Citation: Yu, Y.; Wang, R.; Li, D.; Lu,
Y. Monitoring Physiological
Performance over 4 Weeks Moderate
Altitude Training in Elite Chinese
Cross-Country Skiers: An
Observational Study. Int. J. Environ.
Res. Public Health 2023, 20, 266.
https://doi.org/10.3390/
ijerph20010266
Academic Editor: Roberto Cejuela
Anta
Received: 20 November 2022
Revised: 18 December 2022
Accepted: 20 December 2022
Published: 24 December 2022
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International  Journal  of
Environmental Research
and Public Health
Article
Monitoring Physiological Performance over 4 Weeks Moderate
Altitude Training in Elite Chinese Cross-Country Skiers:
An Observational Study
Yichao Yu 1,2, Ruolin Wang 3, Dongye Li 1,2 and Yifan Lu 1,4,*
1
The School of Sports Medicine and Rehabilitation, Beijing Sports University, Beijing 100084, China
2
The Graduate School, Beijing Sport University, Beijing 100084, China
3
Melbourne School of Population and Global Health, The University of Melbourne,
Carlton, VIC 3053, Australia
4
Key Laboratory of Sports and Physical Fitness of the Ministry of Education, Beijing Sport University,
Beijing 100084, China
*
Correspondence: luyifan@bsu.edu.cn; Tel.: +86-137-0110-2889
Abstract: The 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
signiﬁcantly (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 signiﬁcantly increase after athletes returned to sea level (p > 0.05). These
ﬁndings suggest that 4 weeks of moderate altitude training can signiﬁcantly improve athletes’ lactate
threshold and upper body muscle mass; no signiﬁcant 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 Health 2023, 20, 266. https://doi.org/10.3390/ijerph20010266
https://www.mdpi.com/journal/ijerph
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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 1α (HIF-1α) [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
signiﬁcantly 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 beneﬁts of training at moderate altitudes include
increased haemoglobin mass, body metabolic efﬁciency, 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.
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 1):
the pre-test (55 m above sea level) was completed ﬁve 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 ﬁve 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 was 69.73 ± 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 Health 2023, 20, 266
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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 
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) [26,27]. The treadmill incline angle for the VO2max 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 VO2max 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 VO2max 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
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) [26,27]. The treadmill incline angle for the VO2max 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 VO2max 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 VO2max 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 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 Health 2023, 20, 266
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(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 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].
Int. J. Environ. Res. Public Health 2023, 20, x FOR PEER REVIEW 
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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 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 Health 2023, 20, 266
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were used the Bonferroni post hoc test with signiﬁcant changes at p < 0.05 and highly
signiﬁcant differences at p < 0.01. Differences in body composition between pre- and post-
tests were using the paired sample T-test with signiﬁcant changes at p < 0.05 and highly
signiﬁcant differences at p < 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 2).
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 was p < 0.01. The ﬁndings in Table 3 show the following. The athletes’
lactate threshold velocity, lactate threshold heart rate, VO2max, and maximal heart rate at
VO2max were statistically signiﬁcant before and after the three tests (p < 0.01). The lactate
threshold velocity was 0.24 m·s−1 higher in the post-test compared to the pre-test (95% CI:
0.046–0.436, p < 0.05) and 0.31 m·s−1 higher in the post-test compared to the mid-test
(95% CI: 0.020–0.056, p < 0.05). The lactate threshold heart rate was 9.0 beats·min−1 higher
in the post-test compared to the pre-test (95% CI: 5.079–12.921, p < 0.05) and 9.4 beats·min−1
higher in the post-test compared to the mid-test (95% CI: 0.020–0.056, p < 0.05). The VO2max
were 6.11 L·min−1 (95% CI: 2.293–9.932, p < 0.05) and 0.57 L·min−1 (95% CI: 0.213–0.917,
p < 0.05) higher in the pre-test compared to the mid-test. The post-test result of VO2max
increased separately 3.50 mL·min−1·kg−1 (95% CI: 0.545–6.455, p < 0.05) and 0.39 L·min−1
(95% CI: 0.108–0.674, p < 0.05) compared to the mid-test. The maximum heart rate of the
athletes tested was 9.0 beats·min−1 higher in the post-test than in the mid-test (95% CI:
2.763–9.23, p < 0.01).
The heart rate–velocity and lactate–velocity curves plotted from the lactate threshold
test data are shown in Figure 3. After training at moderate altitude (post-test), the lactate
threshold heart rate was signiﬁcantly higher than pre-test and mid-test at all speed levels
(p < 0.05).
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Table 3. Changes in Lactate Threshold and Maximum Oxygen Uptake after 4 weeks Moderate
Altitude Training.
Pre-Test
Mid-Test
Post-Test
Time Effect
Lactate threshold velocity(m·s−1)
3.02 ± 0.18
2.96 ± 0.14
3.27 ± 0.24 *$
F(2,14) = 10.40, p = 0.002
Lactate threshold HR(beats·min−1)
179.5 ± 9.8
179.1 ± 10.3
188.5 ± 9.2 **$$
F(2,14) = 26.46, p < 0.001
VO2max (mL·min−1·kg−1)
73.74 ± 3.63
67.63 ± 2.13 **
71.12 ± 3.14 $
F(2,14) = 15.48, p < 0.001
VO2max (L·min−1)
4.82 ± 0.36
4.25 ± 0.36 **
4.64 ± 0.43 $
F(2,14) = 19.70, p < 0.001
RER
1.23 ± 0.09
1.23 ± 0.05
1.11 ± 0.03 **$
F(2,14) = 14.53, p < 0.001
Maximum HR(beats·min−1)
197.4 ± 11.6
192.9 ± 8.6
198.9 ± 9.1 $$
F(2,14) = 9.41, p = 0.003
Maximum Lactate(mmol·L−1)
13.31 ± 1.42
11.69 ± 2.34
11.33 ± 2.79
F(2,14) = 1.84, p = 0.196
* indicates a signiﬁcant difference compared to pre-test (* p < 0.05, ** p < 0.01). $ indicates a signiﬁcant difference
compared to mid-test ($ p < 0.05, $$ p < 0.01). Abbreviations: HR = heart rate, VO2max = maximal oxygen uptake,
RER = respiratory exchange ratio.
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The heart rate–velocity and lactate–velocity curves plotted from the lactate threshold 
test data are shown in Figure 3. After training at moderate altitude (post-test), the lactate 
threshold heart rate was significantly higher than pre-test and mid-test at all speed levels 
(p < 0.05). 
 
 
 
(a) 
(b) 
Figure 3. Eight elite Chinese cross-country skiers’ lactate-velocity curve (a) and heart rate-curve (b) 
from lactate threshold test. * indicates a significant difference compared to pre-test (* p < 0.05). Ab-
breviations: La = lactate, HR = heart rate. 
As demonstrated in Table 4, there were significant changes (p < 0.05 or p < 0.01) in 
RBC, BUN, and %MXD in the three tests. The RBC increased by 0.29 × 106·μL−1 in the pre-
test compared to the mid-test (95% CI: 0.041–0.534, p < 0.05). The BUN increased 1.16 
mmol·L−1 after two weeks of exposure to the moderate altitude (mid-test) compared to the 
pre-test (95% CI: 0.195–2.130, p < 0.05). The % MXD increased separately by 3.26% and 
3.36% in the pre-test (95% CI: 1.246–5.279, p < 0.01) and mid-test (95% CI: 1.720–5.005, p < 
0.01) compared to the post-test. For HCT and WBC, the F-test achieved statistical signifi-
cance. There was no significant difference between the measurements. This could happen 
if the sample size is small, leading to unstable statistical results 
A paired sample T-test of body composition data during four weeks at moderate al-
titude (Table 5) showed that the percentage of upper-body muscle mass increased from 
11.74% to 12.03% after altitude training, with a significant increase in upper-body muscle 
mass before and after training (95% CI: 0.093–0.307, p < 0.01). 
Table 3. Changes in Lactate Threshold and Maximum Oxygen Uptake after 4 weeks Moderate Al-
titude Training. 
 
Pre-Test  
Mid-Test 
Post-Test 
Time Effect 
Lactate threshold velocity(m·s−1) 
3.02 ± 0.18 
2.96 ± 0.14 
3.27 ± 0.24 *$ 
F(2,14) = 10.40, p = 0.002 
Lactate threshold HR(beats·min−1) 
179.5 ± 9.8 
179.1 ± 10.3 
188.5 ± 9.2 **$$ 
F(2,14) = 26.46, p < 0.001 
VO2max (mL·min−1·kg−1) 
73.74 ± 3.63 
67.63 ± 2.13 ** 
71.12 ± 3.14 $ 
F(2,14) = 15.48, p < 0.001 
VO2max (L·min−1) 
4.82 ± 0.36 
4.25 ± 0.36 ** 
4.64 ± 0.43 $ 
F(2,14) = 19.70, p < 0.001 
RER 
1.23 ± 0.09 
1.23 ± 0.05 
1.11 ± 0.03 **$ 
F(2,14) = 14.53, p < 0.001 
Maximum HR(beats·min−1) 
197.4 ± 11.6 
192.9 ± 8.6 
198.9 ± 9.1 $$ 
F(2,14) = 9.41, p = 0.003 
Maximum Lactate(mmol·L−1) 
13.31 ± 1.42 
11.69 ± 2.34 
11.33 ± 2.79 
F(2,14) = 1.84, p = 0.196 
* indicates a significant difference compared to pre-test (* p < 0.05, ** p < 0.01). $ indicates a significant 
difference compared to mid-test ($ p < 0.05, $$ p < 0.01). Abbreviations: HR = heart rate, VO2max = 
maximal oxygen uptake, RER = respiratory exchange ratio. 
Figure 3. Eight elite Chinese cross-country skiers’ lactate-velocity curve (a) and heart rate-curve
(b) from lactate threshold test. * indicates a signiﬁcant difference compared to pre-test (* p < 0.05).
Abbreviations: La = lactate, HR = heart rate.
As demonstrated in Table 4, there were signiﬁcant changes (p < 0.05 or p < 0.01) in RBC,
BUN, and %MXD in the three tests. The RBC increased by 0.29 × 106·µL−1 in the pre-test
compared to the mid-test (95% CI: 0.041–0.534, p < 0.05). The BUN increased 1.16 mmol·L−1
after two weeks of exposure to the moderate altitude (mid-test) compared to the pre-test
(95% CI: 0.195–2.130, p < 0.05). The % MXD increased separately by 3.26% and 3.36% in
the pre-test (95% CI: 1.246–5.279, p < 0.01) and mid-test (95% CI: 1.720–5.005, p < 0.01)
compared to the post-test. For HCT and WBC, the F-test achieved statistical signiﬁcance.
There was no signiﬁcant difference between the measurements. This could happen if the
sample size is small, leading to unstable statistical results
Table 4. Changes in Blood Indicators after 4-weeks Moderate Altitude Training.
Pre-Test
Mid-Test
Post-Test
Time Effect
HCT
0.48 ± 0.02
0.46 ± 0.02
0.46 ± 0.02
F(2,14) = 6.85, p = 0.008
HGB/(g·L−1)
160.00 ± 10.90
156.00 ± 7.25
160.88 ± 10.48
F(2,14) = 2.71, p = 0.101
RBC/(×106·µL−1)
5.40 ± 0.32
5.11 ± 0.29 *
5.23 ± 0.27
F(2,14) = 8.70, p = 0.004
CK/(U·L−1)
207.63 ± 122.47
233.38 ± 101.77
181.38 ± 80.15
F(2,14) = 1.46, p = 0.265
BUN/(mmol·L−1)
6.65 ± 1.10
7.81 ± 0.74 *
6.71 ± 0.96
F(2,14) = 5.96, p = 0.013
%LYM
38.6 ± 6.3
39.3 ± 8.2
40.6 ± 7.8
F(2,14) = 0.39, p = 0.683
%MXD
7.8 ± 2.7
7.9 ± 1.8
4.6 ± 1.4 **$$
F(2,14) = 16.26, p < 0.001
%NEUT
53.6 ± 6.3
50.8 ± 9.0
53.1 ± 9.3
F(2,14) = 18.41, p = 0.518
WBC/(×103·µL−1)
5.84 ± 1.30
4.93 ± 0.84
5.35 ± 0.63
F(2,14) = 4.86, p = 0.025
* indicates a signiﬁcant difference compared to pre-test (* p < 0.05, ** p < 0.01). $ indicates a signiﬁcant difference
compared to mid-test ($$ p < 0.01). Abbreviations: HCT = hematocrit, HGB = hemoglobin, RBC= red blood cell,
CK = creatine kinase, BUN = blood urea nitrogen, %LYM = percentage of lymphocyte, %MXD = percentage of
mononucleosi, %NEUT = percentage of neutrophil, WBC = white blood cell.
Int. J. Environ. Res. Public Health 2023, 20, 266
7 of 11
A paired sample T-test of body composition data during four weeks at moderate
altitude (Table 5) showed that the percentage of upper-body muscle mass increased from
11.74% to 12.03% after altitude training, with a signiﬁcant increase in upper-body muscle
mass before and after training (95% CI: 0.093–0.307, p < 0.01).
Table 5. Changes in Body Composition after 4-weeks Moderate Altitude Training.
Pre-Test
Post-Test
p Values
Muscle mass (kg)
Whole body
55.66 ± 4.01
56.01 ± 4.02
p = 0.071
Upper body
6.54 ± 0.42
6.74 ± 0.36 ##
p = 0.004
Trunk
27.20 ± 2.18
27.47 ± 2.00
p = 0.485
Lower body
18.60 ± 1.82
18.57 ± 1.68
p = 0.924
Fat mass (kg)
Whole body
7.81 ± 1.51
7.91 ± 1.14
p = 0.193
Upper body
0.91 ± 0.16
0.97 ± 0.18
p = 0.172
Trunk
3.13 ± 0.84
3.06 ± 0.60
p = 0.649
Lower body
2.91 ± 0.56
3.03 ± 0.47
p = 0.139
Bone mass (kg)
Whole body
2.83 ± 0.16
2.84 ± 0.13
p = 0.356
Upper body
0.40 ± 0.01
0.41 ± 0.04
p = 0.172
Trunk
0.84 ± 0.05
0.81 ± 0.04
p = 0.356
Lower body
1.16 ± 0.08
1.16 ± 0.08
p > 0.05
# indicates a signiﬁcant difference compared to pre-test (## p < 0.01).
4. Discussion
The primary contribution of our study is to provide relevant and new data regarding
the effects of four weeks of moderate altitude training on the physiological performance
of elite Chinese XC-skiers. This study indicates that four weeks of living and training at a
moderate altitude leads to elevated lactate threshold and upper-body muscle mass.
In the case of lactic acid build-up, the athletes’ cells’ removal of lactic acid somewhat
reﬂects the athletes’ aerobic and lactate metabolism capacity [30]. This study found that
lactate threshold velocity and heart rate were signiﬁcantly higher after four weeks of
moderate altitude training, reﬂecting an increase in the athletes’ lactate threshold and
aerobic capacity. The lactate–velocity curve shows that the blood lactate concentration after
four weeks of moderate altitude training was consistently lower than the ﬁrst two tests at
the velocity level prior to reaching the lactate threshold. This improvement is consistent
with Ingjer et al. [31], who discovered that excellent XC-skiers had signiﬁcantly reduced
lactate levels in the submaximal test after three weeks of moderate altitude training at
1900 m. In addition, a study of a group of British national team distance runners showed
a 12% increase in lactate threshold velocity after four weeks of 1500–2000 m endurance
training [32].
High biochemical blood indicators are the primary reason for applying altitude train-
ing to increase lactate metabolism [33]. However, no similar situation occurred in this
study. Unlike the traditional LH-TH plan (over 3000 m), 1550 m may be too low to increase
biochemical blood indicators, such as Hb [19]. We speculate that the increase in lactate
metabolism capacity has resulted from non-haematological hypoxia-induced changes. In
LH-TL, elite athletes living at high altitudes of 2000–3000 m while simultaneously training
below 1500 m can enhance muscle buffering capacity [34]. In this study, the living altitude
of athletes was much lower, which is more conducive to recovery and may further improve
muscle buffering capacity and lactate metabolism. Meanwhile, with the continuous promo-
tion of training, the further improvement of the athletes’ economy of action may also have
some impacts. Based on the current experimental design, we could not determine whether
the effect was due to hypoxic exposure or a training plan.
Maximal oxygen uptake is one of the most important indicators of XC-skiers [1,35,36].
We found that the maximum oxygen uptake test on the plateaus was signiﬁcantly lower
Int. J. Environ. Res. Public Health 2023, 20, 266
8 of 11
than on the plains. This is common in altitude training; as the partial pressure of oxygen
decreases, oxygen from arterial blood is difﬁcult to deliver to tissue cells, with negative
effects on muscle metabolism and contraction, leading to increased peripheral fatigue [12].
Contrary to the traditional LH-TH plan, maximal oxygen uptake levels do not change
signiﬁcantly after returning to sea level. Ingjer et al. [31] and Chapman et al. [37] also
found the same results in elite endurance athletes when they were training at 2000–3000 m.
Compared with the altitude used in the traditional altitude training modality, this may
be because a moderate altitude environment does not cause a strong enough stimulus
to the athletes’ bodies [19]. In addition, insufﬁcient training intensity may lead to this
phenomenon. According to the training load monitoring results, the lack of HIT training
may have contributed to the lack of cardiorespiratory and physiological stimulation. In
addition to no changes in physiological indicators, training at 1550 m may not suffer any
change in lung diffusing, which was proved in 1800 m swimming training [38]. A detailed
examination applied in a meta-analysis about altitude training reveals a lag in the change
in maximal oxygen uptake after altitude training. The longer the time after returning to
sea level, the more pronounced the increase in maximal oxygen uptake due to altitude
training [3]. Whether similar mechanisms exist at this altitude could be further veriﬁed by
future experiments.
RBCs are the body’s carriers of oxygen and play an important role in endurance
training. This study found that after two weeks of moderate altitude training stimulation,
athletes’ RBC counts and haemoglobin levels tended to decrease compared to the pre-
test. After returning to sea level, neither increased relative to pre-training levels. The
same result was reported in a study with XC-skiers under long-term moderate altitude
training (1500–1800 m) [39]. While no EPO could be measured in the present study, the
study of altitude training indicated that higher altitude had a more positive effect on RBC
production in athletes [9,40]. Meanwhile, several prior studies have reported a correlation
between maximal oxygen uptake values and haemoglobin concentrations in athletes [31].
Maximal oxygen uptake and haemoglobin change trends in this case study are also relevant.
Contrary to previous research in other sports, our results indicate that training at 1550 m
may not stimulate red cell production with concurrent amelioration of aerobic performance
like VO2max. The insufﬁcient hypoxia exposure and some hypoxia-induced disturbances in
physiological function will cause it [40].
At the same time, attention should be paid to the exposure time to hypoxia and the
environment; the terrain, wind speed and load could have altered this outcome. The training
camp environment is windy all year round, which may lead to the loss of athletes’ body fluids,
resulting in negative effects. Typically, the duration of moderate altitude training is generally
7 to 21 days [23], and 28 days of hypoxic exposure may have a negative impact. Moreover,
we did not control plasma and blood volume, which may have affected the results. BUN is
more sensitive to changes in training volume, as the higher the training load, the greater the
rise in BUN and the slower the recovery rate early the following morning [12]. In the mid-test
of this study, BUN was significantly higher than in the pre-test, which is also consistent with
the accumulation of fatigue in athletes as the load increases. After moderate altitude training,
the percentage of monocytes in the athletes in this study was much lower than before, and
throughout training, the athletes’ bodies likely showed signs of infection-like conditions at the
beginning, and the accumulated load of altitude training contributed to this phenomenon [32].
There is evidence that athletes exposed to hypoxic environments for training may
experience signiﬁcant changes in body composition [41]. MacDougall J et al. [42] found
that long-term exposure to chronic hypoxia at high altitudes (over 5000 m) can disrupt the
body’s protein synthesis, which in turn can result in lower body weight, skeletal muscle
mass, and fat mass. In contrast, we did not ﬁnd signiﬁcant body mass, bone, or muscle
loss in this case study, which is likely to be related to insufﬁcient exposure time. The
upper-body muscle mass change showed an upward trend after four weeks of moderate
altitude training. It is worth noting that during this period, the Chinese XC-ski team
coaching team speciﬁcally strengthened the athletes’ strength base. The training load
Int. J. Environ. Res. Public Health 2023, 20, 266
9 of 11
intervention created a certain intervention mechanism on the body, resulting in the athletes’
body muscle mass being well maintained, which has been proven in elite winter sports
athletes [43,44]. Another possible explanation is that specialist nutrition supplies from
the national team may have led to this result. It is dangerous for endurance athletes to
lose muscle and body weight during altitude training. Nevertheless, a solid nutritional
approach prevents signiﬁcant changes in body composition [45,46]. The study of Kayser
et al. [47] has demonstrated that it is possible to maintain body composition at altitudes
below 5000 metres if people intake sufﬁcient energy. These results suggest that the athletes
could maintain energy balance throughout the moderate altitude camp. Both training load
and nutritional strategy may have inﬂuenced the results of this study, which can be veriﬁed
in the future through well-controlled studies.
Some limitations of this study should be considered. Firstly, Due to the limitations
of the design, this study lacked a control group, and the observations in this study were
relatively small. Nevertheless, this issue is a standard limitation of observational studies in
real-world competitive sports settings. Secondly, there was a high degree of uncertainty in
training during the ﬁeld follow-up observations, and we needed to fully account for these
confounding factors’ effects. Thirdly, due to realistic conditions, we have only discussed
the physiological performance of athletes. Competitive sports are a results-driven business,
and more data on sports performance are needed. Regarding further research, it would be
interesting to replicate the present study with more athletes, different genders, different
exposure times at moderate altitudes, or even different kinds of altitude training modalities
with a control group.
5. Conclusions
Four weeks of training at a moderate altitude positively affected the athletes’ lactate
threshold and upper-body muscle mass. Maximum oxygen uptake was reduced in athletes
tested at high altitudes compared to sea level due to being in a hypoxic environment. The
aerobic capacity indicators (maximal oxygen uptake, haemoglobin, and RBC count) did
not improve signiﬁcantly after the athletes returned to sea level compared to pre-moderate
altitude training, which may be related to altitudes that were too low, the duration of
exposure to the hypoxic environment, and the training load design. When imposing
moderate altitude training on athletes, exposure time, training load characteristics, and
nutritional strategies must be meticulously designed to optimise training outcomes.
Author Contributions: Data curation, Y.Y., R.W. and D.L.; formal analysis, Y.Y. and R.W.; investi-
gation, Y.Y., R.W. and D.L.; methodology, Y.Y. and Y.L.; project administration, Y.L.; supervision,
Y.L.; writing—original draft, Y.Y. and R.W; writing—review and editing, Y.Y., R.W., D.L. and Y.L. All
authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: The study was conducted in accordance with the Declaration
of Helsinki, and approved by the Sports Science Experimental Ethics Committee of Beijing Sport
University (Grant No. 201906711, approved on 5 July 2019).
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Not applicable.
Acknowledgments: Thanks to the athletes who participated in the study and the Chinese National
Olympic Committee.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
Abbreviations
LIT: low-intensity training; MIT, moderate-intensity training, HIT, high-intensity training, HR = heart
rate, VO2max = maximal oxygen uptake, RER = respiratory exchange ratio, HCT = hematocrit,
HGB = hemoglobin, RBC = red blood cell, CK = creatine kinase, BUN = blood urea, %LYM = percentage
Int. J. Environ. Res. Public Health 2023, 20, 266
10 of 11
of lymphocyte, %MXD = percentage of mononucleosi, %NEUT = percentage of neutrophil, WBC = white
blood cell.
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Monitoring Physiological Performance over 4 Weeks Moderate Altitude Training in Elite Chinese Cross-Country Skiers: An Observational Study.
Yu, Yichao,Wang, Ruolin,Li, Dongye,Lu, Yifan