Shinshu Med J, 69⑹：355～362, 2021

Respiratory and Metabolic Responses as Determinantsof Exercise Endurance Time in Constant-load Exercise Testin Healthy Adult Volunteers : A Prospective Cross-sectional Study
Shohei Kawachi1） and Keisaku Fujimoto2）＊1） Department of Biomedical Laboratory Science, Graduate School of Medicine, Shinshu University2） Department of Clinical Laboratory Sciences, Shinshu University School of Health Sciences

Background : Constant-load exercise (CLE) tests have been used to evaluate the effectiveness of treatment withlong-acting bronchodilators or pulmonary rehabilitation in patients with chronic obstructive pulmonary disease(COPD). However, exercise limitation in the CLE test is due not only to pulmonary dysfunction but also to leg
muscular strength and metabolic efficiency of the muscle, which are affected by daily physical activity andexercise habits. The present study was performed to examine the factors involved in exercise limitation in theCLE test other than pulmonary dysfunction in healthy adult volunteers to establish baselines before applying
the study in COPD patients.
Methods : Twenty-seven healthy volunteers performed the CLE test using a treadmill at 80 ％ of maximum load
determined by incremental-load exercise (ILE) test up to a maximum of 20 minutes. Subjects were divided into
those who were and were not able to complete 20 minutes of the CLE test. The relative exercise load was calcu­
lated as : peak oxygen uptake (VO2) in the CLE test/VO2 at anaerobic threshold in the ILE test. The metabolic
variables and relative exercise load were compared between the two groups.
Results : In the CLE test, the non-completion group showed significantly higher respiratory exchange ratio(RER, ＞1) as a variable of the anaerobic threshold. The relative exercise load of the CLE test and the actual
exercise load in the non-completion group were higher compared to the completion group.
Conclusion : This study showed that the high relative exercise load in the CLE test causes the subject to exceed
anaerobic threshold and limits maintenance of exercise in the CLE test. Shinshu Med J 69 : 355―362, 2021
(Received for publication October 15, 2020 ; accepted in revised form August 10, 2021)
Key words : cardiopulmonary exercise test, anaerobic threshold, ventilatory inefficiency,
exercise tolerance

Ⅰ Introductionease (COPD)1）-3）. The CPE test is usually performed
as an incremental-load exercise (ILE) test or a con­

The cardiopulmonary exercise (CPE) test is per­stant-load exercise (CLE) test. Generally, the ILE test

formed to determine factors limiting exercise toler­is used to evaluate maximum oxygen intake as exer­

ance and to evaluate the efficacy of pulmonary reha­cise tolerance and respiratory metabolism by symp­

bilitation and bronchodilators for exercise tolerance

tom-limited exercise and the limiting factors forin patients with chronic obstructive pulmonary dis­

exercise tolerance. The CLE test is usually performed
at approximately 70 ％-80 ％ of peak VO2 determined＊

Corresponding author : Keisaku Fujimoto
Department of Clinical Laboratory Sciences,
Shinshu University School of Health Sciences,3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan
E-mail : keisaku@shinshu-u.ac.jp

No. 6, 2021

in the ILE test4）5）, and is also used to evaluate not
only the efficacy of treatment with bronchodilatorsand pulmonary rehabilitation as in the ILE test, butalso dynamic lung hyperinflation in patients with355

Kawachi・Fujimoto

COPD. In the CLE test, the metabolic kinetics have

tom-limited ILE test was performed, and then thebeen reported to reach a plateau at 5-10 minutes

CLE test was conducted at a load of 80 ％ of theafter commencement of exercise, and healthy sub­maximum exercise capacity determined in the ILE1）

jects can maintain exercising for at least 10 minutes .

test. The endurance time of the CLE test had anHowever, most patients with COPD are unable toupper limit of 20 minutes. The subjects were divided

maintain exercising for more than 10 minutes eveninto those who were and were not able to completewith the exercise load of the CLE test based on indi­

20 minutes of exercise in the CLE test designated as6）

vidual exercise capacity determined in the ILE test .

the completion group and the non-completion group,Therefore, exercise limitation in the CLE test may berespectively, and metabolic variables in each exercise

due not only to pulmonary dysfunction but also totest were compared between the two groups. A

leg muscular strength and metabolic efficiency of thethreshold of 20 minutes was used to divide the sub­muscle, which are affected by daily physical activi­

jects into the two groups based on a previous study

ty and exercise habits. The present study was per­in which the CLE test was performed for at least 20

formed in healthy adult volunteers to determine theminutes in healthy subjects8）.

factors responsible for exercise limitation in the CLE

ＣEvaluation methods

test while excluding the effects of pulmonary dys­１

Spirometryfunction prior to applying the study in COPD patients.

Spirometry was performed using a spirometer

The CLE test is characterized by more reflective(Fukuda Denshi Co., Ltd., Tokyo, Japan). All mea­

muscle metabolism and greater promotion of lactic

surements were performed according to the Japa­

7）

acidosis than the ILE test . We postulated that there

nese Respiratory Society guidelines for lung function

may be some differences in muscle metabolism andmeasurements. For predicted values of FEV1 andthe threshold of aerobic metabolism in the CLE testVC, Japanese local reference data developed by thebetween subjects who can maintain CLE for a long

Japanese Respiratory Society were adopted9）.

time and those who cannot. In the present study,２Symptom-limited incremental-load exercise test

pulmonary metabolic variables were compared be­

Both exercise tests were performed using a tread­

tween healthy volunteers without pulmonary dys­

mill (Auto Runner AT-200 ; Minato Medical Science

function divided into two groups according to wheth­Co., Ltd., Osaka, Japan), and methods were as de­er they could maintain exercise for more than 20

scribed previously10）. Oxygen uptake (VO2, mL/kg/

minutes or not in the CLE test.

min), carbon dioxide production (VCO2, mL/kg/min),ⅡＡ

Materials and Methods

Subjects

respiratory exchange ratio (RER＝VCO2 /VO2 ), endtidal oxygen (ETO2 , ％ ), end-tidal carbon dioxide(ETCO2, ％), minute ventilation/oxygen uptake (VE/The study population consisted of 27 healthy vol­VO2), minute ventilation/carbon dioxide production

unteers (24 males and 3 females) from 18 to 60 years(VE/VCO2), minute ventilation (VE, L/min), tidal volume

old with normal spirometry results who performed(VT, L), and respiratory rate (RR, times/min) were

exercise tests at Shinshu University between March

recorded using the breath-by-breath measure­2017 and December 2018. Subjects were included inments in each exercise test determined using a breath

the study regardless of their exercise habits. Exclu­

analyzer system (AE-310S Aeromonitor ; Minato

sion criteria were current treatment for asthma or

Medical Science). These metabolic variables during

heart failure, and musculoskeletal diseases that could

exercise measured in a breath-by-breath manner

affect exercise testing.

were defined as the average value for 30s before theＢend of the exercise, which was called the “peakStudy design
This study had a prospective cross-sectional de­value.” We evaluated the anaerobic threshold (AT)sign. All subjects underwent spirometry. The symp­

point by the V-Slope method, and the respiratory

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Shinshu Med JVol. 69

Determinants of exercise tolerance in constant-load testcompensation (RC) point was determined by thethe CLE test/VO2 at the AT in the ILE test.

VE-VCO2 slope method according to the ATS/ACCPＤ3）

Statistical analysisstatement . Pulse rate (PR, beats/min) during exer­

As there have been no similar previous studies,cise was measured continuously with a fingertip

the required sample size could not be calculated.

monitor (AE-310S Aeromonitor ; Minato Medical

Therefore, we calculated the statistical power byScience). Percutaneous oxygen saturation (SpO2, ％)post hoc analysis. Normality was analyzed by thewas measured with a wrist-worn pulse oximeter

method of Kolmogorov-Smirnov and then Student’s tＴＭ

test was used for comparison of metabolic variables

Co., Ltd., Tokyo, Japan).

between the completion group and the non-comple­(WristOX2, Model 3150 ; Philips Electronics JapanThe ILE test protocol was performed on a treadmilltion group. Finally, stepwise multiple regression

adopting the TR-3 protocol, which is designed toanalysis was performed to determine the factors

increase VO2 linearly. In this protocol, the speed andaffecting exercise tolerance for 20 minutes in theslope changed continuously and this was the same

CLE test with the completion group or the non-method used for determination of the AT referencecompletion group as independent variables and the11）value in Japanese subjects . Subjects continued tovariables that were significantly different between

exercise until limited by symptoms, and maximum

the two groups in the CLE test and the relative ex­

exercise capacity and maximum exercise load wereercise load as explanatory variables. The number ofevaluated. Subjects walked on a treadmill at a speed

explanatory variables in multiple regression analysisof 1 km/h with a slope of 0 ％ for 3 minutes before

was calculated based on the sample size of theand after the ILE test as warm-up and cool-down,present study. In all analyses, P＜0.05 was taken torespectively.

indicate statistical significance. SPSS ver. 25 was３

used for statistical analyses (SPSS Inc., Chicago, IL).

Symptom-limited constant-load exercise test
After sufficient rest, the symptom-limited CLE

ＥEthics approval and consent to participate

test was conducted at a load of 80 ％ of the maximum

All subjects were given an adequate explanationload in the ILE test as described previously10）. First,of the study and provided written informed consent

the subjects walked for 3 minutes as a warm-up at ato participation. This study was conducted in accor­

speed of 1 km/h with a slope of 0 ％ on the treadmill.

dance with the ethical principles for medical re­

Next, they began to exercise at up to 80 ％ of thesearch involving human subjects of the Declaration

maximum load determined in the ILE test. The CLEof Helsinki after obtaining approval from the Shinshu

test was performed to the point when the subject

University Medical Ethics Committee (approval

could no longer maintain exercise or a time limit ofnumber : 3705).

10）

20 minutes had been reached . After the CLE test,
the subjects walked for 3 minutes as a cool-downwith the same load as in the warm-up. The subjects

ⅢResults

Ａ Characteristics of physical findings, lung

who were not able to complete 20 minutes of exercise

function, and complicationsin the CLE test were evaluated to determine whether

Table 1 shows the characteristics of physical find­they had performed maximum-effort exercise in con­ings and spirometry. The study population consisted

sideration of 85 ％ of the maximal age-predicted heartof a total of 27 subjects. Eleven of the 27 subjects12）

rate . The subjects were divided into those whowere university students who exercised regularly as

were and were not able to complete 20 minutes ofsports club members, while the remaining 16 subjects

exercise in the CLE test, defined as the completion

were working persons with no regular exercise hab­group and the non-completion group, respectively.

its. Physical findings and lung function were not sig­

The relative exercise load of the CLE test to the AT

nificantly different between the two groups, and all

point in the ILE test was calculated as : peak VO2 invalues of spirometry were within reference ranges inNo. 6, 2021

357

Kawachi・Fujimoto
Table 1 Subject characteristics of basic physicalfindings, lung function
nAge, years
Sex (male/female)
Height, cm
Body weight
BMI, kg/m2％VC, ％％FEV1, ％FEV1/FVC, ％all subjects.
Ｂ Comparison of metabolic variables in27
29.3±2.2
24/3
170.8±1.5
65.0±1.822.3±0.6
91.5±2.4
89.7±2.287.1±2.6

Notes : Values represent the means±standard error ofthe mean.
Abbreviations : BMI, body mass variable ; VC, vital
capacity ; FEV1, forced expiratory volume in 1 second ;FVC, forced vital capacity.

incremental-load exercise test
Table 2 shows the results of the ILE test. The
peak VO2 as a general variable of exercise tolerance
was 44.9±1.7 ml/kg/min, which was higher in thenon-completion group, although the difference was
not significant (43.4±8.7 vs. 47.5±3.0 ml/kg/min,respectively). The AT points did not differ between
the two groups and were within the reference ranges
in Japanese subjects (26.0±4.7 vs. 25.7±4.2 ml/kg/min, respectively) There were also no significant differences in other metabolic variables between thetwo groups. The speed and angle of the treadmill
were higher in the non-completion group, although
the differences were not significant. The relationships between these exercise loads, AT points, andTable 2 Results of metabolic variables in incrementalload exercise testnVO2, ml/kg/min

VCO2, ml/kg/min
RER
ETO2, ％ETCO2, ％VE/VO2

VE/VCO2

VE, L/min
VT, L
VD/VT
RR, breaths/min
PR, beats/min
SpO2, ％

AT point, ml/kg/min
RC point, ml/kg/minMinimum VE/VCO2

peak VO2 are shown in Fig 1.
Ｃ Comparison of metabolic variables in 2744.9±1.756.5±2.2
1.26±0.01
15.4±0.1
6.2±0.1

constant-load exercise test
Table 3 shows a comparison of metabolic variablesin the CLE tests. Although the actual exercise load
was significantly higher in the non-completion groupthan the completion group, the relative exercise load38.7±1.4of the CLE test was also higher in the non-completion30.3±0.5
109.7±4.1
2.4±0.1
0.27±0.01
44.7±1.2
167.2±4.9group, although the difference was not significant

89.7±5.1
25.9±0.938.7±1.427.9±0.6

Notes : All values of metabolic variables except the ATpoint, RC point, and minimum VE/VCO2 are shown at thepeak of exercise. Values represent the means±standard
error of the mean.
Abbreviations : VO2, oxygen uptake ; VCO2, carbon dioxide
output ; RER, respiratory exchange ratio ; ETO2, end-tidal

oxygen, ETCO2 , end-tidal carbon dioxide ; VE, ventila-tion ; VT, tidal volume ; VD, dead-space gas volume ; RR,respiratory rate ; PR, pulse rate ; SpO 2 , percutaneous
oxygen saturation ; AT, anaerobic threshold ; RC, respiratory compensation.

(140.2±28.9 ％ vs. 158.3±19.2 ％ , respectively, P＝0.052). The non-completion group showed significantly higher values of peak VCO2, RER, and VE than thecompletion group. The RER in the non-completiongroup exceeded 1 (Fig. 2). The minimum VE/VCO2, asa variable of ventilatory inefficiency, was significantly
higher in the non-completion group than in the completion group.
ＤResults of stepwise multiple regression analysis

Stepwise multiple regression analysis was performed with the completion or the non-completiongroup as independent variables and relative exercise
load, RER, minimum VE/VCO2, and VE as explanatory variables. The minimum VE/VCO2 remained as thefinal variable (P＜0.05).
ⅣDiscussion

The non-completion group showed significantly
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Determinants of exercise tolerance in constant-load test

Fig. 1 Relationships between actual exercise load, AT point, and peak VO2 in the incremental load
exercise test (ILE test).
Note : Panel A (left) shows VO2 and treadmill speed in the ILE test. Panel B (right) shows VO2 and treadmill angle

in the ILE test. Open circles (○), completion group ; closed circles (●), non-completion group ; open triangles (△),
AT point of the completion group ; closed triangles (▲), AT point of the non-completion group. The AT point isplotted at the intersection of the VO2 lines.

Abbreviations : VO2, oxygen uptake ; AT, anaerobic threshold.
Table 3Comparison of metabolic variables in constant load exercise test
Completion

Non-CompletionnTreadmill speed, km/h

175.0±0.2

105.5±0.3＊Treadmill angle, ％14.2±0.516.2±5.5＊

Relative exercise load, ％Endurance time, min
VO2, ml/kg/min

140.2±28.9-158.3±19.2
9.9±2.035.2±1.841.5±2.6＊34.3±1.942.7±3.1＊

RER

0.97±0.01

1.01±0.02＊

ETO2, ％ETCO2, ％14.1±0.615.2±0.8

VCO2, ml/kg/min

5.7±0.6

5.4±0.633.3±5.0

35.9±4.9

VE/VCO234.6±4.1
76.4±4.1

35.4±3.692.2±7.0＊

VT, L
VD/VTRR, breaths/min
PR, beats/min
SpO2, ％

1.76±0.08
0.27±0.02
43.9±1.9177.3±4.4

2.19±0.20
0.26±0.03
43.7±3.8
185.2±2.2

VE/VO2VE, L/minMinimum VE/VCO2

91.2±1.0

90.7±2.3

27.0±2.929.5±2.6＊

Notes : All values of metabolic variables except minimum VE/VCO2 are shown at the peak of exercise. Values represent
the means±standard error of the mean. ＊P＜0.05 vs. the completion group. The relative exercise load of the CLE test to
the AT point in the ILE test was calculated as : peak VO2 in the CLE test/VO2 at the AT in the ILE test. The completiongroup consisted of those who continued to exercise for 20 minutes in the constant load exercise test, while the noncompletion group consisted of those who could not.
Abbreviations : VO2, oxygen uptake ; VCO2, carbon dioxide output ; RER, respiratory exchange ratio ; ETO2, end-tidal

oxygen, ETCO2, end-tidal carbon dioxide ; VE, ventilation ; VT, tidal volume ; VD, dead-space gas volume ; RR, respiratory
rate ; PR, pulse rate ; SpO2, percutaneous oxygen saturation ; AT, anaerobic threshold ; RC, respiratory compensation.
No. 6, 2021359

Kawachi・Fujimoto

Fig. 2 Comparison of gas exchange ratio and ventilation inefficiency
variables in constant load exercise test (CLE test).
Note : Panel A (left) shows a comparison of RER (VCO2/VO2) as a variable of AT in

the CLE test. Panel B (right) shows a comparison of minimum VE/V CO2 as avariable of ventilatory inefficiency in the CLE test.
Abbreviations : AT, anaerobic threshold ; RER, respiratory exchange ratio ; VE/VCO2, minute ventilation/carbon dioxide production.

higher RER and peak VE than the completion group(Fig. 1). As peak VO2 of the ILE test is affected byin the CLE test, and the RER exceeded 1 as the stan­motivation and subjective symptoms3）, it may havedard of the AT point13）. Ventilation is more enhanced

been more appropriate to determine the exercise

to compensate for increased CO2 when exercisingload for the CLE test based on the AT point, which

beyond the AT point, which promotes lactic acido­

is an objective variable that does not depend on14）

sis . Therefore, the non-completion group may havemotivation16）. A previous study adopted the AT levelexceeded the AT to a greater extent than the com­as the exercise load for the CLE test in healthy

pletion group in the CLE test, which may have pre­subjects17）. However, the AT point is 50 ％-60 ％ ofvented them from completing the CLE test. Themaximal capacity3）, and it is recommended to per­non-completion group exceeded the AT point in theform the CLE test at 75 ％-80 ％ of maximal exercise

CLE test to such a great extent because the relative

capacity in patients with COPD who perform theexercise load of the CLE test was higher in the non-CLE test frequently18）. Therefore, it is necessary tocompletion group than the completion group. It wasconduct the CLE test at 75 ％-80 ％ of the maximal

reported previously that a higher relative exercise

capacity calculated by the AT point, and to examineload calculated by the AT point was associatedwhether the relative exercise load and exercise dura­

15）

with shorter exercise duration . In the present study,tion can be standardized among subjects in such a

the relative exercise load was higher in the non-com­

CLE test in a future study.

pletion group, but the difference between groups was

The results of stepwise multiple regression analysis

not significant. This may have been because theshowed that the minimum VE/VCO2 was the variable

non-completion group exercised to a higher exercise

with the greatest influence on exercise duration.

load and showed a higher peak VO2 compared to theMinimum VE/VCO2, which is a variable of ventilatorycompletion group in the ILE test, even though theinefficiency in patients with heart failure, was signifi­

AT point was comparable between the two groups

cantly higher in the non-completion group, but was360

Shinshu Med Jol. 69

Determinants of exercise tolerance in constant-load test

within the reference range and less than the stan­fatigue was responsible for exercise limitation.

3）

dard value of 36 . Keller-Ross et al. reported thatⅤ Conclusion

minimum VE/VCO2 and RER affected by increasingThis study showed that the high relative exercise

VCO2 were related to the type of fibers in skeletal
19）

muscle . As daily physical activity influences theload in the CLE test causes the subject to exceed theproportions of the different types of fibers in skeletalanaerobic threshold and limits maintenance of exer­

20）

muscle , the present study suggested that dailycise in the test. In future, we will conduct a study on

physical activity should be assessed when performing

the validity of the CLE test in which the exercise

the CLE test to standardize the proportions of types

load is determined by the AT point to standardizeof fibers in skeletal muscle and minimum VE/VCO2

the relative exercise load in the CLE test.

among subjects.

ⅥThis study had several limitations. First, theConflict of interest

The authors declare no conflicts of interest.

changes in blood lactate, lower limb muscle strength,
and cardiac function, and the differences in relative

Ⅶ Acknowledgments

exercise load between the two groups, which may
affect exercise tolerance and ventilation inefficiency,

The authors wish to thank Koganei Co., Ltd. (Tokyo,were not measured. Second, we did not evaluate

Japan) for providing the measurement equipmenttraining status or fitness level for modulation of en­

and facilities for this study, and M. Oyamada, a stu­durance exercise performance. Third, the healthydent of Shinshu University School of Health Sciences,subjects varied widely in age over the range of 18-for help and support. The authors thank the healthy

60 years old, and the results of the exercise test may

adult volunteers who participated in the present

have been affected by the lack of adjustment for age.

study for their effort and enthusiastic cooperation.

Fourth, we did not evaluate the reasons why theThis research received no specific grants from any

subjects stopped exercising in the CLE test, and it

funding agency in the public, commercial, or not-for-was not determined whether dyspnea or muscle

profit sectors.

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