The validity and reliability of the modified Four Square Step Test (mFSST) in individuals with ankylosing spondylitis

This study aimed to examine the test-retest reliability and concurrent validity of the modified Four Square Step Test (mFSST) in individuals with ankylosing spondylitis (AS).

Forty-eight participants diagnosed with AS were included. The mFSST was administered twice on the same day with a 1-h seated rest to assess reliability. Concurrent validity was tested with the Timed Up and Go (TUG), Five Times Sit-to-Stand (5xSTS), and Functional Reach (FR) tests, as well as disease activity (Bath Ankylosing Spondylitis Disease Activity Index (BASDAI)) and functional status (Bath Ankylosing Spondylitis Functional Index (BASFI)) indices. Intraclass correlation coefficients (ICC), standard error of measurement (SEM), minimal detectable change (MDC₉₅), Bland-Altman plots, and Spearman correlations were calculated.

The mFSST demonstrated excellent test-retest reliability (ICC = 0.952), with SEM = 0.42 s and MDC₉₅ = 1.17 s. Bland-Altman analysis showed narrow agreement limits (- 0.57 to + 1.31 s). For validity, the mFSST correlated strongly with TUG (r = 0.771, p < 0.001) and moderately with 5xSTS (r = 0.473, p < 0.001), while no significant association was found with FR (p > 0.05). Retest values confirmed these results, showing strong correlation with TUG (r = 0.823, p < 0.001), moderate with 5xSTS (r = 0.606, p < 0.001), and a weak negative correlation with FR (r = - 0.334, p < 0.05). Furthermore, both mFSST test and retest scores showed moderate positive correlations with BASDAI (r = 0.501 and r = 0.543, respectively; p < 0.001) and BASFI (r = 0.605 and r = 0.681, respectively; p < 0.001).

The mFSST is a reliable and valid tool for assessing dynamic balance in AS. Its brevity, minimal equipment needs, and interpretable error metrics support integration into clinical practice for baseline profiling and outcome monitoring. The MDC₉₅ threshold offers clinicians a benchmark for interpreting meaningful change in rehabilitation. Key Points • The modified Four Square Step Test (mFSST) showed excellent test-retest reliability (ICC = 0.952) with a minimal detectable change of 1.17 s in individuals with ankylosing spondylitis. • The mFSST demonstrated strong correlations with the Timed Up and Go test and moderate correlations with the Five Times Sit-to-Stand test, BASDAI, and BASFI, supporting its concurrent validity. • The mFSST is a brief, equipment-free, and clinically useful tool for assessing and monitoring dynamic balance and agility in ankylosing spondylitis.