Six-minute walk distance in healthy young adults

Abstract

Background: The 6-min walk test (6MWT) is a commonly used clinical assessment of exercise capacity in patients with cardiopulmonary or neuromuscular disease, but normal values are lacking for young adults, who are frequent subjects of testing.

Methods: In a two-center study, 272 young adults, ages 18-50, underwent American Thoracic Society protocolized 6-min walk testing, and 56 underwent repeat testing. A linear regression model was developed based on anthropomorphic data. This model was compared to existing prediction equations.

Results: Median 6MWD for the cohort was 637 m (IQR 584-686 m) and was not significantly impacted by age. This is in contrast to existing equations extrapolated from older subjects that predict increasing 6MWD in younger subjects. We found weak correlation of 6MWD with height, weight, BMI, and resting heart rate. Heart rate at completion correlated most strongly with 6MWD (rho 0.53 p < 0.0001). Repeat 6MWD was surprisingly variable, with a median difference between tests of 32.5 ± 31.9 m. Established reference equations performed poorly in this population, largely because age has much less effect on 6MWD in this group than in older adults.

Conclusions: Established reference equations should be reconfigured to include data from young adults, as age has minimal effect on 6MWD in this population. Heart rate response may be a valuable measure of effort in normal subjects. Six-minute walk distance, as with pulmonary function and exercise testing, should have predictive equations across the spectrum of age to allow for accurate assessment of exercise limitation.

Keywords: Cardiorespiratory fitness; Exercise test; Reference values.

Figure 1:

Prediction models for six-minute walk distance versus age in the study population. Lines represent quadratic regression fit, and shaded areas represent 95% CI of the mean.

Figure 2:

Bland-Altman plot of change in six-minute walk distance (6MWD) between the first and second tests versus initial 6MWD among participants whom underwent repeat testing (n=56). Dotted lines represent the mean change in 6MWD and 95% confidence interval.

Figure 3:

Univariate plots of observed 6MWD vs. predictor variables. Lines represent fitted spline regression with 95% confidence interval of the mean.

Figure 4 A&B:

A: Six-minute walk distance by number of self-reported days of exercise per week. B: Six-minute walk distance versus average daily step count obtained from participants’ smartphone or wearable fitness device. The line represents a linear prediction with 95% confidence interval of the mean.