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Of those with obesity, those with a BMI≥40 kg/m2 (classified class III obesity) have the highest risk of comorbidity and Type 2 diabetes [1]. Additionally, a reduction in mobility is often seen in those living with obesity, due to excessive adiposity [2,3]. Therefore, exhaustive exercise tasks, like the VO2 max test, are often impractical. Instead, walking tasks are a feasible measure that reflect day to day activities, and can be used to demonstrate overall quality of life [4].

Within a clinical setting, the six-minute walk test (6MWT) has been used to assess the severity of obesity and risk of obesity related co-morbidities [5-7]. As well, the 6MWT has been shown to be a valid indicator of functional ability [8, 9]. In the clinical setting, the 6MWT is advantageous because the test is practical to setup with no equipment or training required for the participant, relative to VO2 max testing [4, 6]. The 6MWT is often well tolerated and more closely related to daily living than other walk tests [10]. In addition, the 6MWT provides insight on the pulmonary and cardiovascular system, systematic circulation, peripheral circulation, blood, neuromuscular units and muscle metabolism at a whole body level during exercise [6].

For those living with class lll obesity and other diseases which limit physical capacity, the 6MWT can be lengthy and difficult for some individuals. Thus, in certain cases, a 2-minute walk test (2MWT) may be more appropriate. For instance, Leung, Chan [11] found that the 2MWT was well tolerated for patients with severe COPD and reported a strong intraclass correlation coefficient between repeated 2MWT scores (r=0.99). Yuksel, Kalkan [12] found both a strong test-retest reliability and intraclass correlation coefficient (r=0.97) within the 2MWT for patients with total knee arthroplasty. Although studies have found strong validity and reproducibility amongst individuals with obesity and the 6MWT [8, 9], the 2MWT has not yet been validated amongst those living with class lll obesity. The objective of this study was to therefore examine if the 2MWT is a comparable alternative to the 6MWT in individuals with class III obesity.

Methods

Participants

Participants (67% female) with class III obesity (body mass index [BMI] > 40kg/m2) were recruited from the bariatric surgery clinic of the McGill University Health Centres. Participants were excluded if they had diseases or conditions that would affect protein metabolism or muscle function. They were also excluded if they were unable to walk or needed an aid to walk. The study was approved by the Comité Central D’éthique de la Recherche du Ministre de la Santé et des Services Sociaux and the Research Ethics Board of the MUHC. All participants provided written informed consent.

2 and 6 minute walk tests

Walk tests were conducted according to the standards set by the American Thoracic Society [6] without providing the time remaining in the test. Pylons were placed at 0 m and 30 m in a flat hallway, making a 60m lap. Participants were asked to cover as much distance as possible without running between the pylons for 6 minutes. Distance was recorded at the 2-minute and 6-minute marks discretely. Participants were informed to stop at any time if they felt discomfort. To ensure consistency, each participant was given the same evaluator.

Statistics

Descriptive statistics are mean ± standard deviation (Table 1). Data analyses were conducted using SPSS ((IBM SPSS Statistics v22 (Armonk, NY). Data was tested for normality using Shapiro-Wilks. To compare the distances walked, the 2MWT was multiplied by 3 to extrapolate the distance to a 6 minute period (assuming constant speed) [13]. The 6MWT and 3 x 2MWT was compared using a Bland-Altman plot with 95% confidence intervals (42 m and -86 m). A paired t-test was also used to test the difference between the 2 and 6MWT. Pearson correlation was used to test for an association between the two variables (2MWT & 6MWT).

Results

Participants (67% female, n=18) were an average age of 44±8 y and had an average BMI of 47.1±4.8 kg/m2. The average distance covered in the 2MWT x 3 was greater (p<0.001) than the 6 MWT (Table 1). There was a strong correlation (r=0.89, p<0.01) between the two tests (Figure 1). When examining the distributions using the Bland-Altman plots, all but one participant fell within the limits of agreement (42 m and -86 m) (Figure 2). However, the plot showed a potential clinically significant mean difference of -22 m [14]. The plot also showed the distance covered in the 3 x 2MWT to be up to 109m greater than the 6MWT (Figures 1 and 2).

Table 1: Comparison of distance walked in the 2MWT and 6MWT.

Figure 1: Correlation between 6MWT vs. 2MWT in participants with class III obesity.

Figure 2: Bland-Altman plot of 6MWT vs. 3 x 2MWT comparison.

Discussion

To our knowledge, this is the first study to examine whether the 2MWT can be used as an alternative to the 6MWT in individuals with class III obesity. We found that the 2 MWT overestimates the distance that would be walked in the 6MWT by an average of 22m. Despite there being a strong correlation between the two tests, a potential clinically significant mean difference demonstrates that the tests cannot be used interchangeably [14]. A systematic review by Bohannon and Crouch [14], reported that across various patient populations, a 6MWT difference of 14.0 to 30.5m was clinically meaningful.

The strong correlation between the 2 and 6MWT that we observed is in accordance with a study by Witherspoon et al., [13] who established that the 2MWT and 6MWT were strongly correlated in a population with neuromuscular disease [13]. Although they also observed that the 2MWT overestimated the 6MWT, their mean difference of 5.07m was not considered clinically significant. Thus, in those with neuromuscular disease, the 2MWT provides a sufficient estimate of the 6 MWT. Similar to our results, Bohannon et al. [15] found a strong (r=0.968) relationship between the distance covered in a 2 versus 6 MWT amongst individuals aged 3-85 years. However, their analyses did not include a Bland-Altman plot which is problematic because a strong correlation does not reflect the accuracy of the estimate between the two methods as a mean difference would [16]. Additionally, implications such as arthritis, chronic pain, joint stress and gait disturbances are all factors associated with obesity [3], which may influence the distance covered as time progresses.

A study by Motl et al. [17] suggests that there are physiological differences behind the 2MWT and 6MWT. During the two walk tests, 95 participants with Multiple-Sclerosis wore a metabolic unit to gather VO2 data [17]. They reported a shift within VO2 max as the test progressed from minutes 3 to 6, suggesting that the 2MWT uses predominantly anaerobic systems and the 6MWT uses predominantly aerobic systems [17]. Therefore, the 2 and 6MWT appear to test different metabolic systems and thus, the use of one as an indicator of the other may not be appropriate. These results are in accordance with our findings, despite being strongly correlated, show differences of up to 109m between the two walk tests. Moreover, the limits of agreement are wide indicating there can be considerable differences between outcomes of the 2MWT and 6MWT.

A limitation to our study is assuming that constant speed was maintained when extrapolating the 2MWT from the 6MWT [13]. Future studies may want to have participants perform each walk test on different occasions in a randomised order. Performing tests on different days or on multiple occasions could also affect outcomes. For example, of four walk tests performed on the same day in 20-80 year olds, the difference between the first to best 6 MWT was 43 m [18]. Though encouragement was given throughout the test to minimize effect of multiple testing or day to day variability participants were not informed of the time remaining in the test. They were asked to cover as much ground as possible over the period of the test. In taking the 2 minute measurement, we were careful to discretely mark the 2 minute distance. The same was done at 6 minutes, after which the participants were asked to stop.

Thus, the distances were marked consistently between the 2 and 6 minute tests. It is possible that if the tests were conducted separately, gait speed could be affected with gait speed being slightly faster in the 2 than 6 MWT [19]. Though we did not measure gait speed, we observed a greater distance covered in the initial 2 minutes of the test compared to over the entire 6 minute period, reflective of a faster gait speed during 2 minutes. Conducting separate 2 and 6 MWT could have exacerbated our observed differences and thus, would unlikely have changed our conclusions that the 2 and 6 MWT are not comparable in our population.

Conclusion

When compared to the 6MWT, the 2MWT overestimates the distance walked in individuals with class III obesity. Although a strong correlation was seen between the 2MWT and 6MWT, different physiological systems and populations with varying capability could affect the outcomes. These findings indicate that these two tests should not be used interchangeably in class III obesity. Future studies are necessary to determine the interchangeability between the 2MWT and 6MWT in different clinical settings.

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