; an `all- out' derivative of HIIT) may also be considered under the HIIT umbrella. HIIT is typically around or below 100% VO2maxwhilst SIT is all-out, an intensity above VO2max[15–22]. Both HIIT and SIT are reportedly ef cacious in improving VO2maxat sea level [19,21–26]. The understanding that exercise intensity is a potent stimuli for improving VO2maxis not a new discovery [27–30]. To these ends, HIIT is generally considered a more potent stimulus than moderate-intensity continuous training (MICT) for improving VO2max(especially over a short intervention duration) [23,25,31–33]. In this context, a recent meta-analysis by Su and colleagues [25] indicated that HIIT in normoxia increased VO2maxmore than MICT in normoxia, in overweight/obese individuals (aged 18–48 years), speci cally when intervals were >2 min in duration, with a standard mean difference of 0.444 (95% con dence intervals [CI] = 0.037–0.851; Small magnitude of effect). Moreover, a seminal RCT [33] observed greater improvement in VO2maxfollowing 4 4 min running with

[25] indicated that HIIT in normoxia increased VO2maxmore than MICT in normoxia, in overweight/obese individuals (aged 18–48 years), speci cally when intervals were >2 min in duration, with a standard mean difference of 0.444 (95% con dence intervals [CI] = 0.037–0.851; Small magnitude of effect). Moreover, a seminal RCT [33] observed greater improvement in VO2maxfollowing 4 4 min running with 3 min rests (Cohen's d= 0.66) and 47 15 s of running with 15 s rests (Cohen'sd= 0.79) at 90–95% heart rate maximum compared to lactate threshold (Cohen'sd= 0.16) and sub-threshold training (Cohen'sd= 0.13). Recently, HIIT training has been combined with hypoxia training with the aim of eliciting optimal training adaptations. LLTH methods allow athletes to continue to live at normoxia, whilst exposed to acute periods of hypoxia during training. Within LLTH there are different training methodologies, including continuous hypoxic training (CPT), interval hypoxic training (IHT), and repeated sprint training in hypoxia (RST). Several original in- vestigations have now been conducted examining either HIIT or RST in hypoxia[12,34–36] . Gatterer et al. [37] published results from a pilot study noting HIIT and RST in hypoxia improved sea-level performance of the repeated sprint ability (RSA) and a Yo-Yo intermit- tent recovery test 2 (YYIR2), in addition to muscle re-oxygenation. Studies investigating physiological adaptations during HIIT in hypoxia have reported that different training methods in hypoxia elicit different training effects, including increased oxidative capacity (CPT), buffering capacity (IHT), and compensatory ber-selective vasodilation (RST), re- spectively [12]. Overall, LLTH methods have been shown to stimulate non-haematological peripheral adaptations, such as muscular adaptations which promote energy metabolism, alongside increased perfusion, improving O2utilisation and delivery, favouring sporting performance [11]. Despite previous reports suggesting training in hypoxia can augment HIIT-induced adaptations in VO2max[37], a systematic review by Hopeller et al. [38] reported hypoxia supplementary to exercise training was not consistently advantageous for performance at sea level. Hamlin et al.'s [39] meta-analysis of HIIT-hypoxia training focusses speci cally on populations participating in team sports, and on high intensity running (Yo-Yo IRT) after a hypoxic interventions. Therefore, there is a need for a meta-analysis with wider inclusion criteria

by Hopeller et al. [38] reported hypoxia supplementary to exercise training was not consistently advantageous for performance at sea level. Hamlin et al.'s [39] meta-analysis of HIIT-hypoxia training focusses speci cally on populations participating in team sports, and on high intensity running (Yo-Yo IRT) after a hypoxic interventions. Therefore, there is a need for a meta-analysis with wider inclusion criteria (i.e., not just team sports players) with VO2maxas the primary outcome variable, as VO2maxis the gold standard of cardiorespiratory tness measurement. Therefore, for

Int. J. Environ. Res. Public Health2022,19, 14261 3 of 15 a more coherent interpretation of the effects of HIIT in hypoxia vs. HIIT in normoxia, a quantitative pooled analysis of previous studies was necessary. 1.2. Objectives Despite the abundance of studies investigating and reviewing hypoxia and HIIT separately, there was a lack of literature focusing on the effects of HIIT in hypoxia on VO2max. Therefore, the aim of this investigation was to conduct a meta-analysis on the effect of HIIT in hypoxia compared to HIIT in normoxia on VO2max. A secondary aim was to investigate study characteristics (i.e., degree of hypoxia, study duration) on magnitude of effect through meta-regressions. 2. Materials and Methods 2.1. Eligibility Criteria This meta-analysis was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines. Studies which met the following criteria were included: (1) full text manuscript; (2) not a review; (3) studies were required to have a control group within normoxic/sea-level environment or include pre-exercise intervention measures; (4) healthy participants of any sex aged 16–65 years; (5) studies were required to employ a HIIT intervention/programme for a minimum of 14 days. Furthermore, studies were required to have reported descriptive data, such as mean, standard deviation (SD), and sample size (n). If required, requests for details and full papers were submitted to the author(s). The primary aim was to investigate whether VO2maxwas affected by HIIT in hypoxia (environmental or simulated). Therefore, only studies which directly measured (i.e., not estimated) VO2max(ml kg min 1 or l min 1 pre- and post-intervention) were included. Within this review both randomised control trials (RCTs) and non-randomised control trials (CTs) were considered. Thus, studies without a control group (i.e., uncontrolled trials with a pre- to post-exposure design) were excluded from analysis. 2.2. Information Sources PubMed, ScienceDirect, and SPORTDiscus were searched with no start date up until 10 February 2021. The search was performed within all elds and terms were “HIIT” AND “hypoxia”, “HIIT” AND “hypoxic”, “HIIT” AND “altitude”, “high-intensity interval train- ing” AND “hypoxia”, “high-intensity interval training” AND “hypoxic”, “high-intensity interval training” AND “altitude”, “Sprint interval training”

were excluded from analysis. 2.2. Information Sources PubMed, ScienceDirect, and SPORTDiscus were searched with no start date up until 10 February 2021. The search was performed within all elds and terms were “HIIT” AND “hypoxia”, “HIIT” AND “hypoxic”, “HIIT” AND “altitude”, “high-intensity interval train- ing” AND “hypoxia”, “high-intensity interval training” AND “hypoxic”, “high-intensity interval training” AND “altitude”, “Sprint interval training” AND “hypoxia”, “Sprint interval training” AND “hypoxic”, and “Sprint interval training” AND “altitude”. 2.3. Study Selection Following searches, obtained manuscripts were downloaded into a single reference manager (Zotero, 2016, Zotero version 5.0.96.1). Prior to eligibility screening the papers were sorted into a single reference list, with duplicates removed. Title and abstracts for all papers were screened for eligibility by two authors (A.W. and L.D.H.) with those that did not meet inclusion criteria excluded. Any disagreement between both reviewers was discussed in a consensus meeting. Out of the remaining manuscripts, those which examined HIIT in hypoxia were collated. Full text manuscripts were screened in depth and compared against inclusion and exclusion criteria. Following full text eligibility screening authors extracted participant data sets (sample size,n; age, mean SD), exercise modality (cycling, running, swimming, etc.), intervention method (HIIT, SIT, multi-component training, and interval training), intervention duration, altitude conditions (FiO2, or height above sea level, e.g., 3000 m) and VO2maxanalysis method (Douglas bag, breath by breath gas analysis). Furthermore, manuscripts were coded as RCTs or CTs (Figure). Subsequently, all remaining papers were assessed against the Physiotherapy Evidence Database (PEDro) scale. The PEDro scale objectively assesses methodological quality of each study [40].

Int. J. Environ. Res. Public Health2022,19, 14261 4 of 15Int. J. Environ. Res. Public Health 2022, 19, x 4 of 16 Subsequently, all remaining papers were assessed against the Physiotherapy Evidence Database (PEDro) scale. The PEDro scale objectively assesses methodological quality of each study [40]. Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) flow di- agram detailing inclusion and exclusion of potential studies as well as final number of studies in- cluded in the systematic review and meta-analysis. RCT = randomised control trial; CT = control trial. 2.4. Data Collection Process Information was imported into a spreadsheet, which was specifically designed for meta-analyses (Jamovi version 2.3.0.0, MAJOR package, https://www.jamovi.org, 2 Octo- ber 2022). Data from both hypoxic and control groups were extracted from manuscripts: Change in (Δ)VO2max (ml·kg·min −1 or l·min −1 ), effective FiO2, intervention duration, and sample size (n). For clarity, in an RCT or CT, the mean and SD ΔVO2max from pre- to post- training in the experimental group and control group, plus the n of each group is entered into the spreadsheet (six data items). Where the mean ΔVO2max was not reported, we sub- tracted pre-training VO2max from post-training VO2max. Where the SD ΔVO2max was not re- ported, it was calculated thusly: Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) ow diagram detailing inclusion and exclusion of potential studies as well as nal number of studies included in the systematic review and meta-analysis. RCT = randomised control trial; CT = control trial. 2.4. Data Collection Process Information was imported into a spreadsheet, which was speci cally designed for meta-analyses (Jamovi version 2.3.0.0, MAJOR package,, 2 Octo- ber 2022). Data from both hypoxic and control groups were extracted from manuscripts: Change in (D)VO2max(ml kg min 1 or l min 1 ), effective FiO2, intervention duration, and sample size (n). For clarity, in an RCT or CT, the mean and SDDVO2maxfrom pre- to post-training in the experimental group and control group, plus thenof each group is entered into the spreadsheet (six data items). Where the meanDVO2maxwas not reported, we subtracted pre-training VO2maxfrom

in (D)VO2max(ml kg min 1 or l min 1 ), effective FiO2, intervention duration, and sample size (n). For clarity, in an RCT or CT, the mean and SDDVO2maxfrom pre- to post-training in the experimental group and control group, plus thenof each group is entered into the spreadsheet (six data items). Where the meanDVO2maxwas not reported, we subtracted pre-training VO2maxfrom post-training VO2max. Where the SDDVO2max was not reported, it was calculated thusly: s change= q s 2 1 +s 2 2 (2 corr s1 s2) whereby:corr= correlation coef cient, a value of which describes the relationship between baseline and nal VO2maxmeasurements over time. We used the correlation coef cient from Lawler et al. [41] (0.94) which was the correlation coef cient of pre-and post-intervention

Int. J. Environ. Res. Public Health2022,19, 14261 5 of 15 VO2maxin a group of trained and untrained male athletes (n= 13). In cases of missing data, authors were contacted via email and asked to provide necessary information. If no response was received, means and SDs were estimated from gures using computer software (Image J, Towson, MD, USA, 2.5. Data Items Standardised mean differences (SMD) expressed the intervention effect within each study [42] using a restricted maximum-likelihood model estimate. For clarity, the mean change in (D) VO2maxin the hypoxic group, the SD ofDVO2maxin the hypoxic group, thenof the hypoxic group, the meanDVO2maxin the normoxic (i.e., control) group, the SD ofDVO2maxin the normoxic group, and thenof the normoxic group were used to calculate SMD. All studies had a control group so no uncontrolled trials were analysed. The alpha level (p) describes the probability of a type I error, and 95% was used as the con dence interval (CI) level. Statistical heterogeneity was quanti ed using theI 2 statistics. AnI 2 value greater than 50% is classi ed as moderate to high between study heterogeneity. Due to the included studies being considered heterogeneous (I 2 = 63%) a random effects meta-analysis was conducted. Funnel plots and the trim and ll method [43] assessed publication bias. The trim and ll method determines the number of studies necessary to eradicate publication bias from the funnel plot. 3. Results 3.1. Study Selection Combined results from the three database searches identi ed 441 articles (Figure). After duplicates were removed a total of 119 titles and abstracts were screened for eligibility using the inclusion and exclusion criteria. We attempted to retrieve 37 records, and 33 re- ports were successfully retrieved and assessed for eligibility. Of the 33 screened, 24 papers were excluded, leaving nine full text manuscripts included within the nal quantitative synthesis. 3.2. Study Characteristics On completion of data pooling, nine studies were included in the analysis: seven were RCTs and two were control trials (Table). Within the nine studies, a total of 194 participants (men = 139, women = 55) were included. Studies were 2–13 weeks

24 papers were excluded, leaving nine full text manuscripts included within the nal quantitative synthesis. 3.2. Study Characteristics On completion of data pooling, nine studies were included in the analysis: seven were RCTs and two were control trials (Table). Within the nine studies, a total of 194 participants (men = 139, women = 55) were included. Studies were 2–13 weeks in duration (Table), and included running, cycling, swimming, or multi-component training. The PEDro scale determined quality of studies, and results indicated a mean score of 4 1.

This systematic review analyzes the effects of HIIT in hypoxia on VO2max.