Effects of Eight Weeks Altitude Training on the Aerobic Capacity and Microcirculation Function in Trained Rowers

Abstract

Meng, Zhijun, Huan Gao, Tao Li, Peng Ge, Yixiao Xu, and Binghong Gao. Effects of eight weeks altitude training on the aerobic capacity and microcirculation function in trained rowers. High Alt Med Biol. 22:24–31, 2021.

Background:

The mechanism of aerobic improvement after altitude training (AT) has not been resolved yet. Few studies have looked at microcirculation changes after AT in athletes.

Materials and Methods:

Thirty-three male rowers were recruited and divided into either the AT (n = 18, altitude 2,280 m) or the sea level training (ST group, n = 15, altitude 50 m) for 8 weeks training. Microcirculation function was monitored using a laser Doppler flowmeter. VO_2peak and ergometer 5 km time trial (Er5k) were conducted.

Results:

Within the AT group there was an 8.8% increment in VO_2peak from pre- to post-training (4,708.9 ± 455.2 vs. 5,123.3 ± 391.2 ml/min, p < 0.01), whereas in ST group there was a 3.1% increase of VO_2peak from pre- to post-training (4,975.4 ± 501.1 vs. 5,128.0 ± 499.3 m/min, p = 0.125). Er5k performance in AT group was significantly improved (1,040.3 ± 26.3 vs. 1,033.2 ± 27.5 seconds, p = 0.038), whereas in ST group Er5k performance was not improved (1,059.6 ± 30.9 vs. 1,060.4 ± 33.2 seconds, p = 0.819). Postocclusive reactive hyperemia reserve and heat reserve in the forearm of AT subjects increased significantly after 8 weeks. Meanwhile, the AT group's resting blood flow and cutaneous vascular conductance (CVC) of the thigh were higher after AT. For the ST group, resting blood flow and CVC in the thigh decreased significantly at third week post-training. There was a low correlation between the change of VO_2peak and blood flow of the thigh (r = 0.45, p = 0.01).

Conclusions:

Trained rowers benefit more from 8 weeks of AT than from 8 weeks ST in terms of aerobic capacity. We have found that 8 weeks of AT increases thigh blood flow and improves endothelial function.

Introduction

The main aim of performing altitude training (AT) was to improve the aerobic capacity of athletes at sea level (Suzuki et al., 2014; Rodriguez et al., 2015). However, whether athletes benefit from AT or not is still controversial (Robach and Lundby, 2012; Lundby and Robach, 2016; Diebel et al., 2017; Millet and Brocherie, 2020). The reasons for varied results of AT in different studies are complicated. Differences in altitude levels, training programs, individual differences in blood iron levels, and other parameters may all lead to differences in the findings (Bailey and Davies, 1997). Moreover, the mechanism of improvement in aerobic capacity due to AT has not been conclusively determined yet.

Studies have shown that the hypoxia condition during AT can increase erythropoietin (EPO) and total hemoglobin mass (Hbmass) (Heinicke et al., 2003; Robach et al., 2004). Levels of Hbmass correlates (r = 0.97) with the maximum oxygen uptake (VO_2max) (Schmidt and Prommer, 2010). During altitude Hbmass was estimated to increase by ∼1.1%/100 hours for living high-training low and classic altitude. Camps as short as 2 weeks of classic and living high-training low altitude will quite likely increase Hbmass and most athletes can expect benefit (Gore et al., 2013). The linear relationship between hypoxic exposure and percentage increase in Hbmass, and for a total hypoxic exposure of ∼300 hours, Hb mass increases ranged from +2.4% to +6.1% (Millet et al., 2019).

However, it was reported that AT and hypoxia training does not increase Hbmass (Gore et al., 1998; Robach and Lundby, 2012). Therefore, the beneficial effects of AT in athletes are not only derived from hematological factors, but also derived from some nonhematological factors, such as angiogenesis, glucose transport, glycolysis (Gore et al., 2007), and vascular function (Asano et al., 1998).

Hypoxia condition is a common characteristic seen in individuals during exposure to high-altitude environments. Studies have shown that microcirculation is involved in adaptation to hypoxia (Ovadia-Blechman et al., 2015; Treml et al., 2018). Hypoxia causes cutaneous vasodilation and robust increases in cutaneous blood flow in healthy humans (Simmons et al., 2007; Lawley et al., 2014). Intermittent hypoxia training has a positive effect on hemodynamics, microvascular endothelial function, and work capacity in untrained healthy men (Shatilo et al., 2008).

However, the issue of how a hypoxic stimulus influences microcirculation and microcirculatory blood flow is still debating and under investigation (Simmons et al., 2007; Siebenmann et al., 2017; Treml et al., 2018). Paparde et al. found that acute systemic hypoxia causes sympathetic stimulation to the heart, which results in an increased heart rate and enlarged cardiac output, and that may lead to increment in forearm skin blood flow increment during acute systemic hypoxia (Paparde et al., 2015). Studies suggest that under hypoxia, a decrease in oxygen delivery to the cells could result in vasodilation in the capillaries to enhance blood flow in microcirculation (Delashaw and Duling, 1988). The role of the cutaneous microcirculation is to maintain the supply of oxygen and various nutrients to the tissues and to remove waste products of muscle metabolism.

Results from several studies show that aerobic and/or resistance training have a positive effect on microcirculation function and aerobic capacity in diabetic or overweight women (Hodges et al., 2010; Suksom et al., 2015). However, there are few studies on microcirculation changes after AT of trained athletes and its relation with changes in aerobic capacity. Therefore, the purpose of this study was to examine in a controlled setting whether 8 weeks of AT could improve the aerobic capacity and microcirculation function in trained rowers and to further explore the relationship between these two parameters. We hypothesized that AT would improve the rowers' aerobic capacity and microcirculation function, and the improved microcirculation may be the cause of the improved aerobic capacity.

Methods

Study design

The research question is whether the AT can improve the aerobic capacity and microcirculation function of the subjects, and whether the change of aerobic capacity is related to the change of microcirculation function. We recruited rowers to attend both AT camp and sea level training (ST) camp for 8 weeks.

The aerobic capacity of AT and ST group were conducted before and at third week post-training at sea level. The microcirculation function was measured five times: at baseline, on the third and the sixth week of training, and on the third day and third week post-training. It would not be practical to transfer AT group to sea level for testing microcirculation function at the third and sixth week of training, because the two training camps are about 2,000 km apart. Microcirculation function was tested at third and sixth week at altitude to observe its changing characteristics and trends at altitude compared with the microcirculation of ST group at sea level. The aerobic capacity and microcirculation function were measured at third week post-training as it is the recommended time to compete after AT (Chapman et al., 2014b).

Ethical Approval

All of the subjects were made aware of the potential risks and benefits of the research study before them providing written informed consent, according to the guidelines of Shanghai University of Sport. All of the aspects of this research study complied with the Declaration of Helsinki. This study was approved by the Institutional Review Board of Shanghai University of Sport (approval no.# 102772019RT033).

Subjects

Thirty-three male rowers were recruited from the Shanghai Rowing Team and were divided into either the AT group (training high and living high, AT, n = 18) or the ST group (training low and living low, ST, n = 18, three participants had to quit because of injuries). Based on an updated panorama of the different AT methods (Girard et al., 2017), the AT method of AT group was living high and training high (LHTH). Characteristics of the participants in the study are presented in Table 1.

Table 1.

Characteristics of Participants

	AT	ST
n	18	15
Age/years	18.4 ± 2.0	19.3 ± 3.3
Height/cm	188.7 ± 6.2	188.9 ± 5.8
Preweight/kg	80.0 ± 8.0	82.5 ± 13.6
Postweight/kg	78.1 ± 8.0	82.7 ± 12.8
Prebody fat/%	14.7 ± 3.0	14.0 ± 4.2
Postbody fat/%	14.1 ± 2.4	13.4 ± 2.4

AT, altitude training; ST, sea level training.

Measurement and calculation of body fat

A trained operator performed skinfold thickness measurements twice with a caliper (Lange) on the subject's right side at the triceps and subscapular. If the two measurements differed by >2 mm, a third measurement was taken, and the two closest values were then averaged as the final value. Equation of Drunin and Womersley was used to estimate body density. Body density values were then converted to % body fat values according to Siri equations (Bacchi et al., 2017).

Training program

The participants completed the 8-week AT or ST program. The AT group lived and trained daily at altitude (Huize, Yunnan, China, 2,280 m, hypobaric hypoxia), whereas the ST group also lived and trained together at a training camp for 8 weeks, which located in the aquatic training base (50 m), Qiandao Lake, Zhejiang province, China. The training volume and the training intensity for the two groups were similar (Table 2).

Table 2.

Altitude Training and Sea Level Training Program

	Group	Training volume (km)	Objectives of training
Week 1	AT	210	Adapt to hypoxia training, large training volume
Week 1	ST	200	Endurance training to improve aerobic capacity
Week 2	AT	246	Maintain training volume
Week 2	ST	230	Maintain training volume
Week 3	AT	150	Decrease training volume
Week 3	ST	185	Decrease training volume
Week 4	AT	190	Increase training volume
Week 4	ST	203	Increase training volume
Week 5	AT	218	Increase training volume
Week 5	ST	220	Increase training volume
Week 6	AT	205	Maintain training volume
Week 6	ST	210	Maintain training volume
Week 7	AT	167	Increase training intensity
Week 7	ST	190	Increase training intensity
Week 8	AT	133	Decrease training volume and prepare for test after intervention
Week 8	ST	170	Decrease training volume and prepare for test after intervention
Total volume	AT	1,519
Total volume	ST	1,608

Measurement of microcirculation

Forearm and leg cutaneous blood flow, concentration of moving blood cells (CMBC), velocity and transcutaneous oxygen pressure (TcPO₂) were monitored using a laser Doppler flowmeter (PeriFlux6000, Perimed, Sweden) at room temperature (22°C), with the subject in a supine position. The forearm and leg blood flow were also measured after exposure to localized heating of 44°C for 3 minutes. Microcirculation function was evaluated from the maximal postocclusive reactive hyperemia (PORH) after forearm ischemia for 3 minutes through cuff inflation (200 mmHg). Similar procedures have been used by other investigators (Lenasi and Strucl, 2004; Shatilo et al., 2008; Tew et al., 2010). Blood pressure was measured by brachial auscultation. Pulse oxygen saturation (SpO₂) was measured by a handheld pulse oximeter (9500; NONIN).

Measurement of VO_2peak

VO_2peak tests were conducted before training and at third week post-training using an incremental rowing ergometer (Concept 2 Model D) protocol through a portable metabolic analyzer (Cortex MetaMax 3BR2, Germany). The relative VO_2peak (RVO_2peak) is the VO_2peak divided by the participants' body weight. The test started at the workload of 180 W. Thereafter, the power was increased by 30 W every 2 minutes until the participant reached volitional exhaustion.

Measurement of ergometer 5 km

Subjects performed an all-out efforts 5 km time trial on a rowing ergometer (Er5k) (Concept 2 Model D), which was used in other studies to evaluate rowers' aerobic performance (Volianitis et al., 2001; Hinckson et al., 2006). First of all, the environmental factors (wind, waves, water temperature, etc.), which are likely to influence rowing performance, were controlled in this simulated condition. Furthermore, rowing ergometer is a typical equipment used by coaches for training and fitness/performance testing. Finally, the aerobic capacity of rowers was evaluated by a 5,000 m race on the rowing ergometer. Before testing, participants warmed up for 30 minutes on the ergometer and adjusted the drag factor according to their usual habits. After testing, results from the monitors were recorded.

Statistics

A signal processing software was used for offline analysis of laser Doppler data (Perisoft for Windows 2.5.5; Perimed AB). Microcirculation function were defined as follows: (1) baseline, that is, the arithmetic mean of the fifth and sixth minutes, perfusion unit (PU); (2) the lowest and highest blood flow of PORH; (3) plateau, that is, the arithmetic mean of the last 2 minutes of heating at 44°C; and (4) the skin blood flow data (recorded in arbitrary PU). The baseline PU was divided by the mean arterial pressure (in millimeters of mercury) to give cutaneous vascular conductance (CVC; in PU per millimeter of mercury) (Lawley et al., 2014).

The data were analyzed using SPSS 25.0 (IBM). The Kolmogorov–Smirnov test was used to establish normality of the data. The differences between the AT and ST group were analyzed using a linear mixed model for repeated measures. The interaction of group by time was evaluated to examine the time effect in responses to the intervention between groups. Different times in the AT/ST group were analyzed using a Wilcoxon signed-rank test for non-normally distributed data. Significance was accepted at p < 0.05, with a trend of significance at 0.05 < p < 0.1. Pearson's correlation analysis was used to identify relationship of blood flow and endothelial function with the VO_2peak. The magnitude of correlation (r) was rated as follows: little (0.00 < |r| < 0.25), low (0.26 < |r| < 0.49), moderate (0.50 < |r| < 0.69), high (0.70 < |r| < 0.89), and very high (|r| > 0.90) (Imai and Kaneoka, 2016).

All of the normally distributed values are expressed as mean ± standard deviation, and all of the non-normally distributed values are expressed as the interquartile range and median (P₂₅, P₇₅). For effect size, the partial eta squared statistic was calculated, and 0.01, 0.06, and 0.14 were interpreted as small, medium, and large effect sizes, respectively (Richardson, 2011).

Results

VO_2peak and relative VO_2peak

General linear mixed model with repeated measures has one between-subject factor, (group with two levels, AT and ST) and one within-subject factor (time, pre, and post). As shown in Table 3, there was a significant time by group interaction effect on VO_2peak, p < 0.01. In AT group VO_2peak was increased by 8.8% after the intervention (4,708.9 ± 455.2 vs. 5,123.3 ± 391.2 ml/min, p < 0.01), whereas in ST group there was a 3.1% increase of VO_2peak from pre- to post-training (4,975.4 ± 501.1 vs. 5,128.0 ± 499.3 ml/min, p = 0.125). There was a significant time by group interaction effect on the relative VO_2peak, p < 0.01. Relative VO_2peak of AT was significantly improved [58.9 ± 4.9 vs. 66.0 ± 5.1 ml/(min·kg), p < 0.01], whereas the relative VO_2peak in ST group did not change significantly [61.3 ± 7.4 vs. 62.8 ± 7.4 ml/(min·kg), p = 0.217].

Table 3.

The VO₂_pea_k, RVO₂_pea_k, and Er5k of Participants

	AT		ST		Interaction effect	Effect size
	Pre	Post	Pre	Post	p	η²
VO_2peak (ml/min)	4,708.9 ± 455.2	5,123.3 ± 391.2^a	4,975.4 ± 501.1	5,128.0 ± 499.3	0.001	0.167
RVO_2peak [ml/(min·kg)]	58.9 ± 4.9	66.0 ± 5.1^a	61.3 ± 7.4	62.8 ± 7.4	0.001	0.332
Er5k (seconds)	1,040.3 ± 26.3	1,033.2 ± 27.5^a	1,059.6 ± 30.9	1,060.4 ± 33.2	0.105	0.083

p < 0.05 versus baseline.

Rowing ergometer 5 km performance

For rowing Er5k performance, no significant time by group interaction effect was observed (p = 0.105). However, performance in AT group was significantly improved (1,040.3 ± 26.3 vs. 1,033.2 ± 27.5 seconds, p = 0.038). In contrast, performance in ST group was not significantly improved (1,059.6 ± 30.9 vs. 1,060.4 ± 33.2 seconds, p = 0.819).

Microcirculation function

As shown in Table 4, there was a statistically significant difference in SpO₂, resting blood flow, TcPO₂, heat reserve, and PORH reserve measurements from the forearm, between different time points in the two groups. Similarly, there was a statistically significant difference in resting blood flow, CMBC, heat reserve, and CVC in thigh between different times points in the two groups. This was determined by repeated measure linear mixed models (p < 0.05).

Table 4.

Characteristics of Microcirculation-Related Parameters

		AT					ST					IE
		Baseline	Week 3	Week 6	Post-3rd day	Post-3rd week	Baseline	Week 3	Week 6	Post-3rd day	Post-3rd week	p
	SpO₂ (%)	98.1 ± 0.8	94.7 ± 1.2^a	94.8 ± 1.9^a	98.3 ± 0.6	97.8 ± 1.3	97.7 ± 0.5	97.7 ± 0.8	98.0 ± 0.6	97.8 ± 0.6	98.2 ± 0.8	0.001
Forearm	Resting (PU)	12.0 (9.9, 18.5)	14.0 (11.0, 17.5)	13.0 (9.9, 17.3)	10.0 (8.6, 13.8)	14.5 (8.2, 19.0)	11.5 (7.6, 13.0)	8.6 (8.2, 10.8)	11.0 (9.3, 13.0)	8.3 (7.4, 13.3)	10.0 (8.3, 13.0)	0.035
	CMBC	112.0 (78.0, 147.5)	122.0 (97.3, 164.5)	112.0 (89.3, 128.8)	125.0 (91.8, 156.0)	125.5 (82.8, 155.5)	80.0 (69.3, 128.5)	114.5 (93.0, 143.0)	126.0 (99.0, 142.0)	108.0 (93.0, 148.0)	108.0 (75.0, 132.0)	0.753
	Velocity	12.5 (9.2, 14.3)	11.5 (10.7, 15.0)	12.0 (10.8, 15.3)	9.5 (8.9, 11.0)^a	11.5 (9.0, 15.0)	11.5 (10.3, 14.0)	8.7 (7.0, 9.6)^a	9.9 (8.9, 11.0)^a	8.7 (8.2, 10.0)^a	11.0 (9.0, 12.0)	0.001
	TcPO₂	80.0 (69.0, 84.3)	60.0 (56.8, 75.0)^a	64.0 (60.3, 73.3)^a	88.0 (86.0, 103.5)^a	76.5 (66.0, 83.3)	101.0 (83.0, 104.0)	93.5 (84.0, 106.8)	86.0 (74.0, 102.0)	79.5 (72.0, 93.3)^a	70.0 (59.0, 78.0)^a	0.001
	Heat (PU)	104.5 (87.3, 141.0)	131.5 (87.0, 167.3)	111.0 (97.3, 145.5)	112.5 (89.5, 170.5)	123.5 (96.0, 148.3)^b	110.5 (80.3, 144.5)	121.0 (86.3, 159.3)	123.0 (109.0, 179.0)	101.0 (75.5, 149.3)	132.0 (90.0, 162.0)	0.987
	Heat reserve	8.2 (7.0, 12.1)	9.3 (6.3, 12.7)	9.1 (7.5, 12.1)	11.6 (10.7, 14.9)^a	10.4 (6.7, 14.8)	11.8 (8.5, 13.6)	13.2 (10.7, 16.3)	10.5 (8.9, 16.8)	10.3 (9.3, 14.5)	12.5 (7.0, 13.9)	0.068
	PORH (lowest PU)	3.2 (3.0, 3.5)	2.6 (2.4, 2.8)^a	3.3 (2.9, 3.8)	2.8 (2.5, 3.3)^b	3.0 (2.6, 3.2)^b	2.9 (2.6, 3.4)	2.7 (2.5, 3.6)	3.7 (2.8, 3.9)	3.2 (2.9, 3.5)	3.3 (2.7, 3.6)	0.008
	PORH (highest PU)	44.5 (35.0, 60.0)	48.5 (41.3, 71.5)	55.0 (47.3, 70.8)^b	53.0 (36.8, 65.3)	54.0 (38.0, 83.5)^b	48.5 (36.5, 81.0)	47.0 (37.3, 57.8)	70.0 (37.0, 88.0)	45.0 (39.3, 76.8)	58.0 (42.0, 77.0)	0.686
	PORH reserve	3.6 (3.2, 4.3)	3.6 (3.1, 5.1)	4.5 (3.5, 5.8)^a	4.9 (4.4, 5.8)^a	4.4 (3.7, 5.7)^a	5.5 (4.0, 7.4)	5.0 (4.4, 5.8)	4.0 (3.8, 6.0)	5.5 (4.1, 6.5)	6.1 (3.5, 7.8)	0.013
	CVC (PU/mmHg)	0.13 (0.11, 0.21)	0.15 (0.12, 0.20)	0.14 (0.11, 0.17)	0.11 (0.09, 0.14)^b	0.15 (0.09, 0.19)	0.11 (0.08, 0.13)	0.09 (0.08, 0.11)	0.11 (0.09, 0.12)	0.10 (0.08, 0.14)	0.11 (0.09, 0.15)	0.011
Thigh	Resting (PU)	9.5 (8.3, 13.0)	14.5 (9.8, 20.3)^a	12.0 (8.9, 15.8)	10.9 (8.9, 15.3)	12.0 (9.3, 15.3)^a	10.5 (7.7, 15.8)	7.6 (6.5, 9.2)^b	10.0 (7.5, 13.0)	9.7 (8.3, 19.3)	8.2 (6.8, 12.0)^a	0.003
	CMBC	95.5 (66.5, 113.8)	142.0 (88.8, 176.3)^a	111.0 (85.0, 146.0)^b	96.0 (71.3, 121.0)	108.0 (91.0, 137.5)^b	104.5 (87.5, 117.5)	80.5 (59.3, 118.5)	94.0 (70.0, 128.0)	95.5 (74.0, 131.3)	72.0 (68.0, 86.0)^a	0.004
	Velocity	12.5 (10.8, 14.0)	12.0 (10.0, 13.3)	11.5 (9.0, 16.3)	12.0 (9.3, 713.3)	11.5 (9.8, 13.3)	10.5 (8.7, 16.5)	10.0 (6.8, 13.3)	12.0 (8.4, 14.0)	13.5 (9.3, 18.5)	12.0 (9.4, 14.0)	0.625
	Heat (PU)	93.0 (65.5, 117.3)	91.5 (70.8, 115.6)	89.0 (66.8, 110.5)	91.0 (75.8, 120.0)	109.5 (59.3, 135.0)	91.5 (80.3, 144.5)	81.0 (69.3, 168.0)	67.0 (49.0, 84.0)^a	85.0 (58.8, 102.0)	80.0 (68.0, 105.0)^b	0.268
	Heat reserve	9.1 (7.1, 10.9)	7.0 (4.3, 9.5)	7.8 (6.0, 9.1)	9.2 (6.2, 11.7)	7.6 (6.1, 10.8)	11.1 (5.0, 13.4)	12.0 (10.2, 17.9)	6.5 (6.0, 7.9)	6.4 (4.5, 10.2)	10.4 (7.2, 13.0)	0.017
	CVC (PU/mmHg)	0.11 (0.09, 0.14)	0.15 (0.11, 0.22)^a	0.12 (0.09, 0.17)	0.11 (0.09, 0.16)	0.13 (0.09, 0.16)^b	0.09 (0.08, 0.17)	0.08 (0.07, 0.10)^b	0.09 (0.07, 0.13)^b	0.10 (0.09, 0.20)	0.09 (0.07, 0.13)^a	0.003

IE: group by time interaction effect.

p < 0.05 versus baseline.

0.05 < p < 0.1 versus baseline.

Heat reserve: (heat PU)/(resting PU); PORH reserve: PORH (highest PU)/(resting PU).

CMBC, concentration of moving blood cells; CVC, cutaneous vascular conductance; PORH, postocclusive reactive hyperemia; PU, perfusion unit; TcPO₂, transcutaneous oxygen pressure.

Two related samples in a nonparametric test showed that heat reserve and PORH reserve in the forearm of AT subjects increased significantly at post-third day post-training. At post-third week post-training, PORH reserve also increased significantly from baseline, 3.6 (3.2, 4.3) versus 4.4 (3.7, 5.7). Meanwhile, in the AT group, the resting blood flow in the thigh increased significantly at third week post-training, 9.5 (8.3, 13.0) versus 12.0 (9.3, 15.3). For the ST group, resting blood flow, CMBC and CVC in the thigh decreased significantly at third week post-training.

Correlation analysis was conducted for change in VO_2peak and microcirculation data. We found that there was a low correlation between the change of VO_2peak (post vs. baseline) and change of thigh blood flow (week 6 vs. baseline; r = 0.45, p = 0.01). Also, a low correlation was observed between the change of VO_2peak (post vs. baseline) and change of thigh CVC (week 6 vs. baseline; r = 0.43, p = 0.01). However, we did not find any correlation between forearm blood flow and aerobic capacity.

Discussion

The main findings of this study are as follows: (1) 8 weeks AT (2,280 m from sea level) improves VO_2peak, RVO_2peak, and Er5k significantly, unlike 8 weeks ST that did not change these measurements significantly; (2) 8 weeks AT improves heat reserve and PORH reserve of forearm, resting blood flow, and CVC of the thigh significantly; and (3) the change of VO_2peak of rowers correlates with the change in blood flow, and CVC of the thigh, which may be one of the mechanisms for the AT-induced improvement in rowers' aerobic capacity.

Since the 1968 Mexico City Olympics, studies that use AT to enhance the endurance performance of athletes have emerged; currently, these training methods are commonly being applied by many athletes and coaches. In fact, whether athletes' performance improved from AT or not is still controversial (Robach and Lundby, 2012; Lundby and Robach, 2016; Diebel et al., 2017; Millet and Brocherie, 2020). Using meta-analysis, Park et al. (2016) found that AT appears to be more effective than ST for improvement of aerobic capacity (Park et al., 2016). Our results are in accordance with this finding. We suppose there are two main factors that influence the effect of AT: objective factors and subjective factors. Objective factors mainly include the height and duration of training. Subjective factors mainly include the implementation of training program.

During this study, the rowers conducted 8 weeks AT (2,280 m above sea level). In China, the rowers have been using this training camp for many years. The altitude of this training camp and duration are appropriate “dose” of hypoxia exposure (Levine and Stray-Gundersen, 2006; Gore et al., 2013; Chapman et al., 2014a). Coaches and athletes have a wealth of successful AT experience. These can guarantee the success of this AT. The VO_2peak tests of AT and ST groups were conducted before and after intervention at sea level, so the VO_2peak of AT after AT was improved at sea level at third week after intervention. The aerobic capacity were measured at third week post-training as it is the recommended time to compete after AT (Chapman et al., 2014b).The rowers kept normal training during the 3 weeks after they return from AT camp to sea level.

Majority of sport scientists believe that erythropoiesis is the main contributor to the AT-induced improvement in aerobic capacity (Garvican et al., 2012; Wachsmuth et al., 2013; Swenson and Bartsch, 2015; Hauser et al., 2016). Studies show that more red blood cells carry more oxygen to tissues to maintain the oxygen supply under hypoxic conditions (Mairbaurl, 1994). However, some studies found that AT improves endurance performance without erythropoiesis (Gore et al., 1998) or increases hematological parameters without improving performance (Martinez-Bello et al., 2011). Therefore, erythropoiesis is not the one only cause for the AT-induced improvement in endurance performance.

To our knowledge, the interaction among cardiac output, microcirculation, and O₂-transport capacity in hypoxic condition is not completely understood. Jung et al. (2016) mentioned that the microcirculation in hypoxia is the center of the battlefield competing for oxygen, and the microvasculature plays an important role in inflammation, hyperviscosity, endothelial function, hemodynamic changes, and blood flow regulation.

However, Siebenmann et al. reported that cutaneous exposure to hypoxia has no effect on skin perfusion in humans (Siebenmann et al., 2017). A meta-analysis shows that athletes present enhanced microvascular function compared with untrained but otherwise healthy subjects (Montero et al., 2015). Lanting et al. have explained the reason of this difference is that various exercise intervention improves cutaneous microvascular reactivity in adults (Lanting et al., 2017). PORH reserve, heat reserve, and CVC are important indicators for assessment of cutaneous microvascular reactivity and endothelial function (Shatilo et al., 2008; Tew et al., 2010; Suksom et al., 2011; Lawley et al., 2014; Smith et al., 2014).

In this study, PORH reserve of the forearm and resting blood flow of the thigh are improved after intervention, and the blood flow of the thigh correlated with VO_2peak. Indeed, lower limbs are likely to have larger influence on rowing performance and whole body VO_2peak (Lawton et al., 2013). Microcirculation function was tested at third and sixth week at altitude to observe its changing characteristics and trends at altitude compared with the microcirculation of ST group at sea level. We compared the values at baseline and post-third day and post-third week (measured at sea level) between the groups and found the significant group by time interaction effect of heat reserve of forearm, blood flow of thigh, and CVC of thigh as compared with the values at baseline, third week, sixth week, post-third day, and post-third week.

So, only the values measured at sea level between the groups were compared; we also found the significant interaction effect of microcirculation function between group by time. Maione et al. reported that oxygen uptake kinetics during the constant load exercise subanaerobic threshold are highly sensitive to endothelial function in muscular microcirculation (Maione et al., 2015). Therefore, the improved blood flow and CVC of the thigh may contribute to the increase in VO_2peak.

Conclusion

VO_2peak and ergometer 5 km performance were improved in the AT group. Trained rowers benefit more from 8 weeks of AT than that of ST with regard to aerobic capacity. In addition to traditional research on the hematological system in AT, we have found that 8 weeks of AT increases thigh blood flow and improves microvascular reactivity and endothelial function (PORH reserve and CVC). The blood flow and CVC of the thigh is correlated with the aerobic capacity. The improved microcirculation function appears to be one of the mechanisms for improving the performance. However, more studies are needed to confirm this result.

Limitations

This study compares 8 weeks of AT used by trained rowers with a control group who trained at the sea level. Owing to the difficulty to recruit more subjects, the study lacked another control group, just suffering altitude hypoxic condition, without any endurance training. We did not distinguish the individual effects of altitude condition and an 8-week training period on microcirculation function. Another limitation of this study is that the microcirculation function was not tested at the same altitude at third and sixth week. It would not be practical to transfer AT group to sea level for testing microcirculation function at third and sixth week, because the two training camps are about 2,000 km apart.

Footnotes

Acknowledgments

The authors thank the rowers and their coaches for their immense cooperation.

Authors' Contributions

Conception and design of the article by Z.M., B.G., and H.G. Data acquisition, analysis, and interpretation by T.L., P.G., and Y.X. All authors contributed to the drafting of the article and the revision for critically important content. All authors approved the article and agree to be accountable for the study herein. All persons who qualify for authorship have been listed and all authors meet the criteria for authorship. The corresponding author and all coauthors have reviewed and approved of the article before submission.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The financial support for the study was provided by National Natural Science Foundation of China (No. 31771316) and Shanghai Key Lab of Human Performance, Shanghai University of Sport (No. 11DZ2261100).

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Effects of Eight Weeks Altitude Training on the Aerobic Capacity and Microcirculation Function in Trained Rowers

Abstract

Background:

Materials and Methods:

Results:

Conclusions:

Introduction

Methods

Study design

Ethical Approval

Subjects

Measurement and calculation of body fat

Training program

Measurement of microcirculation

Measurement of VO2peak

Measurement of ergometer 5 km

Statistics

Results

VO2peak and relative VO2peak

Rowing ergometer 5 km performance

Microcirculation function

Discussion

Conclusion

Limitations

Footnotes

Acknowledgments

Authors' Contributions

Author Disclosure Statement

Funding Information

References

Measurement of VO_2peak

VO_2peak and relative VO_2peak