Monitoring Physiological Performance over 4 Weeks Moderate Altitude Training in Elite Chinese Cross-Country Skiers: An Observational Study

he current observational study aimed to monitor the physiological performance over 4 weeks of living and training at a moderate altitude in elite Chinese cross-country skiers (8 males, mean age 20.83 1.08 years). Lactate threshold, maximal oxygen uptake, blood, and body com- position tests were performed at different time points to investigate the changes in physiological performance. The data were analysed by a one-way repeated measures ANOVA and a paired sample T-test between the test results. During the training camp, systematic load monitoring was carried out. Lactate threshold velocity, lactate threshold heart rate, and upper body muscle mass increased signi cantly (p< 0.01) after moderate altitude training. Maximum oxygen uptake was reduced compared to pre-tests (p< 0.05). Aerobic capacity parameters (maximal oxygen uptake, haemoglobin, red blood cell count) did not signi cantly increase after athletes returned to sea level (p> 0.05). These ndings suggest that 4 weeks of moderate altitude training can signi cantly improve athletes' lactate threshold

increased signi cantly (p< 0.01) after moderate altitude training. Maximum oxygen uptake was reduced compared to pre-tests (p< 0.05). Aerobic capacity parameters (maximal oxygen uptake, haemoglobin, red blood cell count) did not signi cantly increase after athletes returned to sea level (p> 0.05). These ndings suggest that 4 weeks of moderate altitude training can signi cantly improve athletes' lactate threshold and upper body muscle mass; no signi cant improvement in other aerobic capacity was seen. Exposure time, training load, and nutritional strategies should be thoroughly planned for optimal training of skiers at moderate altitudes. Keywords:cross-country skiing; physiological performance; moderate altitude training 1. Introduction One of the most demanding endurance sports, cross-country skiing (XC-skiing), re- quires athletes to compete on courses between 1.5 km and 50 km long over various terrains. The different techniques of classical and skating styles require a high level of technical and physiological performance in athletes [1]. In sports training, physiological performance refers to the physiological adaptation of the athletes' bodies for training or competition. Us- ing physiological and biochemical indicators to monitor athletes' performance has become an essential part of training in endurance sports [2]. Altitude training is widely used in endurance sports, which can induce different responses depending on the modality used [3,4]. It can generally be divided into three main types of training: living high training high (LH-TH), living high training low (LH- TL), and living low training high (LL-TH). Coaches and sports researchers use LH-TH to increase red blood cell (RBC) count, haemoglobin (Hb), maximal oxygen uptake (VO2max), and other performance measures at sea level [5]. However, athletes cannot maintain the same intensities as those at sea level. To avoid the limitations of LH-TH, LH-TL is used to improve athletes' physiological performance while maintaining training intensity [6,7]. Some studies have found that LH-TL can enhance haematological and neuromuscular Int. J. Environ. Res. Public Health2023,20, 266.

Int. J. Environ. Res. Public Health2023,20, 266 2 of 11 adaptations [8,9]. In LL-TH, athletes live at sea level, with hypoxic exposure lasting for several seconds to several hours during training and repeated over several days to weeks. LL-TH has been proven to improve athletes' erythropoietin (EPO) and skeletal muscle mitochondrial density of hypoxia-inducible factor 1 (HIF-1 ) [10,11]. The core aim of altitude training is to enhance the athletes' physiological performance through hypoxic exposure in different ways. Meanwhile, unreasonable altitude training can lead to overreaching and even overtraining of athletes [12]. In recent years, many sports researchers have explored the effects of living and training at moderate altitudes (1500–3000 m), which is closer to the traditional LH-TH modality but lower in altitude [7,13,14]. From the point of view of combining exercise physiology and sports training, such an environment provides a certain level of hypoxic stimulation to the body while approaching the intensity of basic training, thus improving physiological performance [15,16]. Research has demonstrated that three weeks of training at 1800 m signi cantly increased haemoglobin levels in well-trained runners [17]. Czuba et al. [18] found that prolonged exposure to hypoxic conditions (simulated moderate altitude) would continue to promote the synthesis of erythropoietin, improving RBCs' ability to carry more oxygen. Karlsson et al. reported that 17–21 days of training at 1800 m increased elite XC-skiers and biathletes' lactate threshold velocity [19]. Four weeks of moderate altitude training (2200 m) increased the resting metabolic rate and haemoglobin in highly trained middle-distance runners [20]. The bene ts of training at moderate altitudes include increased haemoglobin mass, body metabolic ef ciency, and enhanced lactate threshold [21]. In the past two decades, most Olympic XC-skiing events have been held at 1500–1800 metres, and training at moderate altitudes has become common [22,23]. However, little systematic research has focused on changes in the physiological performance of XC-skiers at moderate altitudes. This observational study aims to measure changes in physiological performance after four weeks of living and training at moderate altitude, meanwhile providing referenceable physiological evidence to help XC-skiing coaches and practitioners when using moderate altitude training.

at moderate altitudes has become common [22,23]. However, little systematic research has focused on changes in the physiological performance of XC-skiers at moderate altitudes. This observational study aims to measure changes in physiological performance after four weeks of living and training at moderate altitude, meanwhile providing referenceable physiological evidence to help XC-skiing coaches and practitioners when using moderate altitude training. 2. Materials and Methods 2.1. Subjects and Study Design This research is an observational study of eight elite Chinese cross-country skiers over four weeks living and training at the Chinese national snow sports training base in BaShang, Chengde, Hebei Province (average sea level at 1510–1700 m, latitude at 44.5 N). A `polarised training' plan based on a traditional Nordic XC-skiing training programme was used, and the daily training data were compiled by the scientists accompanying the team [23,24]. Throughout the design of the study, three tests were conducted (Figure): the pre-test (55 m above sea level) was completed ve days before the moderate altitude training, the mid-test (1550 m above sea level) was performed on day 15 of the moderate altitude training, and the post-test (55 m above sea level) was conducted ve days after the moderate altitude training. The participants in this study included eight male athletes involved in prepara- tion for the 2022 Winter Olympics in Beijing. The mean age was 20.83 1.08 years; weight was69.73 5.12 kg ; height was 179.63 5.93 cm, and mean training years was 4.18 1.92 years . All the athletes involved in the experiment were at the elite-level [25], were in excellent physical condition, and were uninjured. Prior to data collection, all athletes were given an informed permission form and a description of the experiment's objective and potential dangers. Signing both agreements demonstrated a willingness to participate voluntarily. The Sports Science Experimental Ethics Committee of Beijing Sport University approved the research protocol (Grant No. 201906711).

Int. J. Environ. Res. Public Health2023,20, 266 3 of 11Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 3 of 12 Figure 1. Study Design of the Research. 2.2. Physiological Performance Test Physiological performance tests in treadmill running were conducted using protocols developed by the Norwegian Top Sport Centre. First, an “intermittent” incremental test was used to obtain the lactate threshold. After a 5 min recovery, the athletes conducted an incremental test to determine VO 2max [26,27]. All athletes implemented a standardised warm-up process prior to the lactate threshold test under the supervision of a professional fitness coach. The warm-up routine consisted of 10 min of low-intensity jogging on a treadmill with an athlete rating of perceived exertion (RPE) of 2, followed by 10 push-ups and five squat jumps. After the athletes warmed up, the lactate threshold test was implemented on the treadmill (RL2500E, Rodby, Södertalje, Sweden) using the incremental load test method. The incline angle of the treadmill was set at 10.5% and maintained throughout the test, with the starting speed of the treadmill set at 7 km/h. Athletes ran at a constant speed for five minutes at each level of speed, with a 30 s rest interval at the end of the run. The treadmill’s speed was increased by 1 km/h between runs [26]. The heart rate level of each athlete was recorded in the last 30 s of each stage. The athlete’s blood lactate level was tested immediately after each level of the running platform test. Blood lactate concentration was measured immediately after exercise using an EKF benchtop blood lactate metre (Boisen, EKF Industrial Electronics, Magdeburg, Germany). Furthermore, the athlete’s RPE was recorded using a 0–10 scale. The lactate threshold was defined as a blood lactate level of 4 mmol/L −1 [28]; when the threshold exceeded 4 mmol/L −1 , the test was stopped. Treadmill speed at 4 mmol/L −1 was calculated using linear interpolation [28]. Following the lactate threshold test, the athletes rested for 5 min before assessing their maximum oxygen uptake using a portable gas metabolism analyser (MetaMax 3B, Cortex, Leipzig, Germany)

blood lactate level of 4 mmol/L −1 [28]; when the threshold exceeded 4 mmol/L −1 , the test was stopped. Treadmill speed at 4 mmol/L −1 was calculated using linear interpolation [28]. Following the lactate threshold test, the athletes rested for 5 min before assessing their maximum oxygen uptake using a portable gas metabolism analyser (MetaMax 3B, Cortex, Leipzig, Germany) [26,27]. The treadmill incline angle for the VO 2max test was 10.5%, and the treadmill start speed was 1 km/h below the end speed of the lactate threshold test. The treadmill’s speed was increased by 1 km/h every minute from the beginning of the test until the participant was exhausted. Throughout the test, the athlete wore a ventilation mask to evaluate his oxygen uptake volume. The VO 2max was defined as the average of the two highest and consecutive 30 s measurements. A heart rate belt (H10, Polar, Finland) was used to monitor the athlete’s heart rate. Maximum HR was defined as the highest 5 s heart rate measurement during the VO 2max test. The blood lactate concentration was measured 1 min after completing the test, and the RPE values were recorded, with the RPE counted on a 0–10 scale. The athlete’s final treadmill speed, maximum oxygen uptake, and respiratory exchange ratio (RER) were recorded. Blood tests were performed between 6:00 am and 7:00 am on each test day, with venous blood drawn by medical staff in the morning while the athlete was awake and Figure 1.Study Design of the Research. 2.2. Physiological Performance Test Physiological performance tests in treadmill running were conducted using protocols developed by the Norwegian Top Sport Centre. First, an “intermittent” incremental test was used to obtain the lactate threshold. After a 5 min recovery, the athletes conducted an incremental test to determine VO2max[26,27]. All athletes implemented a standardised warm-up process prior to the lactate threshold test under the supervision of a professional tness coach. The warm-up routine consisted of 10 min of low-intensity jogging on a treadmill with an athlete rating of perceived exertion (RPE) of 2, followed by 10 push- ups and ve squat

athletes conducted an incremental test to determine VO2max[26,27]. All athletes implemented a standardised warm-up process prior to the lactate threshold test under the supervision of a professional tness coach. The warm-up routine consisted of 10 min of low-intensity jogging on a treadmill with an athlete rating of perceived exertion (RPE) of 2, followed by 10 push- ups and ve squat jumps. After the athletes warmed up, the lactate threshold test was implemented on the treadmill (RL2500E, Rodby, Södertalje, Sweden) using the incremental load test method. The incline angle of the treadmill was set at 10.5% and maintained throughout the test, with the starting speed of the treadmill set at 7 km/h. Athletes ran at a constant speed for ve minutes at each level of speed, with a 30 s rest interval at the end of the run. The treadmill's speed was increased by 1 km/h between runs [26]. The heart rate level of each athlete was recorded in the last 30 s of each stage. The athlete's blood lactate level was tested immediately after each level of the running platform test. Blood lactate concentration was measured immediately after exercise using an EKF benchtop blood lactate metre (Boisen, EKF Industrial Electronics, Magdeburg, Germany). Furthermore, the athlete's RPE was recorded using a 0–10 scale. The lactate threshold was de ned as a blood lactate level of 4 mmol/L 1 [28]; when the threshold exceeded 4 mmol/L 1 , the test was stopped. Treadmill speed at 4 mmol/L 1 was calculated using linear interpolation [28]. Following the lactate threshold test, the athletes rested for 5 min before assessing their maximum oxygen uptake using a portable gas metabolism analyser (MetaMax 3B, Cortex, Leipzig, Germany) [26,27]. The treadmill incline angle for the VO2maxtest was 10.5%, and the treadmill start speed was 1 km/h below the end speed of the lactate threshold test. The treadmill's speed was increased by 1 km/h every minute from the beginning of the test until the participant was exhausted. Throughout the test, the athlete wore a ventilation mask to evaluate his oxygen uptake volume. The VO2maxwas de ned as the average

and the treadmill start speed was 1 km/h below the end speed of the lactate threshold test. The treadmill's speed was increased by 1 km/h every minute from the beginning of the test until the participant was exhausted. Throughout the test, the athlete wore a ventilation mask to evaluate his oxygen uptake volume. The VO2maxwas de ned as the average of the two highest and consecutive 30 s measurements. A heart rate belt (H10, Polar, Finland) was used to monitor the athlete's heart rate. Maximum HR was de ned as the highest 5 s heart rate measurement during the VO2maxtest. The blood lactate concentration was measured 1 min after completing the test, and the RPE values were recorded, with the RPE counted on a 0–10 scale. The athlete's nal treadmill speed, maximum oxygen uptake, and respiratory exchange ratio (RER) were recorded. Blood tests were performed between 6:00 a.m. and 7:00 a.m. on each test day, with venous blood drawn by medical staff in the morning while the athlete was awake and fasting. Routine blood tests were analysed using a fully automated haematology analyser (BC-5180CRP Automatic Haematology Analyser, Myriad, Shenzhen, China). Blood urea

Int. J. Environ. Res. Public Health2023,20, 266 4 of 11 (BUN) and creatine kinase (CK) were analysed using a fully automated biochemistry analyser (AU680 Automatic Biochemical Analyser, Beckman Coulter, Brea, CA, USA). All instruments were standardised using the original and matching reagents. Body composition testing was conducted only on the morning of the pre-tests and post-tests. The fat, muscle, and bone mass of the athlete's body and all body segments (upper body, trunk, and lower body) were measured using a dual-energy X-ray bone density analyser (Luna iDXA, General Electric Company, Schenectady, NY, USA) after the athlete had nished the venous blood draw. 2.3. Training Load Monitoring This study's four-week moderate altitude training programme involved six days of training per week, with Monday for rest and recovery. Low-intensity aerobic training was conducted ve days before and after four weeks of moderate altitude training. All training loads, including volumes and intensities, were developed and implemented by the heart rate zones obtained from the athletes' baseline physiological tests. Load monitoring used a 5-zone load intensity model (Table) designed by the Norwegian National Olympic Committee based on a combination of laboratory test results and actual training [24,29]. Table 1.The 5-zone model was used in the current study.Intensity Zone Blood Lactate (mmol/L) Heart Rate (% Max) RPE 5 HIT 6.0–10.0 92–97 8–10 4 4.0–6.0 87–92 7–9 3 MIT 2.5–4.0 82–87 4–7 2 LIT 1.5–2.5 72–82 3–5 1 0.8–1.5 55–72 4 Abbreviations: LIT: low-intensity training; MIT, moderate-intensity training, HIT, high-intensity training. The training statistics format adhered to the training recording format suggested by the Norwegian National Olympic Committee, using exercise forms, training forms, and exercise intensity for load recording for all workouts (Figure) [24].Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 4 of 12 fasting. Routine blood tests were analysed using a fully automated haematology analyser (BC-5180CRP Automatic Haematology Analyser, Myriad, Shenzhen, China). Blood urea (BUN) and creatine kinase (CK) were analysed using a fully automated biochemistry analyser (AU680 Automatic Biochemical Analyser, Beckman Coulter, Brea, CA, USA). All instruments were standardised using the original and matching reagents. Body composition testing was conducted

of 12 fasting. Routine blood tests were analysed using a fully automated haematology analyser (BC-5180CRP Automatic Haematology Analyser, Myriad, Shenzhen, China). Blood urea (BUN) and creatine kinase (CK) were analysed using a fully automated biochemistry analyser (AU680 Automatic Biochemical Analyser, Beckman Coulter, Brea, CA, USA). All instruments were standardised using the original and matching reagents. Body composition testing was conducted only on the morning of the pre-tests and post-tests. The fat, muscle, and bone mass of the athlete’s body and all body segments (upper body, trunk, and lower body) were measured using a dual-energy X-ray bone den- sity analyser (Luna iDXA, General Electric Company, Schenectady, NY, USA) after the athlete had finished the venous blood draw. 2.3. Training Load Monitoring This study’s four-week moderate altitude training programme involved six days of training per week, with Monday for rest and recovery. Low-intensity aerobic training was conducted five days before and after four weeks of moderate altitude training. All training loads, including volumes and intensities, were developed and implemented by the heart rate zones obtained from the athletes’ baseline physiological tests. Load monitoring used a 5-zone load intensity model (Table 1) designed by the Norwegian National Olympic Committee based on a combination of laboratory test results and actual training [24,29]. Table 1. The 5-zone model was used in the current study. Intensity Zone Blood Lactate (mmol/L) Heart Rate (% Max) RPE 5 HIT 6.0–10.0 92–97 8–10 4 4.0–6.0 87–92 7–9 3 MIT 2.5–4.0 82–87 4–7 2 LIT 1.5–2.5 72–82 3–5 1 0.8–1.5 55–72 ≤4 Abbreviations: LIT: low-intensity training; MIT, moderate-intensity training, HIT, high-intensity training. The training statistics format adhered to the training recording format suggested by the Norwegian National Olympic Committee, using exercise forms, training forms, and exercise intensity for load recording for all workouts (Figure 2) [24]. Figure 2. Training distribution methods. abbreviations: ACT. FORMS = activity forms. Figure 2.Training distribution methods. abbreviations: ACT. FORMS = activity forms. 2.4. Statistical Analyses Data statistics and analysis were performed using IBM SPSS 25.0 and Excel 2019 software. All data were presented as mean SD and were tested for normality using the Shapiro–Wilk test

for all workouts (Figure 2) [24]. Figure 2. Training distribution methods. abbreviations: ACT. FORMS = activity forms. Figure 2.Training distribution methods. abbreviations: ACT. FORMS = activity forms. 2.4. Statistical Analyses Data statistics and analysis were performed using IBM SPSS 25.0 and Excel 2019 software. All data were presented as mean SD and were tested for normality using the Shapiro–Wilk test before processing. Differences in the lactate threshold test, maximum oxygen uptake test, and blood test between the three tests were assessed using repeated measures ANOVA. Mauchly's Test of Sphericity was used, and in the multiple comparisons

Int. J. Environ. Res. Public Health2023,20, 266 5 of 11 were used the Bonferroni post hoc test with signi cant changes atp< 0.05 and highly signi cant differences atp< 0.01. Differences in body composition between pre- and post- tests were using the paired sample T-test with signi cant changes atp< 0.05 and highly signi cant differences atp< 0.01. 3. Results During four weeks of moderate altitude training, the eight elite Chinese male XC- skiers recorded a total training time of 67 h, with an average of 16.7 h per week. The average percentage of time spent in the i1-i5 intensity interval over four weeks was 63.6%, 28.5%, 5.8%, 1.4%, and 0.7%. The average LIT (i1–i2), MIT (i3), and HIT (i4–i5) intensities were 92.1%, 5.8%, and 2.1%, respectively. Endurance training throughout the cycle included roller skiing, skiing, running, and Nordic walking (Table). Table 2.Training Characteristics of 4 weeks Moderate Altitude Training.Weekly Training Patterns Week1 Week2 Week3 Week4 Total Training Total Training Time (h wk 1 ) 13.0 0.6 17.4 1.2 16.9 2.7 19.7 1.4 Training Sessions wk 1 10.4 1.1 12 2.3 10.5 2.4 12.6 1.4 Endurance Distance (km wk 1 ) 155.4 19.5 205.3 35.1 208.9 38.3 237.6 17.6 Training forms Endurance Training Time (h wk 1 ) 11.8 1.0 14.3 1.2 14.2 1.6 16.9 0.7 Strength Training Time (h wk 1 ) 1.1 0.9 3 0.5 2.5 1.3 2.7 1.1 Sprint Training Time (h wk 1 ) 0.1 0.1 0.2 0.1 0.2 0.1 0.1 0.1 Endurance Intensity distribution Zone 1 (h wk 1 ) 7.5 1.6 9.2 3.4 9.9 2.4 9.8 3.2 Zone 2 (h wk 1 ) 3.3 1.5 3.9 1.8 3.7 2.5 5.4 2.6 Zone 3 (h wk 1 ) 0.6 0.4 0.8 1 0.5 0.4 1.4 0.9 Zone 4 (h wk 1 ) 0.2 0.2 0.3 0.3 0.1 0.1 0.2 0.2 Zone 5 (h wk 1 ) 0.2 0.1 0.1 0.1 0 0.1 0.1 0.1 According to Mauchly's spherical hypothesis test, the variance–covariance matrix of all dependent variables wasp< 0.01. The ndings in Table lactate threshold velocity, lactate threshold heart rate, VO2max, and maximal heart rate

This study provides data on the effects of moderate altitude training on physiological performance.