To the best of our knowledge, this is the first study to describe 6MWD in obese Chinese subjects older than 40 years. As shown in our study, there was a significant difference in the walking distance samples for men and women. Men tend to walk longer distances than women, probably due to their higher muscle mass and greater athletic ability.
There was a correlation between the 6MWD and independent variables in both men and women (Figure 1). We can see that age and BMI are negatively correlated with the 6MWD in our study. This probably is related to the loss of muscle mass as we age and the decrease in oxygen intake. Obese subjects have a certain complications with the increase in BMI, which is usually manifested as activity disorders caused by heart and breathing limitations(23, 24), and weakened skeletal muscle strength is also one of the main causes of disability(25, 26). Other possible reasons include skin friction caused by fat deposition in the thighs, increased plantar pressure, and physical discomfort caused by exercise for people with a BMI that is higher than normal(9, 27-29). We can see that height and walking distance are positively correlated in our study. This could be because, in general, taller individuals with larger strides cover more walking distance in the same amount of time than others. However, the target of our study was obese subjects, and BMI was more meaningful than height for the 6MWD, so height was not included in the final regression equation. From this study, we observed that the resting heart rate of male subjects was lower than that of female subjects. Previous studies have shown that the difference in heart rate is correlated with sex, and the resting heart rate of men is lower than that of women(30). This may be because men and women have different abilities to regulate the baroreflex heart rate, and oestrogen contributes to the ability to regulate the baroreflex heart rate in people(31).
Fig. 1. The relationship between the 6MWD and age, height and BMI for females and males.
In this study, categorical variables (age, height and BMI) were used for stepwise multiple regression analysis. Age and height were identified as independent factors that influenced the 6MWD, and they explained 31% and 27% of the variance in walking distance for the male and female groups, respectively.
After applying mathematical analysis, such as the theory proposed by Hulens et al(32), we found that other factors that correlate with walking distance in six minutes would certainly benefit the prediction equations. These factors (heart rate, blood pressure, muscle strength and lifestyle factors) also play an important role in predicting 6MWD, but they exist only in the equations and are impractical for clinical use.
By measuring the walking distance performance on 2 occasions, we can see that the mean 6MWD during the second test period was longer than the mean 6MWD during the first test period. This finding is the same as the previous findings for the 6MWT(5, 33). The reason why the distance can be increased is probably related to overcoming negative emotions, improving coordination and finding an appropriate stride distance. Although it was found that the performance of walking distance in the second 6MWT was higher than that of the first test in our study, the reliability of the two 6MWTs was good (ICC = 0.89). Previous studies have also proven that the two 6MWTs are reliable(33), which is consistent with our research results. The Bland-Altman chart shows the mean difference between the first and second 6MWD (Figure 2). Seventeen participants had error values outside the 95% confidence interval (CI), and seven participants showed an increase in the second 6MWD, which might be due to familiarity with the 6MWT. Ten participants showed shorter walking distances in the second 6MWT, which might be due to greater fatigue during the second test due to better performance in the first 6MWT.
Fig. 2 Bland-Altman plot of the results for performance in the first and second 6MWTs.
These reference equations from previous studies (Ben et al. (18), Iwama et al.(19), Capodaglio et al(17). and Camarri et al.(20)) cannot accurately estimate the distance walked during the 6MWT when used with obese subjects. Judging from the results of our data, the walking distance of obese individuals in our sample is significantly different from the estimated distance of these reference equations mentioned above. These differences were often due to the test protocol, anthropometric factors, ethnic background and demographic differences among the participants. In the studies by Ben et al. (18), Iwama et al.(19) and Camarri et al.(20), healthy subjects were primarily recruited, while obese subjects accounted for only a small percentage of the study population. Even if the workload was small, the walking distance in these studies was higher than the walking distance in our sample. Although both study samples consisted of obese people, our results are not consistent with those provided by Capodaglio et al.(17). The difference may be due to the higher average age (20 years older) and the lower workload of the subjects in the study. In a sense, age represents a proxy of disability, which obviously affects the performance of subjects during the 6MWT. In our sample, 42% of subjects were over 60 years old, and 15% were 70 years old, whereas in the study by Capodaglio et al.(17), age ranged from 20 to 60 years. Even if age was taken into account as an explanatory variable(17), this equation has been verified in samples younger than 60 years old, thus reducing the predictive validity for elderly subjects. In addition to the daily physical activity of the participants, their mood and psychological factors may influence the 6MWD(34).
In our study, there are also some limitations. First, although the sample size of our study was relatively large, the sample was one of convenience, and relatively few subjects were over the age of 70 in the study. Second, we did not recruit obese subjects under 40 years of age. A large multicentre study is needed to address these limitations.
In summary, the study was the first to describe the 6MWD in Chinese obese subjects and to propose predictive equations. These established equations can contribute to improving the assessment of Chinese obese patients with diseases that affect their exercise capacity.
What is already known on this subject?
The 6MWT is a commonly used clinical assessment of exercise capacity in patients with cardiopulmonary disease. Although, the reference equations of the 6MWD for Chinese healthy subjects have been established, some studies have showed that the reference equations for the walking distance derived mainly from healthy, normal-weight people are not suitable for obese subjects. However, there is a lack of standard reference equations for the 6MWD in Chinese obese subjects.
What this study adds?
The study was the first to describe the 6MWD in Chinese obese subjects and to propose predictive equations. These established equations can contribute to improving the assessment of Chinese obese patients with diseases that affect their exercise capacity.