Effect of an Eleven-Day Altitude Training Program on Aerobic and Anaerobic Performance in Adolescent Runners.

Background and Objectives: We evaluated the effect of an eleven-day altitude training camp on aerobic and anaerobic fitness in trained adolescent runners. Materials and Methods: Twenty adolescent (14-18 yrs) middle- and long-distance runners (11 males and 9 females; 16.7 ± 0.8 yrs), with at least two years of self-reported consistent run training, participated in this study. Eight of the subjects (4 females/4 males) constituted the control group, whereas twelve subjects (5 females/7 males) took part in a structured eleven-day altitude training camp, and training load was matched between groups. Primary variables of interest included changes in aerobic (VO2max) and anaerobic (30 s Wingate test) power. We also explored the relationships between running velocity and blood lactate levels before and after the altitude training camp. Results: Following 11 days of altitude training, desirable changes (p < 0.01) in VO2max (+13.6%), peak relative work rate (+9.6%), and running velocity at various blood lactate concentrations (+5.9%-9.6%) were observed. Meanwhile, changes in Wingate anaerobic power (+5.1%) were statistically insignificant (p > 0.05). Conclusions: Short duration altitude appears to yield meaningful improvements in aerobic but not anaerobic power in trained adolescent endurance runners.


Introduction
Endurance training yields morphological and functional adaptations of the human organism [1]. Concomitant adaptations of numerous physiological systems (e.g., cardiovascular, musculoskeletal, etc.) results in an increase in oxygen consumption (VO 2 max) and improvements in power output [1,2]. Coalescence of these adaptations yields superior endurance-related performance outcomes, reflected by improvements in running performance. During maturation, in adolescent age (14-18 years) [3], functional and morphological systems are developing. Data dealing with the effects of endurance training on the development of physiological systems in adolescent runners are sparse [4].
In adults, altitude training provides a unique stimulus to potentiate many of the physiological benefits seen with endurance training [5][6][7][8]. More specifically, the available evidence confirms the development of cardiorespiratory fitness with altitude training [9]. However, fewer studies have sought to examine the influence of altitude training in adolescent individuals. Buchheit et al. [5] provided evidence that high intensity interval training in a hypoxic environment may not be optimal for cardiorespiratory and neuromuscular development in adolescent runners. Meanwhile, findings from our group [10] and Saltin et al. [7] provide evidence for a positive benefit of altitude training on cardiorespiratory fitness parameters in young runners. These contrasting findings highlight the uncertain benefits of altitude training for adolescent endurance athletes, a unique population with highly dynamic and evolving physiological systems.
Endurance training-mediated changes in cardiorespiratory fitness level are most frequently quantified via VO 2 max [11][12][13] and anaerobic threshold [14][15][16][17]. Changes in anaerobic performance following acute altitude exposure, most frequently quantified via evaluation of 30-s Wingate anaerobic power, are less studied [18]. Since there is no doubt that training at higher altitudes is a potent stimulus to achieve peak performance in adults runners [6,8,10,11], and because acclimatization and training become more effective with every stay at higher altitude [19], it is interesting to see how young runners respond to these conditions. In adulthood, acclimatization to a higher altitude can be quicker, and training there can be more efficient. The overarching goal of the present study was to characterize aerobic (VO 2 max) and anaerobic (30-s Wingate test) responses to an eleven-day altitude training camp in adolescent runners. To facilitate an improved understanding of the effects of the training camp on running performance, we also sought to explore whether the training camp modulated relationships between running velocity and blood lactate levels, a significant indicator of running performance [20][21][22]. We hypothesized that an appropriately structured, short-duration (i.e., 11-day) altitude training camp would improve aerobic and anaerobic power as well as running velocity at a given blood lactate level.

Measures
To facilitate an improved understanding of the effects of short-duration altitude training on aerobic and anaerobic performance indicators in adolescent individuals, we assessed VO 2 max and 30-s Wingate performance in 14-18 yr old middle-and long-distance runners before and after an eleven-day altitude (1850 m above sea level) training camp. Running velocity at various blood lactate levels (2, 4, 6, and 9 mmol·L −1 ) were also measured before and after altitude exposure. In the altitude and control groups alike, we monitored morning heart rate (HR) values for evaluation of training load and physiological adaptation.

Design and Procedures
Runners took part in an eleven-day training camp in Livigno, Italy, a territory in the province of Sondrio at an altitude of~1850 m above sea level. Training plans of the altitude and control groups were matched for intensity and duration and based on relative heart rate values. The runners in the control group trained in the place of their residence, i.e., at the altitude of~400 m above sea level. These runners experienced the same training load in terms of intensity (determined by heart rate values) and volume as the members of the altitude group. Thus, the altitude training in the altitude group was the only difference between the groups over the study duration. The training block consisted mostly of low-intensity running below aerobic threshold (AeT) with two higher intensity anaerobic threshold (Ant) [23] training sessions. Over the first three training days, all subjects preformed strictly low intensity training so as to provide the altitude group with a chance to acclimatize to their new environment. Over the ensuing days, the load was gradually increased. In the eleven-days of training, the runners completed~100 kilometers of running. We used morning heart rate values to evaluate acclimatization to the altitude exposure and training load. Heart rate (HR) was recorded using a Garmin Fenix 3 device (Garmin, Olathe, KS, USA) immediately post-waking and in the seated position. HR values were recorded 10 days before the camp, during the camp, and for 10 days after returning from the camp. Training after returning from training camp was similar in both groups. On the first day after training camp was a day off. The second and third days were low-intensity running below AeT. After this, participants resumed their normal pre-camp training routine. Moreover, all subjects performed a lactate field test one day before the maximal oxygen consumption tests.
Changes in aerobic and anaerobic power were assessed by laboratory testing. Cardiorespiratory fitness (VO 2 max) was assessed via indirect calorimetry (Metalyzer B3, Cortex, Leipzig, Germany) on a cycle ergometer. The following parameters were evaluated at the moment of subjective exhaustion: absolute and relative VO 2 max, tidal volume (V T ), minute respiratory volume (VE), respiratory frequency (BF), relative work rate (WR) at VO 2 max (WR·kg −1 ), and heart rate at VO 2 max (HR VO 2 max).
The tests were carried out three days before leaving for the camp and nine days after returning. In both cases, the same testing protocol was utilized. The test began with a 2-minute warm-up at a load 25 W, after which the load was subsequently adjusted from 2 W·kg −1 until volitional exhaustion. Total test duration was between 7-9 min. Criteria used to verify attainment of VO 2 max included: respiratory exchange ratio (RER) > 1.1, rating of perceived exertion (RPE) > 17, achievement of plateau in VO 2 , and achievement of 90% age-predicted maximal heart rate [24].
Anaerobic performance was characterized by means of the 30 s Wingate test [18]. The test is used to determine the maximum relative power in 1 s, 5 s, and 30 s. Wingate testing was conducted on an Excalibur sport bicycle ergometer (Lode, Groningen, The Netherlands), 30 min before indirect calorimetry. A five-minute warm-up on the bicycle was followed by a 30 s all-out test (30 s of riding at maximum pedaling frequency, with a load equivalent to 7.5% body mass).
Running speeds at various blood lactate concentrations (2, 4, 6, and 9 mmol·L −1 ) were calculated from a field test (4 × 1600 m on a track), with each repeat performed at a faster velocity. Following each 1600-m interval, participants rested for two minutes while blood lactate was assessed via fingerstick using a Lactate Scout+ (EKF Diagnostics, Cardiff, UK). Blood lactate values at various running velocities were processed by Winlactat (Mesics, Munster, Germany) and an exponential function was used to generate a blood lactate curve, which provided running speeds at a range of blood lactate concentrations of 2, 4, 6, and 9 mmol·L −1 [1,25]. Blood lactate measurements were made at an elevation of~400 m above sea level, two days before leaving for the camp and eight days after return.

Statistical Analysis
Data were represented as the average and the standard deviation (SD), unless otherwise indicated. To verify the normality of the data for both groups (altitude and control), we used the Shapiro-Wilk test, which showed that these data differed significantly from the normal distribution. This led us to use nonparametric methods for further statistical processing. Between group changes were analyzed using the non-parametric Wilcoxon two-sample test and effect sizes (Cohen's d) were used to provide insight into the magnitude of the differences, where d > 0.8 expresses large effect, a value of 0.5-0.8 expresses an intermediate effect, a value of 0.2-0.5 expresses a small effect, and d < 0.2 signifies a slight effect [26]. Ten-day heart rate value means before, during, and after training camp were calculated using a Wilcoxon matched pairs test. Significance was set at the p < 0.05 level. Data processing was performed in Statistica 12 (Dell, Round Rock, TX, USA) and Microsoft Excel (Oregon, WA, USA).

Ethical Approval
All participants, or parents in case of minors, signed an informed consent form. The research was carried out with consent of the Ethics Committee, Faculty of Education, University of South Bohemia, Ref.
No.: 001/2018. All procedures performed in the study were in accordance with the ethical standards of the institutional research committee and with the Helsinki declaration.     Results from maximal exercise testing are provided in Table 1. RER values > 1.10 provide evidence that subjects provided maximal effort [24]. After the return from the training camp, maximum oxygen significantly increased (p < 0.01; d = 0.92) in the altitude versus the control group, as well as the peak relative work rate (+9.6%). Significant (p < 0.05) improvements in the altitude training group compared with the control group were also observed in minute ventilation (+5.2%) and peak heart rate (+1.4%). Results from maximal exercise testing are provided in Table 1. RER values > 1.10 provide evidence that subjects provided maximal effort [24]. After the return from the training camp, maximum oxygen significantly increased (p < 0.01; d = 0.92) in the altitude versus the control group, as well as the peak relative work rate (+9.6%). Significant (p < 0.05) improvements in the altitude training group compared with the control group were also observed in minute ventilation (+5.2%) and peak heart rate (+1.4%). Anaerobic testing results are provided in Table 2. No statistically significant (p > 0.05) increases were observed in the altitude versus control group. However, moderate effect sizes were observed for changes in relative peak power (d = 0.47) and relative 5-s peak power (d = 0.59) in the altitude group.  Table 3 provides theoretical running velocities computed at AeT (2 mmol·L −1 ), AnT (4 mmol·L −1 ), 6, and 9 mmol·L −1 , measured before the camp and after the return. Altitude training camp appeared to positively benefit (p < 0.01) running velocity at all blood lactate levels when compared with the control condition. Table 3. Estimated running speeds at 2, 4, 6, and 9 mmol·L −1 and SD, and changes before and after the camp determined from field testing by Winlactat software.

Discussion
Short-duration (i.e., 11 days) altitude training significantly improved VO 2 max [8,11,27,28] and velocity at a given blood lactate concentration in trained adolescent runners [11,17,28]. Under optimal load, submaximal parameters (lactate thresholds) can be improved faster than maximum parameters, which was also reflected in our research [10,15,17,29]. These findings mirror what may be expected in their adult counterparts, suggesting similar adaptation of cardiorespiratory fitness from short-duration altitude exposure in trained adolescents and adults. Importantly, the trained adolescent runners from the perspective of individual acclimatization well mastered the planned training at higher altitudes and they appeared to tolerate the training well (see Figure 1). Altitude training is more demanding and requires a reduction in training intensity. Failure to do so may also reduce some fitness parameters [30].
In some cases, a reduction in VO 2 max may occur. [17]. These contrasting findings may be explained by the mastered or not initial three-day adaptation period used in the current study that allowed the runners to acclimate to the training environment.
We checked whether altitude training may serve as a stimulus to enhance anaerobic performance, reflected by an improvement on the Wingate test. Contrary to our hypothesis, 30 s anaerobic power was not affected by the altitude training camp. This finding is in agreement with previous research demonstrating a lack of 30 s Wingate power enhancement in eight young (~20 yrs) team sports players following 4-weeks of simulated altitude (~2750 m) training [31]. More specific training regimens featuring high intensity intervals with sufficient rest (e.g., 2 min+) may be needed if improvements in anaerobic power are desired.
Our use of a cycle ergometer, rather than treadmill for maximal exercise testing is an undeniable limitation of the present study, as runners would likely achieve a superior VO 2 max value on a treadmill. [32][33][34]. Due to the nature of our population (i.e., developing adolescents), we felt that a cycle ergometer was more appropriate to reduce the risk of injury. Despite this limitation, we still observed a 13.6% improvement in VO 2 max following the return from the training camp. In trained runners, this magnitude of change is quite robust over such a short time period, considering 3-6 months of exercise training may only confer improvements of~20% [35]. In conjunction with greater running velocity at various blood lactate concentrations, this improvement in VO 2 max portends to the likelihood of significant race performance improvements in these young athletes. Indeed, running velocity-blood lactate relationships are an integral component of running performance [11,[36][37][38]. Continual tracking of these performance variables following the return to sea level would provide insight as to the persistence of these observed adaptations, thus helping to inform training strategies for athletes and coaches looking to employ altitude training as a means to improve race performance. Moreover, it would be interesting to examine the effects of longer-duration altitude training on these performance variables.
Limitations of our study include the use of a cycle ergometer instead of a treadmill, which is more specific for the population, and the relatively small number of participants. However, due to cost and time commitments, it is difficult to recruit a large sample for such a study. Strengths of our study include a control group, which was neglected in many previous altitude training studies, as well as a thorough testing battery including aerobic and anaerobic performance indicators.

Conclusions
An eleven-day altitude training camp at an altitude of~1850 m above sea level proved highly effective at enhancing cardiorespiratory fitness and running velocity at various blood lactate levels, but not anaerobic performance, in trained adolescent runners. These findings are comparable to what may be observed in adult counterparts, and highlight the value of appropriately structured short-duration altitude training for improving fitness level and likely race performance at sea level in this population.