Introduction

The older population is increasing rapidly around the world. Epidemiological estimates indicate that two billion people will be over 60 years old in 2050 [1]. This is an important issue because the aging process reduces functional fitness and increases individuals’ risk of contracting or developing many diseases [2,3].

In light of these data, means of mitigating functional deficits and improving health-related quality of life (HRQoL) in the elderly are required. Physical activity can play a key role in this process. An active lifestyle has been shown to prevent cardiovascular diseases, maintain independence, and improve functional capacity in older people [4,5]. Moreover, evidence has shown that increased PA is associated with improvement in HRQoL.

The six-minute walk test (6MWT) has been widely used to assess functional capacity due to the simplicity of its implementation, ease of interpretation, and representativeness of activity in daily life. The 6MWT performance has become the most common method for evaluating functional capacity in individuals with a variety of health conditions [6,7]. Hsieh et al. reported a significant correlation between distance walked in the 6MWT and HRQoL in post-donor liver transplantation [8]. A similar finding was reported in a study of patients with pulmonary emphysema [9]. These findings indicate a relationship between physical activity and HRQoL.

To our knowledge, no previous studies have analyzed a direct relationship between functional capacity and HRQoL in older individuals without a reported disease. We hypothesized that a higher 6MWT performance (in terms of distance walked) is associated with a better HRQoL in older individuals without chronic diseases or cognitive impairment. We also determined the minimum distance on the 6MWT that would reliably predict a score of 50 on the Short Form (36) Health Survey (SF-36), which indicates good quality of life [10,11].

Materials and Methods

Results

Body mass index was similar between genders (men: 25 ± 3, women: 25 ± 3 kg/m², p = 0.3). Most subjects performed satisfactorily on the 6MWT; predicted distance was achieved for 95 ± 20% of participants. Men walked a greater distance (484 ± 125 m) and a higher proportion achieved the predicted distance (99 ± 20%) compared to women (412 ± 89 m; 91 ± 18%) (p < 0.01). There were no significant differences between genders for HRQoL domains (data not shown).

Significant correlations were found between all HRQoL domains and 6MWT performance in the overall sample (Table 1). For men, the domains correlated with 6MWT performance were physical functioning, role-physical, and social functioning. In women, 6MWT performance was significantly correlated with all domains of HRQoL except mental health.

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Table 1. Association between 6-min walk test performance and quality of life domains.

https://doi.org/10.1371/journal.pone.0117359.t001

Adjusted linear regression models of the relationship between 6MWT performance and HRQoL showed a significant association only for the physical functioning and role-physical domains when potential confounding variables (gender, body weight, height) were included on overall sample (Table 1). Thus, the inclusion of potential confounding variables in the model changed the results of linear correlation tests for several domains. This issue was more pronounced in men, in whom standardized regression coefficients (β) were not significant. In women, adjusted models changed the results of linear correlation tests for the role-emotional and vitality domains.

To perform ROC curve analysis, a score of 50 on the SF-36 was used as a cutoff (Table 2). Test sensitivity and specificity were set at the median 6MWT performance, measured in meters walked. First, the overall sample was tested, and sensitivity and specificity was set at the median distance walked, which was 447 m. Only for the general health and role-physical domains was the AUC over 0.70, despite low sensitivity. Specifically for general health domain, when the cutoff of 447 m in the 6MWT was used, 64% of all participants who scored over 50 on the SF-36 were correctly identified, and 30% of participants who walked 447 m did not score above 50 on the SF-36. For men, median distance walked was 481 m, and AUC was over 0.70 for the physical functioning and role-physical domains. For women, median distance walked was 420 m. The AUC was calculated for the role-emotional, role-physical, and general health domains.

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Table 2. Accuracy of 6MWT distance in predicting health-related quality of life scores.

https://doi.org/10.1371/journal.pone.0117359.t002

Discussion

The main finding of this study was that 6MWT performance may be associated with HRQoL in a selected cohort of older participants. The association between 6MWT performance and HRQoL was also found to be gender-specific; higher 6MWT performance was associated with higher scores for the physical functioning, role-physical, social functioning, and general health domains in women.

Our findings agree with previous studies that have reported an association between functional fitness and self-reported HRQoL in older participants; that is, a superior 6MWT performance was associated with better physical functioning [17,18]. We extend these findings to show that a better 6MWT performance may also be associated with increased scores in the role-physical, social functioning, and general health domains.

It has been shown that body mass index, educational level, and comorbidities are associated with HRQoL [19,20]. Therefore, we selected a suitable sample and included covariates (e.g., sex, body weight, and height) in the regression models to explore independent associations. Notably, we observed significant associations among women but not men. Moreover, even among women, certain domains (role-emotional, mental health and vitality) were not significantly associated with 6MWT performance. It is not clear how to interpret these findings; this interpretational difficulty points to the issue implicit in the fact that there is currently no single theoretical approach capable of clarifying the underlying mechanisms of all the domains of HRQoL (e.g., family life, socioeconomic status, lifestyles, living arrangements, loneliness, race, and widowhood) [21–22]. The possibility that these variables influenced our analysis cannot be excluded.

Finally, further studies are required to determine a target predictive distance for a SF-36 score of 50. Our AUC data did not identify a perfectly accurate cut-off (AUC = 1) for both genders. This is important because a cut-off score of 50 has proved useful when interpreting differences between scales in the SF-36 survey [23–25]. It is therefore possible to use predictive 6MWT scores to analyze individual domains of HRQL.

Conclusion

Our findings suggest that a better 6MWT performance may be associated with improved self-reports of several HRQoL domains in older women. However, further studies should further examine the minimum walking distance that will predict scores above 50 on the SF-36.

Acknowledgments

The authors are grateful to Editage for professionally editing this paper.