Assessment of cardiopulmonary function: updated two-minute walk distance and walk work equations for obese Chinese populations

Abstract

Background

The two-minute walk test (2MWT) is a practical cardiopulmonary function assessment for obese individuals; however, the current reference equations for the two-minute walk distance (2MWD) are limited, and there are no equations for two-minute walk work (2MWW), reducing its clinical applicability. This study aimed to create new reference equations for the 2MWD and 2MWW in obese Chinese adults.

Methods

This cross-sectional study was conducted at Wenzhou People’s Hospital (May 2019–April 2022) and targeted Chinese individuals aged 18–69 years with a BMI greater than 30 kg/m². Following the 2002 American Thoracic Society (ATS) recommendations, the 2MWT was adapted from the 6MWT. Separate equations for the 2MWD and 2MWW were developed by sex via stepwise multiple regression analysis.

Results

Among the 288 participants, the mean 2MWD was 185.9 ± 15.68 m, and the mean 2MWW was 15630.7 ± 2136.38 kg·m, with males showing significantly greater results than females did (P < 0.001). Sex-specific equations explained 35% and 61% of the variance for men, and 33% and 62% for women, respectively.

Conclusion

The newly developed reference equations for obese Chinese adults enhance the accuracy of cardiopulmonary function assessment and may facilitate more precise monitoring and individualized management of obesity-related cardiopulmonary conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-025-03955-9.

Introduction

Obesity affects nearly two billion people worldwide and poses a major public health challenge [1]. As a chronic disease, obesity is associated with increased risks of cardiovascular and pulmonary complications, contributing to increased morbidity and mortality [2]. Assessing cardiopulmonary function is therefore essential, particularly in obese patients with comorbidities.

The six-minute walk test (6MWT) is widely used for cardiopulmonary function evaluations because of its simplicity, practicality, and low cost [3, 4]. However, the 6MWT does not account for body mass, which can affect performance in obese individuals. Six-minute walk work (6MWW)—the product of the 6MWD and body weight—addresses this limitation and is more strongly correlated with peak oxygen uptake than the 6MWD is in patients with pulmonary disease [5–7]. Despite its utility, the 6MWT may be challenging for obese individuals because of early fatigue, lower-limb discomfort, or balance issues, potentially leading to submaximal performance or test termination [8–10].

The two-minute walk test (2MWT), a shorter alternative, has demonstrated strong correlations with the 6MWT across various clinical populations with reduced mobility or exercise tolerance [11]. Although the 2MWT also requires a 30-metre corridor, its shorter duration, quicker recovery, and greater tolerability make it more practical for routine screening and repeated testing in obese individuals. However, the absence of population-specific reference equations for two-minute walk distance (2MWD) and two-minute walk work (2MWW) in obese Chinese adults limits the clinical application of the 2MWT.

The American Thoracic Society (ATS) 2002 guidelines recommend establishing reference equations for walking distances in healthy people [12], yet equations derived from normal-weight individuals are not directly applicable to those with obesity [13]. While several studies have developed 2MWD Eqs [14–18]., most included few or no obese participants [14–17] or had narrow age ranges [18], and none addressed 2MWW in obese Chinese adults.

This study aimed to develop sex-specific reference equations for the 2MWD and 2MWW in obese Chinese adults, compare them with existing equations, and improve the clinical utility of the 2MWT in obesity-related cardiopulmonary assessment.

Methods

Study design

This cross-sectional study was conducted at Wenzhou People’s Hospital from May 2019 to April 2022 to facilitate early screening and intervention for individuals at high risk of cardiovascular disease. The protocol was approved by the Ethics Committee of Wenzhou People’s Hospital (Ethics No. 2019.91), and written informed consent was obtained from all participants.

Participants

Participants were recruited via convenience sampling from community health service centres, physical examination centres, and outpatient clinics in Wenzhou. Individuals aged 18–69 years with a BMI ≥ 30 kg/m² were eligible for this study. Recruitment was based on community health records and data from routine check-ups, with efforts to balance age, sex, and BMI distributions. Before the 2MWT, participants completed a study-specific questionnaire on demographics, medical history, lifestyle, and current health status. The exclusion criteria for this study included a history of cerebrovascular, cardiovascular, pulmonary, or other relevant diseases; walking difficulties or use of assistive devices; resting heart rate ≥ 100 bpm or < 50 bpm; and uncontrolled blood pressure (systolic ≥ 180 mmHg or diastolic ≥ 100 mmHg).

Physical examination

Measurements were performed by trained technicians using standardized protocols. Height was measured within 0.1 cm (RGZ-160; Shanghai JWFU Medical Apparatus Co., Ltd., China), and body weight within 0.1 kg (JH-1995 A; Shandong Dongfang Electronics Co., Ltd., China) with participants wearing light clothing and no shoes. Waist circumference was measured at the midpoint between the lowest rib and iliac crest, and hip circumference was measured at the maximal buttock protuberance. Resting blood pressure and heart rate were measured in a seated position after ≥ 15 min of rest using an automated sphygmomanometer (Yuwell YE660A, Yuyue Medical, China) with an appropriately sized cuff; two readings were averaged.

Two-minute walk test

The 2MWT followed the ATS 2002 guidelines with a 30-metre corridor and two trials to assess repeatability [12]. Participants rested for 15 min before baseline measurements of oxygen saturation, blood pressure, and heart rate were obtained. Blood pressure and heart rate were measured immediately post test with the same sphygmomanometer. The 2MWD was recorded, and the 2MWW was calculated as distance × body mass. Symptoms were monitored, and the Borg dyspnoea scale was recorded before and after the test [19]. A second test was performed two hours later.

Statistical analysis

The Kolmogorov‒Smirnov test confirmed that the predicted data followed a normal distribution. The measurement data are presented as the means ± standard deviations (SDs) to clearly summarize the central tendency and variability. Subject characteristics were analysed via descriptive statistics such as averages, deviations, frequencies, and proportions. Independent Student’s t tests were conducted to examine differences in 2MWT outcomes and categorical variables between sexes, ensuring the statistical significance of the observed differences. Paired-sample Student’s t tests were performed to measure the 2MWD and the 2MWD predicted from previous reference formulas [14–17]. Test–retest reliability was assessed using the intraclass correlation coefficient (ICC, two-way mixed-effects model, absolute agreement) and Bland–Altman analysis [20]. A paired-sample t test was used to compare the 2MWD between the first and second trials to detect potential learning. Additionally, one-way regression and multifactor stepwise regression analyses were performed to establish reference equations. First, Spearman correlation analyses were conducted to assess the relationships between the 2MWD or 2MWW and individual variables (e.g., age, height, body mass, and BMI), identifying potential predictors for further analysis. Reference equations for the 2MWD and 2MWW for males and females were determined via forward stepwise multiple regression analyses. Variables (e.g., age, height, body mass, and BMI) were included individually according to their statistical importance, with the procedure continuing until no more essential variables remained. Significant variables (p-value < 0.05) were added at each step, and the process was repeated until no more significant variables remained. The required sample size for multiple linear regression analysis was determined via G*Power software (version 3.1). With a medium effect size of 0.15, a significance level of 0.05, a statistical power of 0.95, and 4 predictor variables, the analysis revealed that at least 129 samples were needed. SPSS for Windows (version 20.0; SPSS Inc., Chicago, IL) was utilized for the data analysis. A p value < 0.05 was considered indicative of statistical significance for all analyses, ensuring the rigor of the statistical inferences.

Results

Demographic characteristics

Initially, 424 obese individuals were included in the study. After 136 subjects (36 with cardiac disease, 18 with abnormal baseline heart rates, 39 with unstable hypertension, 12 with pulmonary disease, 6 with foot sprain, and 25 with encephalopathy) were excluded, 288 participants (144 males and 144 females) completed the tests successfully. On average, the subjects were 44 years old, with a height of 163.5 ± 7.71 cm, a body mass of 84.0 ± 8.02 kg, and a BMI of 31.4 ± 1.25 kg/m². Table 1 summarizes the demographic characteristics of the participants as well as their performance on the 2MWT. Compared with females, male participants had significantly greater height, weight, and BMI values, and they also presented significantly greater 2MWD and 2MWW values (p < 0.001).

Table 1.

Demographic characteristics and 2MWT results

Characteristic	Males (n = 144)	Females (n = 144)	p value*	All (n = 288)
Age, years	44.3 ± 15.36	43.9 ± 14.98	NS	44.1 ± 15.15
Height, cm	169.2 ± 5.34	157.7 ± 4.86	< 0.001	163.5 ± 7.71
Weight, kg	89.4 ± 6.03	78.6 ± 2.82	< 0.001	84.0 ± 8.02
BMI, kg/m2	31.2 ± 1.13	31.6 ± 1.33	< 0.05	31.4 ± 1.25
2MWD1, m	185.3 ± 15.17	177.9 ± 14.68	< 0.001	181.6 ± 15.36
2MWD2, m	187.8 ± 15.37	179.8 ± 14.82	< 0.001	183.8 ± 15.59
2MWD, m	190.0 ± 15.35	181.9 ± 15.00	< 0.001	185.9 ± 15.68
2MWW, kg·m	16974.5 ± 1754.88	14287.0 ± 1563.34	< 0.001	15630.7 ± 2136.38

Values are expressed as the mean ± SD

2MWT two-minute walk test, 2MWD two-minute walk distance, 2MWW two-minute walk work

*P value between males and females

Two-minute walk test results

In this study, reference values for the 2MWD and 2MWW in a cohort of obese Chinese adults were established, and the results were stratified by age and sex to provide more precise and relevant data. The mean 2MWD and 2MWW for all the subjects were 185.9 ± 15.68 m and 15630.7 ± 2136.38 kg·m, respectively, indicating the average functional capacity of the cohort. The mean 2MWD and 2MWW for males were 190.0 ± 15.35 m and 16974.5 ± 1754.88 kg·m, respectively, and those for females were 181.9 ± 15.00 m and 14287.0 ± 1563.34 kg·m, respectively, indicating a significant sex difference (p < 0.001). The average 2MWDs for the initial and subsequent test sessions were 181.6 ± 15.36 m and 183.8 ± 15.59 m, respectively, indicating progress in the second test, possibly because of a better understanding of the procedure. The ICC for the two 2MWT trials was 0.89, indicating excellent reliability. The mean difference between trials was + 2.19 ± 7.26 m, with the second test yielding a significantly longer distance than the first test did (p < 0.001), suggesting a small but statistically significant learning effect. These findings establish reliable and sex-specific reference values for the 2MWD and 2MWW in obese Chinese adults, highlighting significant differences between males and females. Analysis of the associations between age, sex, and the 2MWT revealed significant differences between male and female participants, as well as across different age groups. Table 2 presents the age- and sex-stratified norms for the 2MWT outcomes, including the 2MWD and 2MWW, which further highlight these differences.

Table 2.

Age- and sex-stratified norms of the 2MWT results

Age, years	Variable	Males (n = 144)	Females (n = 144)	p value*	All (n = 288)
18–29 (n = 64)	2MWD, m	202.6 ± 12.10	192.8 ± 12.81	< 0.05	197.7 ± 13.31
	2MWW, kg·m	18409.0 ± 1280.80	15427.8 ± 1520.82	< 0.05	16918.4 ± 2049.99
30–39 (n = 56)	2MWD, m	191.8 ± 14.16	186.3 ± 13.04	< 0.05	189.1 ± 13.78
	2MWW, kg·m	17546.7 ± 1486.52	14683.6 ± 1600.62	< 0.05	16115.1 ± 2104.55
40–49 (n = 56)	2MWD, m	187.3 ± 13.12	179.4 ± 15.33	< 0.05	183.3 ± 14.70
	2MWW, kg·m	16709.7 ± 1422.46	13941.7 ± 1215.20	< 0.05	15325.7 ± 1915.36
50–59 (n = 56)	2MWD, m	185.2 ± 13.91	177.0 ± 11.96	< 0.05	181.1 ± 13.51
	2MWW, kg·m	16280.9 ± 1852.31	13973.4 ± 1296.39	< 0.05	15127.2 ± 1965.89
60–69 (n = 56)	2MWD, m	181.1 ± 14.45	172.4 ± 13.03	< 0.05	176.8 ± 14.31
	2MWW, kg·m	15721.2 ± 1335.64	13245.6 ± 1230.37	< 0.05	14483.4 ± 1782.92

Values are expressed as the mean ± SD (range)

2MWT two-minute walk test, 2MWD two-minute walk distance, 2MWW two-minute walk work

*P value between males and females

Associations of the study variables with the 2MWD and 2MWW

The relationships between multiple variables and the 2MWT results in men and women were significant, indicating that these variables play crucial roles in determining walking capacity. Univariate linear regression analysis revealed significant correlations between age, height, and BMI and the 2MWD and between age, height, and body mass and the 2MWW, indicating that these factors are vital predictors of walking distance and work. Age and BMI accounted for 35% of the variance in the 2MWD for males and 33% for females, highlighting their substantial impact on walking distance. Age and height independently contributed to the variance in 2MWW, explaining 61% of the variance in males and 62% of the variance in females, demonstrating their importance in predicting walking work. These findings emphasize the significant role of age, BMI, and height in predicting the 2MWD and 2MWW, with variations in these factors explaining a substantial proportion of performance differences. As illustrated in Table 3, the univariate regression analysis demonstrated the individual impact of each variable on the 2MWD and 2MWW. Table 4 further shows the multivariate stepwise regression analysis results, detailing how the combination of variables predicts walking distance more accurately.

Table 3.

One-way regression analyses

2MWD, m
Variable	Males (n = 144)		Females (n = 144)
	r value	p value	r value	p value
Age, years	−0.493	< 0.001	−0.481	< 0.001
Height, cm	0.208	< 0.05	0.264	< 0.05
Weight, kg	−0.029	NS	−0.035	NS
BMI, kg/m2	−0.419	< 0.001	−0.446	< 0.001

2MWW, kg·m
Variable	Males (n = 144)		Females (n = 144)
	r value	p value	r value	p value
Age, years	−0.571	< 0.001	−0.482	< 0.001
Height, cm	0.716	< 0.001	0.763	< 0.001
Weight, kg	0.620	< 0.001	0.638	< 0.001
BMI, kg/m2	−0.095	NS	0.012	NS

2MWD two-minute distance, 2MWW two-minute walk work, r value Pearson’s correlation coefficient, BMI body mass index

Table 4.

Multivariate Stepwise regression analyses

2MWD, m
	Males			Females
	B	SE	p value	B	SE	p value
Constant	355.838	28.693	< 0.001	320.779	24.580	< 0.001
Age, year	−0.434	0.068	< 0.001	−0.390	0.071	< 0.001
BMI, kg/m2	−4.701	0.931	< 0.001	−3.858	0.798	< 0.001
R2	0.359			0.340
Change in R2	0.350			0.331

2MWW, kg·m
	Males			Females
	B	SE	p value	B	SE	p value
Constant	−13648.158	3310.237	< 0.001	−19064.522	2935.538	< 0.001
Height, cm	191.134	18.812	< 0.001	218.029	17.935	< 0.001
Age, year	−38.897	6.538	< 0.001	−23.428	5.816	< 0.001
R2	0.611			0.625
Change in R2	0.605			0.620

B unstandardized coefficients

The sex-specific reference equations for the 2MWD and 2MWWare as follows:

Comparison with published regression equations

We assessed the accuracy and applicability of the predicted 2MWD based on previous reference equations by comparing the measured 2MWD of our cohort [14–17]. However, the reference equations of Zhang, Selman, and Mirza et al. overestimated the walking distances of our participants. In contrast, the reference equations from Bohannon et al. underestimated our participants’ walking distances, suggesting differences in the applicability of these equations to our cohort [14–17]. The discrepancies between the measured 2MWDs and the predicted 2MWDs according to the studies by Zhang et al., Selman et al., Mirza et al., and Bohannon et al. were − 14.5 ± 16.37 m, −25.3 ± 17.70 m, −22.9 ± 21.94 m, and 4.6 ± 15.05 m, respectively [14–17]. These findings highlight the need to develop population-specific reference equations for more accurate predictions of the 2MWD in obese Chinese adults. As shown in Table 5, the discrepancies between the measured and predicted 2MWDs underscore the importance of tailored reference equations that precisely represent the attributes of the research participants.

Table 5.

The 2MWD was measured and predicted using previously reported equations

Study	Measured (m)	Predicted (m)	Measured-predicted (m)
Zhang et al.[14]	185.9 ± 15.68	200.4 ± 17.37	−14.5 ± 16.37*
Selman et al.[15]	185.9 ± 15.68	211.2 ± 20.17	−25.3 ± 17.70*
Mirza et al.[16]	180.4 ± 14.36	203.3 ± 22.82	−22.9 ± 21.94*
Bohannon et al.[17]	185.9 ± 15.68	181.3 ± 16.53	4.6 ± 15.05*

2MWD two-minute walking distance

*p < 0.05 according to the Student’s t test

Discussion

This study developed sex-specific reference equations for predicting the 2MWD and 2MWW in obese Chinese adults aged 18–69 years and compared them with previously published equations. As the first such equations for this population, they address the lack of population-specific reference standards and provide a practical tool to improve the precision and clinical applicability of 2MWT-based cardiopulmonary assessments in obese individuals.

In our study, significant differences were observed between men and women in terms of walking distance, with men generally walking a greater distance. This disparity is likely attributable to differences in muscle mass and physical fitness. As shown in Fig. 1, the 2MWD correlated with several independent variables for both sexes. Notably, age and BMI were negatively correlated with the 2MWD. Age-related muscle atrophy and reduced oxygen uptake likely contributed to the decrease in walking distance with advancing age. In obese individuals, a high BMI is associated with functional limitations, including cardiovascular and respiratory constraints, as well as muscle weakness, all of which hinder their ability to walk [21–24]. Additionally, a positive correlation between height and walking distance was observed, suggesting that taller individuals with longer strides may walk farther in the same timeframe. However, since our study focused on obese individuals, BMI emerged as a more critical determinant of the 2MWD than height, leading to the exclusion of height from the final regression model. Therefore, while height may play a role, BMI is the predominant factor influencing the 2MWD in this population.

Fig. 1.

Correlations of the 2MWD with age, height, and BMI in both males and females

Stepwise multiple regression analysis was employed to develop reference equations for the 2MWT results in obese Chinese adults. For both men and women, age and height were identified as independent factors influencing the 2MWD, explaining 35% and 33% of the variability in walking distance, respectively. These values are consistent with previously reported variances for 2MWD reference equations. Although the explained variance is modest, the use of reference equations that incorporate key influencing factors provides more meaningful insights into exercise capacity than relying solely on absolute outcome measures does. Other factors, such as heart rate, muscle strength, and lifestyle, also contribute to the prediction of the 2MWD, but their inclusion in clinical assessments is limited because of practical constraints.

Age and height were independent factors influencing 2MWW, explaining 61% and 62% of the differences between males and females, respectively. The combination of walking distance and body weight is beneficial for various patient groups [6–26]. This combination has been shown to predict hospitalization in patients with COPD and is a better predictor of carbon monoxide diffusion capacity than walking distance alone is [5, 25]. Additionally, this combined measure is strongly correlated with the anaerobic threshold and maximal oxygen uptake in patients with COPD [5]. Predictive equations for 2MWW can enhance clinical practice by providing a valuable outcome variable, helping healthcare providers better interpret 2MWT results.

We assessed test-retest reliability (Fig. 2) and reported that the second 2MWT resulted in greater walking distance, likely because of improved coordination, reduced negative emotions, and better stride length. Both tests showed good reliability (ICC = 0.89), confirming the consistency of the 2MWT as a measure of physical capacity. These results are consistent with those of previous studies and suggest that further research is needed to identify the factors affecting test-retest reliability and refine assessment protocols [27–29]. Reliable testing methods are essential for accurately monitoring patient progress and treatment outcomes.

Fig. 2.

Bland‒Altman plot comparing the first and second 2MWT results

We compared the actual 2MWD of our participants with values predicted from four previously published reference equations, all of which showed limited predictive accuracy for our obese Chinese cohort in both overall and sex-specific analyses [14–17]. The equations of Selman et al., Mirza et al., and Zhang et al. systematically overestimated 2MWD, likely because they were developed from healthy, normal-weight populations with generally higher fitness levels, different testing environments, and varying encouragement protocols [14–16]. In contrast, the equations proposed by Bohannon et al. yielded smaller prediction errors than the other published formulas did but consistently underestimated the 2MWD in our participants [17]. This may reflect differences in study design—Bohannon’s equations were derived from a large, population-based U.S. cohort aged 18–85 years, with a mean BMI in the overweight range (28.5 kg/m²), tested on a 15.2-m (50-ft) out-and-back course without repeated trials [17]. The relatively good performance of Bohannon’s equations in our cohort may be due to the inclusion of key predictors—age, sex, and BMI—that also strongly influence 2MWT performance in obese individuals [17]. Nevertheless, differences in body composition, gait biomechanics, and exercise tolerance between overweight and obese individuals can still bias predictions when such equations are applied across populations [30].

Our study addresses these limitations by (1) recruiting obese Chinese adults aged 18–69 years; (2) using a standardized 30-m corridor in accordance with ATS guidelines; (3) incorporating test–retest protocols to improve measurement reliability and account for learning effects; and (4) ensuring balanced representation across sex, age, and BMI categories. These methodological refinements enabled the development of the first sex-specific reference equations for 2MWD and 2MWW in this population, improving prediction accuracy and enhancing the clinical applicability of the 2MWT in obesity-related cardiopulmonary assessment.

While our study provides valuable insights, it is essential to acknowledge its limitations. The use of convenience sampling may limit the generalizability of our findings. Future large-scale multicentre studies are needed to validate the reference equations across diverse populations and age groups, ensuring their broad applicability and accuracy.

Conclusions

In conclusion, our study established the first tailored reference equations for the 2MWD and 2MWW specifically for obese Chinese adults aged 18–69 years. These equations will improve patient care and outcomes by providing a valuable tool to address the cardiopulmonary and functional limitations in obese patients, leading to more effective, personalized treatment strategies.

Abbreviations

2MWT

Twominute walk test

2MWW

Twominute walk work

2MWD

Twominute walk distance

6MWT

Sixminute walk test

6MWW

Sixminute walk work

6MWD

Sixminute walk distance

BMI

Body mass index

ICC

Intraclass correlation

SD

Standard deviation

SPSS

Statistical Package for the Social Sciences

Authors’ contributions

LPW, HZ, XSC, JZ and XLW designed the research; LPW, HZ, XSC, JZ, XLW, CL, SXW and YZ performed the research; LPW, HZ, XSC, JZ and XLW were involved in analysing the data; LPW, HZ, XSC, JZ and XLW wrote the paper; and HZ edited the paper. All the authors read and approved the final manuscript.

Funding

This study was funded by the Wenzhou Municipal Science and Technology Bureau, China (project number YC20250331).

Data availability

The datasets generated and analysed during the current study are available in the supplementary material of this article.

Declarations

Ethics approval and consent to participate

The study was approved by the ethics committee of Wenzhou People’s Hospital (Ethics No. 2019.91). All individuals were fully informed about the study and provided written informed consent.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Clinical trial number

Not applicable.