Intermittent Normobaric Hypoxic Exposures at Rest: Effects on Performance in Normoxia and Hypoxia

RESEARCH ARTICLE

942 Aviation, Space, and Environmental Medicine x Vol. 83, No. 10 x October 2012

M EKJAVIC IB, D EBEVEC T, A MON M, K ERAMIDAS ME, K OUNALAKIS SN. Intermittent normobaric hypoxic exposures at rest: effects on perfor-mance in normoxia and hypoxia. Aviat Space Environ Med 2012 83:942–50.

Introduction: It has been speculated that short (~1-h) exposures to intermittent normobaric hypoxia at rest can enhance subsequent exer-cise performance. Thus, the present study investigated the effect of daily resting intermittent hypoxic exposures (IHE) on peak aerobic capacity and performance under both normoxic and hypoxic conditions. Methods: Eighteen subjects were equally assigned to either a control (CON) or IHE group and performed a 4-wk moderate intensity cycling exercise training (1 h z d 2 1 , 5 d z wk 2 1 ). The IHE group additionally performed IHE (60 min) prior to exercise training. IHE consisted of seven cycles alternating between breathing a hypoxic gas mixture (5 min; F I O 2 5 0.12 – 0.09) and room air (3 min; F I O 2 5 0.21). Normoxic and hypoxic peak aerobic capacity (

.V O 2peak ) and endurance performance were evalu-

ated before (PRE), during (MID), upon completion (POST), and 10 d after (AFTER) the training period. Results: Similar improvements were observed in normoxic

.V O 2peak tests in both groups [IHE: D (POST-PRE) 5 1 10%;

CON: D (POST-PRE) 5 1 14%], with no changes in the hypoxic condition. Both groups increased performance time in the normoxic constant power test only [IHE: D (POST-PRE) 5 1 108%; CON: D (POST-PRE) 5 1 114%], whereas only the IHE group retained this improvement in the AFTER test. Higher levels of minute ventilation were noted in the IHE compared to the CON group at the POST and AFTER tests. Conclusion: Based on the results of this study, the IHE does not seem to be benefi cial for normoxic and hypoxic performance enhancement. Keywords: intermittent hypoxemia , endurance training , hypoxic accli-mation , performance .

IT IS NOT UNCOMMON for athletes to perform in-novative training protocols to enhance performance

and the use of different hypoxic training protocols is gaining popularity. However, since the most-often-used normobaric hypoxic training protocol requires at least 4 wk of daily exposures lasting over 12 h z d 2 1 ( 26 ), there is an ongoing pursuit for new, shorter training regimens. The application of relatively short (1 h z d 2 1 ) intermittent normobaric hypoxic exposures (IHE) of relatively high levels of hypoxia at rest (F I o 2 5 0.13 – 0.09) has been sug-gested as a plausible option. IHE has been proposed as a treatment for asthma ( 32 ) and hypertension ( 31 , 33 ), as well as a method for enhancing athletic performance ( 1 ). The main advantage of the IHE protocol over longer hy-poxic/altitude exposures is the shorter exposure time and the simplicity of application. Recent technological advancements have enabled the development of small portable hypoxic devices that made these protocols available to a broad range of users, including athletes ( 35 ). The ability of normobaric IHE to enhance both hy-poxic and normoxic exercise performance and working

ability has direct application in occupational and sports related fi elds. In particular, it can serve as a preacclima-tization strategy for military units and mountain rescue services prior to deployment to high altitude regions ( 24 ). Moreover, IHE could be utilized as a method for enhancing the effects of exercise training and subse-quently improving sports performance at sea level, with as little disturbance to normal daily lifestyle as possible ( ! 1 h z d 2 1 ).

Despite the technological improvements, the effects of IHE on normoxic exercise performance remain ambigu-ous and controversial. Few studies have reported bene-fi cial improvements following IHE in sprint performance ( 22 ), maximal speed ( 36 ), or 3000-m time trial perfor-mance ( 10 ). Moreover, a positive effect of IHE on muscle and systemic oxygenation at maximal exercise intensi-ties has also been suggested ( 12 ). However, the majority of the studies have not detected any benefi ts of IHE on exercise performance in normoxia ( 1 ). The reasons for the disparity in the outcomes reported by different stud-ies include different protocol durations, various hypoxia levels used, and a possible placebo/nocebo effect ( 3 ). There is an obvious lack of data regarding the effect of IHE on hypoxic performance ( 11 ). Hamlin and col-leagues ( 11 ) reported no benefi cial changes for selected performance measures (i.e., 6 3 70 m sprints, maximum speed) at 1550 m altitude following 13 IHE sessions. Since repeated exposures to normobaric hypoxia have been shown to enhance hypoxic ventilatory response ( 34 ), we hypothesized that this increase would occur follow-ing the tested IHE protocol and would subsequently en-hance performance, especially in hypoxic conditions.

From the Department of Automation, Biocybernetics and Robotics, “ Jozef Stefan ” Institute, and Jozef Stefan International Postgraduate School, Ljubljana, Slovenia; Department of Environmental Physiology, School of Technology and Health, Royal Institute of Technology, Stockholm, Sweden; and Human Performance-Rehabilitation Labora-tory, Faculty of Physical and Cultural Education, Hellenic Military University, Vari, Greece.

This manuscript was received for review in February 2012 . It was accepted for publication in April 2012 .

Address correspondence and reprint requests to: Prof. Igor B. Mekjavic, Department of Automation, Biocybernetics and Robotics, Jozef Stefan Institute, Jamova 39, SI-1000, Ljubljana, Slovenia; [email protected].

Reprint & Copyright © by the Aerospace Medical Association, Alexandria, VA.

DOI: 10.3357/ASEM.3332.2012

Intermittent Normobaric Hypoxic Exposures at Rest: Effects on Performance in Normoxia and Hypoxia

Igor B. Mekjavic , Tadej Debevec , Mojca Amon , Michail E. Keramidas , and Stylianos N. Kounalakis

Aviation, Space, and Environmental Medicine x Vol. 83, No. 10 x October 2012 943

INTERMITTENT HYPOXIA AT REST — MEKJAVIC ET AL.

There is also a paucity of data concerning the time course of changes in performance during the deaccli-matization following IHE. Wood and colleagues ( 36 ) showed that following IHE the substantial benefi cial ef-fects on performance were still present 9 d following the cessation of the protocol. A signifi cant decline was shown 3 wk following the continuous intermittent exposures protocol (90 min z d 2 1 3 3 wk) ( 18 ).

We therefore investigated the effects of the IHE proto-col on peak aerobic capacity and time to exhaustion under normoxic and hypoxic conditions, during, post-, and 10-d after the protocol. To eliminate the possible ef-fects of the different training activity (i.e., athletic training) on functional test outcomes, all participants perfor med the same endurance exercise training. We hypothe-sized that long-term IHE in addition to exercise training would provide benefi ts for performance; mainly through enhanced chemo-sensitivity induced ventilatory adap-tations, in both normoxia and hypoxia, as compared to exercise training alone. To our knowledge, this is the fi rst study to date that investigated the effects of IHE on both hypoxic and normoxic performance during and af-ter the IHE protocol.

METHODS

Subjects

Eighteen healthy, young, male subjects were re-cruited for this study. All subjects were free of lung and heart diseases, and were not anemic. Subjects gave their written consent after being familiarized with the study design and the experimental protocols, and in-formed of the risks involved. The protocol was ap-proved by the National Committee for Medical Ethics at the Ministry of Health (Republic of Slovenia) and conformed to the guidelines of the Helsinki declaration. Subjects were instructed to maintain their normal diet and to refrain from alcohol and nicotine during the test-ing periods. Upon selection, the subjects were balanced according to peak aerobic capacity and age and equally assigned to either the control (CON; N 5 9; age 5 22.1 6 4.1 yr; height 5 179.3 6 4.9 cm; body mass 5 72.9 6 9.7 kg; body fat 5 10.4 6 2.9%; V� o 2peak 5 45.8 6 6.4 ml z kg 2 1 z min 2 1 ) or intermittent hypoxic exposure group (IHE; N 5 9; age 5 22.2 6 3.8 yrs; height 5 181.1 6 7.3 cm;

body mass 5 74.2 6 5.7 kg; body fat 5 11.3 6 4.7%; V� o 2peak 5 49.3 6 10.1 ml z kg 2 1 z min 2 1 ).

Procedure

The study outline is presented in Fig. 1 . The experi-mental protocol was comprised of 20 training sessions performed 5 times per week over a 4-wk period for both groups. Tests were performed before (PRE), following 10 training sessions (MID), at the end of the training (POST), and 10 d after the cessation of training (AFTER) and included the following: a) pulmonary function as-sessment; b) hematological tests; c) peak aerobic ca-pacity tests ( V� o 2peak ) in normoxia and hypoxia; and d) constant power tests to exhaustion (CPT) in normoxia and hypoxia ( Fig. 1 ). Each exercise test was performed under similar environmental conditions in the same sea level laboratory (Valdoltra Orthopedics Hospital, Ankaran, Slovenia). The CPTs were performed in a ran-dom and counterbalanced order. On each test day, sub-jects performed V� o 2peak in the morning and CPT in the afternoon. All tests were conducted at the same time of the day for each individual subject to avoid diurnal vari-ations. During the 10-d post protocol period, subjects refrained from any physical training, and followed their normal daily routines.

Training

The training protocol consisted of endurance exercise training performed by both groups, and IHE protocol performed by the IHE group only. The IHE protocol was performed prior to each exercise training session.

All subjects performed endurance exercise training on a cycle ergometer (Bike forma, Technogym, Cesena, Italy). Each training session consisted of a 5-min warm up period, 60-min cycling, and a 5-min cool down period. The intensity of the exercise was maintained so that the exercise heart rate (HR) corresponded to the HR mea-sured at 50% of PRE normoxic peak power output (W peak ). The warm up and cool down periods were per-formed at 60 W. During each training session the sub-ject ’ s HR was constantly monitored and recorded with a telemetric system (Hosand TMpro w , Verbania, Italy). The HR monitoring system provided continuous super-vision of the previously determined individual training

Fig. 1. Study outline. Subjects in the control (CON) and intermittent hypoxic exposure (IHE) groups participated in a series of tests conducted before (PRE), after 10 training sessions (MID), at the end (POST), and 10 d after the training period (AFTER). These included hematology, pulmonary function, anthropometry,

.V O 2peak , and constant power test (CPT). The latter two were conducted under normoxic and hypoxic conditions at a laboratory situated at

sea level. The order of the conditions (normoxia and hypoxia) for the CPT test was counterbalanced.



HR range. The individual range was set within 6 4 bpm of the targeted HR measured during the fi rst V� o 2peak test in normoxia. In the event that the subject ’ s HR was out-side the set HR interval a visual signal prompted the in-vestigator to either decrease or increase the work rate accordingly. Ratings of perceived exertion (RPE) were also reported during training sessions by all subjects using a modifi ed (0-10) Borg scale ( 4 ). The training load was calculated for each individual at each training ses-sion using a heart rate based TRIMP score ( 13 ).

The IHE protocol consisted of 20 IHE sessions over a 4-wk period. During the IHE sessions subjects rested in a seated position. Each session comprised a 4-min pe-riod during which subjects breathed room air, followed by seven IHE cycles. Each IHE cycle consisted of a 5-min period of breathing a hypoxic mixture and a 3-min pe-riod of breathing room air. The level of hypoxia used was based on values used in previous studies ( 10 , 35 ) and was gradually reduced every fi ve training sessions ( Table I ). The inspiratory side of the mask was con-nected to a portable hypoxic gas generator (Everest Summit, Hypoxico, New York, NY) with a Universal mask kit and adjustable high altitude adapter. During each session, the level of F I o 2 in the hypoxic mixture was additionally monitored with an oxygen analyzer (Hy-poxico, New York, NY) attached to the O 2 monitor port on the tube connecting the hypoxic generator with the mask. Capillary oxyhemoglobin saturation (S p o 2 ) was continuously monitored and recorded with a fi nger oxy-meter (Nellcor, BCI 3301, Boulder, CO). The fi nger oxy-metry device had an accuracy of 6 2 units across the range of 70 – 100%, with an acceptable resilience to motion artifact ( 21 ). During each hypoxic period of IHE the sub-jects reported RPE according to a modifi ed Borg scale.

Testing

Subjects ’ body mass (BM), height, and body fat (BF) were measured at PRE and POST testing periods. Body fat was estimated from nine skinfold measurements (subscapular, chest, triceps, suprailiac, abdominal, lower thigh, mid thigh, upper thigh, and inguinal site) accord-ing to the equation of Jackson and Pollock ( 16 ). Pulmonary function was assessed at each testing period ( Fig. 1 ). Forced vital capacity (FVC), forced expiratory volume in

the fi rst second (FEV 1 ), slow vital capacity (SVC), peak expiratory fl ow (PEF), and maximum voluntary venti-lation (MVV) were measured using a pneumotacho-graph (Cardiovit AT-2plus, Schiller, Baar, Switzerland). All tests were performed according to the guidelines published by Miller et al. ( 23 ). The spirometer was calibra ted with a 3-L syringe before each test. Tests were repeated three times and the highest of the three acceptable values obtained was used for the subse-quent analysis.

The blood samples were taken in the morning of the fi rst test day during each test period, as shown in Fig. 1 . Subjects were overnight fasted and blood samples were drawn from the antecubital vein. All samples were ana-lyzed for red blood cell count (RBC), hemoglobin (Hb), hematocrit (Hct), transferrin, and ferritin. For hemogram and transferrin analysis the cythochemical impedance method (Pentra120; Horiba ABX Diagnostics, Montpellier, France) and the turbobidiametrical method (Hitachi 912; Roche Diagnostics, Basel, Switzerland) were used, respec-tively. The subjects did not receive iron supplementation, since the levels of all subject ’ s ferritin were, and remained, above 30 ng z ml 2 1 throughout the experimental period.

Each subject performed two incremental exercise tests to exhaustion ( V� o 2peak ) on an electrically braked cycle-ergometer (ERG 900S, Schiller, Baar, Switzerland). Dur-ing the normoxic test, subjects inspired ambient air (F I o 2 5 0.21; V� o 2peak NORMO ) and during the hypoxic test they inspired a humidifi ed hypoxic gas mixture (F I o 2 5 0.12; V� o 2peak HYPO ) from a 200-L Douglas bag. During all V� o 2peak tests, the subjects breathed through a two-way valve (Model 2, 700 T-Shape, Hans Rudolph, Shawnee, KS), while their oxygen uptake ( V� o 2 ) and ventilation ( V� E ) were measured breath by breath using a metabolic cart (CS-200, Schiller, Baar, Switzerland) and averaged over a 10-s period. The pneumotachograph was cali-brated with a 3-L syringe prior to each test; the gas ana-lyzers were calibrated with two different standard gas mixtures. During the tests, the subjects provided ratings of peripheral (RPE leg ) and central (RPE cen ) sensation of effort on a modifi ed Borg scale (0-10).

The testing protocol consisted of a 10-min resting pe-riod, during which subjects breathed room air. During the hypoxic tests, the rest period comprised two 5-min periods, whereby subjects breathed room air during the fi rst period and the hypoxic gas mixture in the second period. The rest period was followed by a 2-min warm-up at a work rate of 60 W. Thereafter, the work rate was in-creased by 30 W each minute until the subjects could no longer sustain the predetermined pedaling cadence. The criteria for the attainment of the V� o 2peak values that were calculated as the highest average sampled for 60-s were: pedalling cadence , 60 rpm, plateau in V� o 2 , and respi-ratory exchange ratio . 1.1.

Additionally, two constant power tests (CPT) were performed at each test period. In one, subjects breathed ambient air (CPT NORMO ; F I o 2 5 0.21), and in the other a humidifi ed hypoxic mixture (CPT HYPO ; F I o 2 5 0.12). After a 2-min resting period (either in normoxia or hy-poxia for NORMO or HYPO tests, respectively), the

TABLE I. WEEKLY (5 SESSIONS) AVERAGE FRACTION OF INSPIRED OXYGEN (F I O 2 ), CAPILLARY OXYGEN SATURATION (S p O 2 ), HEART RATE (HR), AND RATINGS OF PERCEIVED EXERTION (RPE) OF THE

IHE GROUP DURING THE INTERMITTENT HYPOXIC EXPOSURE SESSIONS.

IHE Sessions

1 - 5 6 - 10 11 - 15 16 - 20

F I O 2 (%) 0.12 0.11 0.10 0.09 S p O 2 (%) 81 6 3 75 6 1 * 73 6 1 * 72 6 1 * , † HR (bpm) 78 6 2 78 6 2 78 6 1 78 6 3 RPE 2 (1 – 4) 2 (1 – 5) 3 (1 – 8) * 3 (1-7) *

Values are mean 6 SD. RPE values are median (range). * Signifi cantly different from wk 1 ( P , 0.05). † Signifi cantly different from wk 2 ( P , 0.05).



subjects performed 2 min of warm-up (60 W) followed by cycling to exhaustion at the predetermined work-load. The exercise workload corresponded to 80% of the PRE V� o 2 peak NORMO for both tests. The number of sec-onds the subject was able to maintain the pedaling ca-dence above 60 rpm determined the fi nal result. During all tests, HR and S p o 2 were continuously monitored with a heart rate monitor (Vantage NVTM, Polar Electro Oy, Kempele, Finland) and a pulse oxymeter (Nellcore, BCI 3301, Boulder, CO), respectively.

Statistical Analysis

All analyses were performed using Statistica 5.0 (StatSoft, Inc., Tulsa, OK). Training variables and group characteristics were compared using a Student’s t -test. The differences in the protocol outcomes were asse-ssed using a 3-way analysis of variance (ANOVA) (group 3 condition 3 testing period). A 4-way ANOVA was employed to analyze the relative values of se-lected variables during the V� o 2 peak and CPT tests (group 3 condition 3 testing period 3 time). A post hoc test (Tukey HSD) was used to identify the specifi c dif-ferences when a signifi cant main effect or interaction was noted. Due to technical problems encountered on four hypoxic PRE V� o 2 peak tests, we used a regression model to predict the missing PRE V� o 2 hypoxic values from the power output- V� o 2 relationship produced from the normoxic and the available hypoxic tests. All data are reported as means 6 SD unless otherwise in-dicated. The alpha level of signifi cance was set a priori at 0.05.

RESULTS

Training

The mean absolute workload (IHE 5 160 6 26 W; CON 5 159 6 19 W) and HR (IHE 5 145 6 9 bpm; CON: 143 6 8 bpm) during the training period were similar for both groups. The average training power output increment between the fi rst and the last training session was 6% and 7% for the IHE and CON group,

respectively. The calculation of the training load based on the TRIMP scores showed no differences between groups, with average values of 31.1 6 3.1 for the CON and 29.9 6 5.9 for the IHE group.

Fig. 2. A) Normoxic and B) hypoxic peak oxygen uptake ( .V O 2peak )

before (PRE), after 10 training sessions (MID), at the end (POST), and 10 d after the training period (AFTER) for the control (CON) and intermit-tent hypoxic exposure (IHE) groups. Values are means 6 SD. Asterisks denote statistically signifi cant differences from PRE values (* P , 0.05).

TABLE II. RESULTS OF PULMONARY FUNCTION AND HEMATOLOGICAL TESTS CONDUCTED BEFORE (PRE), IN THE MIDDLE (MID), AT THE END (POST), AND 10 DAYS AFTER (AFTER) THE TRAINING PERIOD FOR THE CONTROL (CON) AND EXPERIMENTAL

(IHE) GROUP.

CON Group IHE Group

PRE MID POST AFTER PRE MID POST AFTER

FVC (L) 5.6 6 0.9 5.5 6 1.2 5.6 6 1.1 5.5 6 1.7 5.7 6 0.8 5.7 6 0.9 5.8 6 0.7 5.7 6 0.9 FEV 1 (L) 4.9 6 0.5 4.6 6 0.9 4.7 6 0.8 4.7 6 1.3 5 6 0.6 4.9 6 0.7 4.8 6 0.6 4.8 6 0.8 SVC (L) 5.1 6 0.8 5.0 6 0.8 5.6 6 1.3 5.4 6 1.6 5.5 6 0.8 5.3 6 0.8 5.6 6 0.8 5.1 6 0.9 PEF (L) 8.9 6 2.2 10.3 6 1.7 * 10.1 6 1.5 9.7 6 1.4 10.9 6 1.3 † 10.5 6 1.3 11.1 6 1.7 10.4 6 1.6 MVV (L z min 2 1 ) 183 6 34 187 6 34 185 6 29 172 6 75 199 6 32 204 6 36 205 6 39 200.9 6 36 RBC (10 2 12 z L 2 1 ) 5.0 6 0.3 5.1 6 0.3 4.8 6 0.2 5.0 6 0.2 5.1 6 0.3 5.1 6 0.3 5.1 6 0.2 5.2 6 0.15 Hb (g z L 2 1 ) 15.1 6 1.0 15.5 6 0.9 14.3 6 1.2 14.7 6 1.3 15.1 6 0.8 14.5 6 0.9 15.0 6 0.7 15.1 6 0.6 Hct (%) 45 6 3.0 47 6 2.2 44 6 3.0 46 6 4.0 46 6 3.0 46 6 2.0 46 6 3.0 46 6 2.0 Transferrin (g z L 2 1 ) 2.7 6 0.3 2.5 6 0.4 2.7 6 0.2 3.2 6 1.5 2.8 6 0.2 2.6 6 0.1 2.6 6 0.1 2.6 6 0.2 Ferritin (ng z ml 2 1 ) 71.1 6 33.4 67.9 6 66.1 62.2 6 30.8 68.2 6 42.1 79.5 6 44.4 81.6 6 27.8 82.6 6 44.6 89.7 6 36

Values are mean 6 SD; † Signifi cant ( P , 0.05) differences between groups; * Signifi cantly ( P , 0.05) different from PRE test values. FVC, forced vital capacity; FEV 1 , forced expiratory volume in 1 s; SVC, slow vital capacity; PEF, peak expiratory fl ow; MVV, maximum voluntary ventilation; RBCs, Erythrocytes; Hb, Hemoglobin; Hct, Hematocrit.



The average S p o 2 was 75.5 6 5% during the hypoxic exposure bouts, and 95 6 2% during the normoxic bouts. The values of S p o 2 during IHE decreased throughout the training period, concomitantly with the decreasing F I o 2 in the inspired breathing mixture ( Table I ). The ratings of perceived exertion were signifi cantly higher during the last 10 training sessions ( t , 0.05). No changes were observed in HR between sessions.

Testing

There were no changes in the % BF between or within both groups. The CON group had signifi cantly higher BM in the POST compared to the PRE test ( t , 0.05). It is noteworthy that the CON group had signifi cantly lower BM compared to the IHE group before the train-ing protocol. PEF was signifi cantly elevated during the MID test in the CON group (F 5 4.5; P , 0.05). No other signifi cant changes in pulmonary function were noted during and after the protocol ( Table II ).

No signifi cant differences were found within the groups at different testing periods in any of the measured he-matological variables ( Table II ). Also, there were no sig-nifi cant interactions between groups.

V� o 2peak results are presented in Fig. 2 . There was no difference between groups in V� o 2peak NORMO and V� o 2peak HYPO prior to the training protocol. V� o 2peak NORMO was signifi cantly increased in both groups over the course of the training regimen (F 5 3.1; P , 0.05). No signifi cant changes were observed in the V� o 2peak HYPO during the training in either group.

Peak values of the measured cardio-respiratory vari-ables during the V� o 2peak tests are presented in Table III . The only signifi cant increase in V� E peak was noted in the AFTER V� o 2peak NORMO test for the IHE group (F 5 4.2; P , 0.05). HR peak decreased signifi cantly during V� o 2peak NORMO in the IHE group only at the MID test (F 5 3.8; P , 0.05). W peak increased signifi cantly during V� o 2peak NORMO in both

groups in the POST and AFTER tests (F 5 7.1; P , 0.05). No changes were observed in the W peak during the V� o 2peak HYPO test. No signifi cant differences were observed between or within groups in S p o 2 and RPE leg peak . RPE cen peak was signifi cantly lower in the IHE group at the MID testing only during both V� o 2peak NORMO and V� o 2peak HYPO tests (F 5 3.9; P , 0.05).

There was no difference between groups in time to ex-haustion for both CPT NORMO and CPT HYPO before the training protocol ( Fig. 3 ). Both groups signifi cantly im-proved CPT NORMO time at the POST test (F 5 13.9; P , 0.01). Only the IHE group maintained this improvement in the AFTER test (F 5 13.9; P , 0.01) ( Fig. 3 ). No signifi -cant differences were noted in the CPT HYPO results of both groups during the course of the protocol ( Fig. 3 ). Peak values of V� o 2 , S p o 2 , RPE leg , and RPE cen during both CPT NORMO and CPT HYPO were comparable between groups throughout the experimental protocol ( Table IV ). Signifi cant differences were noted in HR peak during the POST CPT NORMO and MID CPT HYPO in the CON and IHE group (F 5 5.2; P , 0.05), respectively.

During the POST and AFTER CPT NORMO tests, a signifi -cantly greater V� E was noted in the IHE compared to the CON group at 60%, 80%, and 100% relative CPT time (F 5 4.5; P , 0.05), respectively ( Fig. 4 ). Comparable in-creases in V� E of both groups were noted at MID, POST, and AFTER CPT NORMO at 20% relative CPT time (F 5 7.3; P , 0.01). A signifi cant decrease in V� E at 80% and 100% of relative CPT NORMO time during the POST testing was noted in the CON group only (F 5 7.3; P , 0.01) ( Fig. 4 ). No differences in relative V� E values were observed within or between groups during the CPT HYPO tests.

DISCUSSION

The present study investigated the effect of repeated intermittent hypoxic exposures at rest, combined with moderate intensity exercise training on V� o 2peak and CPT

TABLE III. PEAK VALUES OF MINUTE VENTILATION ( .V E peak ), HEART RATE (HR peak ), POWER OUTPUT (W peak ), CAPILLARY OXYGEN SATURA-

TION (S p O 2 ), AND PERIPHERAL (RPE leg peak ) AND CENTRAL (RPE cen peak ) RATINGS OF PERCIEVED EXERTION DURING NORMOXIC AND HYPOXIC

.V O 2peak TESTS BEFORE (PRE), IN THE MIDDLE (MID), AT THE END (POST), AND 10 DAYS AFTER (AFTER) THE TRAINING PERIOD

FOR THE CONTROL (CON) AND EXPERIMENTAL (IHE) GROUP.

CON Group IHE Group

PRE MID POST AFTER PRE MIDDLE POST AFTER

.V O 2peak NORMO

.V E peak (L z min 2 1 ) 132 6 18 140 6 19 139 6 21 141 6 13 135 6 30 143 6 30 147 6 28 153 6 30 * HR peak (bpm) 190 6 8 187 6 4 186 6 6 188 6 7 189 6 8 181 6 7.3 * 186 6 7 187 6 8 W peak (W) 294 6 35 309 6 29 322 6 34 * 327 6 31 * 314 6 54 324 6 45 337 6 53 * 342 6 59 * S p O 2 (%) 89 6 8 94 6 2 93 6 6 95 6 2 91 6 3 92 6 5 92 6 6 91 6 5 RPE leg peak 8 (6-10) 9 (7-10) 9 (8-10) 9 (7-10) 10 (7-10) 8 (7-10) 9 (7-10) 10 (7-10) RPE cen peak 7 (4-9) 7 (4-9) 8 (3-10) 8 (3-10) 9 (7-10) 7 (5-9) * 8 (5-10) 8 (5-10)

.V O 2peak HYPO

.V E peak (L z min 2 1 ) 130 6 20 121 6 23 125 6 20 125 6 17 140 6 16 137 6 19 142 6 23 145 6 26 HR peak (bpm) 182 6 8 177 6 6 175 6 6 174 6 6 181 6 7 175 6 7 177 6 7 178 6 11 W peak (W) 250 6 19 247 6 28 247 6 34 244 6 25 271 6 38 263 6 37 280 6 36 283 6 43 S p O 2 (%) 76 6 6 73 6 4 74 6 5 74 6 5 73 6 6 74 6 5 73 6 5 74 6 6 RPE leg peak 7 (1-10) 8 (2-10) 8 (6-10) 8 (7-10) 10 (7-10) 8 (7-10) 8 (7-10) 9 (5-10) RPE cen peak 7 (3-10) 8 (5-10) 8 (3-10) 9 (2-10) 8 (7-10) 7 (4-10) * 8 (5-10) 9 (7-10)

Values are mean 6 SD. RPE values are median (range). * Signifi cantly ( P , 0.05) different from PRE test values.



in normoxia and hypoxia. The results demonstrate no effect of IHE per se on aerobic capacity and endurance performance in normoxia and hypoxia.

Improvements in normoxic V� o 2peak following similar or even lower (45% HR peak ) training intensity protocols have already been demonstrated in healthy young adults ( 9 ). Our results show similar improvements in V� o 2peak NORMO for both the CON and IHE groups ( Fig. 2 ), thus indicating that IHE did not provide any additional benefi ts for sea level aerobic capacity compared to en-durance training per se. Similarly, we observed no addi-tive effect of IHE on endurance performance compared to the exercise training per se at the MID and POST training periods. This is in line with the results of stud-ies using trained athletes showing no additive effects of IHE on normoxic V� o 2peak , running economy ( 17 , 35 ), and 5000-m running performance ( 15 ). On the other hand, the study by Hamlin ( 10 ) showed improvements in 3000-m running time, likely to be benefi cial for non-elite athletes. Nevertheless, their conclusions have also been questioned by Bartsch ( 1 ), who attributed the measured

effect to the inhomogeneous distribution of performance level and gender between the groups, rather than the IHE effects per se. The intriguing fi nding of this study is the retention of the improved performance at the AFTER testing that was only seen in the IHE group. Our fi ndings regarding the postprotocol period are in accordance with the fi ndings of Wood ( 36 ), who showed that benefi ts of IHE were still present 9 d after cessation of IHE. They showed improved sprint performance, reduced exercise blood lactate ([La]) and HR levels POST and AFTER the IHE protocol, thus indicating that IHE can provide ben-efi ts for high intensity running performance. Although these results were promising, they were not confi rmed in a subsequent study performed by the same research group ( 14 ) that showed no benefi t of IHE over compara-ble training and a placebo IHE protocol. On the other hand, Katayama et al. ( 18 ) showed that 3 wk after a IHE protocol (90 min 3 3 z wk 2 1 3 3-wk at ~4500 m), all ben-efi ts were diminished. Therefore, according to our results and those of Katayama et al. ( 18 ), we assumed that the potential benefi ts of IHE could be present for 10 d, but no more than 20 d after the cessation of the intervention.

Although some studies investigating the effects of dif-ferent intermittent hypoxic protocols on performance showed possible benefi ts for hypoxic performance ( 2 ), this was not confi rmed by our study. Namely, no signifi cant improvements in V� o 2peak HYPO or CPT HYPO tests were found in either group during and after the protocol. Our results are in agreement with the fi ndings of Hamlin ( 11 ), who tested the effects of a similar IHE protocol on performance at altitude. The tests were performed at a moderate altitude (1550 m), but suffi ciently high to sig-nifi cantly impair performance in game specifi c tests of rugby players. They found that the IHE protocol pro-vided some benefi ts on submaximal HR and [La] during hypoxic exercise, but induced no changes in the game specifi c test performance variables. Our results do not support the rationale that IHE provides benefi ts for per-formance and working ability in hypoxic conditions and thus cannot be recommended as a preacclimatization model prior to high altitude deployment. Moreover, a recent study also confi rmed that 1 h of continuous se-vere normobaric hypoxia (F I o 2 5 0.12), even if combined with low intensity exercise training, does not provide benefi ts for hypoxic performance ( 6 ).

The unchanged hematological parameters outcome of our study is in line with other studies ( 1 , 17 , 35 ), even though increases in Hb, Hct, and reticulocyte concentra-tions following short term intermittent hypoxia have been shown in one study ( 10 ). It would appear that short intermittent hypoxic exposures do not provide hema-tological benefi ts ( 29 ), most probably due to the insuf-fi cient hypoxic dose ( 1 ). Although hypoxic training modalities mostly focus on erythropoiesis augmenta-tion and subsequent increased total hemoglobin mass, considering this as the main aspect accountable for im-proved performance, other benefi cial factors have also been proposed ( 8 ). These include: ventilatory adjust-ments, improved muscle effi ciency, greater muscle buff-ering capacity and improved lactic acid tolerance ( 1 , 8 ).

Fig. 3. A) Normoxic and B) hypoxic endurance times determined with a constant power (CP) test before (PRE), after 10 training sessions (MID), at the end (POST), and 10 d after the training period (AFTER) for the control (CON) and intermittent hypoxic (IHE) groups. Values are means 6 SD. ** Statistically signifi cant differences from PRE values ( P , 0.01).



The effect of IHE on ventilatory regulation and che-mosensitivity seem to be the main underlying mecha-nism for the potential benefi cial adjustments ( 34 ). According to our fi ndings, the prolongation of improved performance at sea level can be attributed to ventilatory adaptation resulting in increased V� E in the POST and AFTER CPT NORMO tests ( Fig. 4 ). However, this fi nding is

in contrast with the results of Katayama ( 20 ). They showed that continuous IHE did not provide any venti-latory benefi ts during consequent sea level exercise, whereas in our study we found signifi cant increases in maximal and submaximal V� E during the normoxic CPT test following IHE. The different results can probably be ascribed to the different hypoxic doses, since both the

Fig. 4. Minute ventilation ( .V E ) during the normoxic (CPT NORMO ) and hypoxic (CPT HYPO ) constant power test at the same relative endurance time

for the control (CON) and intermittent hypoxic exposure (IHE) groups. Values are means 6 SD. Statistically signifi cant ( P , 0.05) within group (* PRE and MID, ** PRE and POST, *** PRE and AFTER) and between group (## POST, # AFTER) differences are noted. For the purpose of clarity, SDs are not shown.

TABLE IV. PEAK VALUES OF OXYGEN UPTAKE ( .V O 2peak ), HEART RATE (HR peak ), CAPILLARY OXYGEN SATURATION (S p O 2 ), PERIPHERAL

(RPE leg peak ) AND CENTRAL (RPE cen peak ) RATINGS OF PERCIEVED EXERTION DURING THE CPT NORMO AND CPT HYPO TESTS, BEFORE (PRE), IN THE MIDDLE (MID), AT THE END (POST), AND 10 d AFTER (AFTER) THE TRAINING PERIOD FOR THE CONTROL (CON) AND EXPERIMENTAL

(IHE) GROUP.

CON Group IHE Group

PRE MID POST AFTER PRE MIDDLE POST AFTER

CPT NORMO

.V O 2peak (ml z kg 2 1 z min 2 1 ) 45.3 6 6.3 48.4 6 5.6 43.8 6 8.7 46.1 6 5.7 49 6 6.3 53.3 6 6.8 51.7 6 6.4 51.2 6 8.3 HR peak (bpm) 190 6 6 187 6 8 182 6 5 * 185 6 7 187 6 9 180 6 8 185 6 9 184 6 11 S p O 2 (%) 93 6 2 94 6 2 94 6 1 94 6 1 94 6 4 94 6 4 94 6 2 93 RPE leg peak 8 (7-10) - 9 (7-10) 8 (7-10) 9 (8-10) 8 (7-10) 9 (8-10) 9 (7-10) RPE cen peak 8 (8-10) - 8 (6-10) 8 (8-10) 8 (7-10) 7 (6-10) 8 (7-10) 8 (6-10) CPT HYPO

.V O 2peak (ml z kg 2 1 z min 2 1 ) 44.2 6 10.8 39.1 6 3.6 36.5 6 6.2 36.8 6 6.7 42.8 6 5 43.5 6 4.9 43.7 6 5.4 43.1 6 4.4 HR peak (bpm) 181 6 8 178 6 5 175 6 8 176 6 7 179 6 9 170 6 9 * 177 6 8 178 6 8 S p O 2 (%) 72 6 6 72 6 4 73 6 6 72 6 4 72 6 5 75 6 6 73 6 5 72 6 4 RPE leg peak 7 (5-10) 7 (6-10) 8 (7-10) 8 (6-10) 9 (8-10) 8 (7-10) 9 (8-10) 9 (7-10) RPE cen peak 7 (6-10) 7 (7-10) 8 (8-10) 8 (7-10) 9 (8-10) 8 (7-10) 8 (7-10) 8 (8-10)

Values are mean 6 SD. RPE values are median (range). * Signifi cantly ( P , 0.05) different from PRE test values.



above-mentioned studies used only seven continuous 1-h exposures performed during 1 wk. In addition, the increases in maximal normoxic exercise V� E have also been shown following nine 3-5 h exposures to 4000 m ( 27 ). This is consistent with the fi ndings of Sheel et al. ( 34 ), indicating that longer exposure periods, and/or more severe hypoxia, are required to elicit a change in sea level exercise ventilation. However, it seems that no clear link exists between increases in hypoxic ventilatory response and changes in sea level exercise ventilation ( 7 ). In par-ticular, Foster et al. ( 7 ) did not observe any differences in normoxic and hypoxic exercise ventilatory responses between different IHE protocol durations. Currently, the functional effects of increased chemosensitivity follow-ing IHE on sea level exercise ventilation are not clear ( 34 ). Even though our results showed benefi ts in submaximal and maximal ventilation following IHE, they have to be interpreted as a refl ection of concomitant IHE and exer-cise training compared to IHE alone.

Even more interesting is the fact that we did not note any benefi ts of IHE in the ventilatory responses to hypoxic exercise. This is indeed intriguing, since previous studies have shown that increased hypoxic chemosensitivity fol-lowing short hypoxic exposures provides increases in V� E and S p o 2 during exercise in hypoxia ( 19 , 25 ). While these discrepancies could be explained by the shorter duration and the intermittent nature of the hypoxic expo-sures, since both above mentioned studies used continuous exposures ( " 1-h), the exact mechanisms remain to be elucidated.

As can be seen from the results, S p o 2 decreased con-comitantly with decreasing F I o 2 in the inspired air ( Table I ). F I o 2 levels used during our IHE protocol are comparable to those of other studies investigating the effects of IHE on performance and aerobic capacity, al-though the tested hypoxic stimulus was more intense in the last sessions. In particular, the F I o 2 in our study was 9.5% compared to 10% used by Tadibi et al. ( 35 ). The duration of hypoxic and normoxic bouts between the IHE was shorter (5:3 min) compared to those (6:4 min) used in previous studies ( 35 , 36 ), although the total num-ber of sessions was higher (20 vs. 15 sessions).

With regards to the fi ndings of the effect of IHE on hypoxic performance, it should be emphasized that this refers to performance conducted under normobaric hy-poxic conditions. Recent evidence suggests that ambient pressure per se may infl uence the physiological re-sponses to hypoxia ( 5 , 28 , 30 ), thus providing support to the challenge of the Equivalent Air Altitude Model.

The novelty of the present study arises from a strictly controlled and identical training of both groups. Al-though the possible benefi ts induced by IHE could be masked by the training responses per se, we aimed at eliminating the possible performance outcome differ-ences as a result of different exercise training performed concomitantly with the IHE; a possible occurrence in studies where subjects perform their normal training during the testing protocol ( 35 , 36 ).

In conclusion the tested IHE protocol did not enhance performance or peak aerobic capacity in hypoxia or

normoxia during and immediately following the proto-col. However, the prolongation of improved perfor-mance (CPT NORMO ) 10 d after the IHE protocol cessation might indicate a potential IHE benefi t. Further studies investigating the de-acclimatization period following IHE combined with exercise training seem warranted in order to determine the existence, duration, and the mag-nitude of those benefi ts.

ACKNOWLEDGMENTS This study was supported by Grant L7-2413 to Igor B. Mekjavic

from the Slovene Research Agency (ARRS). We are also grateful for the support provided by the Slovene Olympic Committee. We are indebted to Miro Vrhovec for his technical assistance. Appreciation is also extended to all the subjects who participated in the study.

Authors and affi liations: Igor B. Mekjavic, Ph.D., Tadej Debevec, Ph.D., and Michail E. Keramidas, Ph.D., Department of Automation, Biocybernetics and Robotics, “ Jozef Stefan ” Institute, Ljubljana, Slovenia; Tadej Debevec, Ph.D., and Mojca Amon, M.Sc., Jozef Stefan International Postgraduate School, Ljubljana, Slovenia; Michail E. Keramidas, M.Sc., Department of Environmental Physiology, School of Technology and Health, Royal Institute of Technology, Stockholm, Sweden; and Styliano N. Kounalakis, Ph.D., Human Performance-Rehabilitation Laboratory, Faculty of Physical and Cultural Education, Hellenic Military University, Vari, Greece.

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Intermittent Normobaric Hypoxic Exposures at Rest: Effects on Performance in Normoxia and Hypoxia