Reliability and Construct Validity of the Six-Minute Step Test in Individuals With Non-Specific Chronic Low Back Pain.

Abstract

Background and purpose: The six-minute step test (6MST) is a practical, field-based assessment that may provide useful information about stepping performance in individuals with non-specific chronic low back pain (NSCLBP). However, evidence regarding its measurement properties in this population is limited. The aim of this study was to evaluate the inter-rater reliability and construct validity of the 6MST in individuals with NSCLBP.

Methods: We conducted a cross-sectional psychometric study to evaluate the 6MST's inter-rater reliability and construct validity using a known-groups approach. Thirty-one individuals with NSCLBP and 31 age- and sex-matched healthy controls completed the 6MST. Two blinded raters administered the test independently on separate occasions 7 days apart. We assessed inter-rater reliability using the intraclass correlation coefficient (ICC), standard error of measurement (SEM), minimal detectable change (MDC), and Bland-Altman analysis. Construct validity was examined by comparing 6MST performance between groups using hypothesis testing and effect sizes (Cohen's d). Secondary outcomes included heart rate responses, perceived exertion, pain intensity, disability, pain catastrophizing, and kinesiophobia.

Results: The 6MST demonstrated excellent inter-rater reliability, with ICCs of 0.915 (95% CI: 0.83-0.96) for the NSCLBP group and 0.973 (95% CI: 0.94-0.99) for controls. SEM and MDC values were 6 and 17 steps in the NSCLBP group and 3 and 8 steps in the controls, respectively. Bland-Altman analyses revealed good agreement and no evidence of proportional bias. Construct validity was supported by significantly lower performance in the NSCLBP group compared with the control group (mean differences: 37 and 33 steps for raters 1 and 2, respectively, both with p values less than 0.001), with very large effect sizes (Cohen's d = 1.91 and 1.76). Individuals with NSCLBP showed lower attainment of the predefined target heart rate and greater increases in pain intensity following the test, despite similar perceived exertion.

Discussion: The 6MST demonstrated excellent inter-rater reliability and satisfactory construct validity for assessing stepping performance in individuals with NSCLBP. Compared with healthy controls, individuals with NSCLBP completed fewer steps, were less likely to reach the predefined target heart rate during testing, and reported greater increases in pain intensity despite similar perceived exertion. These findings support the use of the 6MST as a practical, low-cost, performance-based assessment of stepping performance in this population. However, the observed heart rate responses should not be interpreted as evidence of differences in exercise tolerance, aerobic capacity, or cardiopulmonary function.