Test-retest reliability and known-groups validity of trunk muscle tests in people with multiple sclerosis: a cross-sectional, case-control study

OBJECTIVE: Research on muscle performance testing reliability in people with multiple sclerosis (PwMS) has focused on limb performance while less is known about trunk strength and endurance. The objective is to 1) establish test-retest reliability of Trunk Flexion, Lateral Flexion and Extension strength tests, and Plank, Side-Bridge and Biering-Sorensen endurance tests in PwMS and matched healthy controls (HC); 2) analyze known-groups validity of these tests in PwMS and HC; 3) to compare groups for side-to-side differences; 4) to describe the relationships between trunk performance and functional mobility tests.METHODS: Fifteen PwMS (median EDSS=3) and 15 HC underwent two trunk isometric strength and endurance testing sessions. Mobility was evaluated by Timed Up-and-Go (TUG) test. Intraclass correlation coefficient (ICC), standard error of measurement (SEM) and minimal detectable change (MDC) were calculated. Between-group differences in trunk performance were tested using the Student's t-test for independent measures. Between-group differences in an asymmetry index were analyzed by independent t-test. Bivariate correlations between trunk tests and mobility were also examined.RESULTS: All trunk tests showed good-to-excellent relative reliability in both groups (ICC>0.71). Regarding absolute reliability, strength tests were associated with low intersession variability in both groups (MDC: MS, 11.23-36.45%; HC, 26.60-31.98%). Conversely, endurance tests showed higher variability scores in PwMS (MDC: MS, 69.55-116.50%; HC, 29.57-54.40%). PwMS displayed significantly lower trunk performance. Likewise, the asymmetry index showed significantly higher scores in PwMS for endurance assessment but not in strength tests. Significant correlations were detected in PwMS between TUG and several trunk tests (r=0.63-0.70).CONCLUSIONS: SEM and MDC scores revealed similar consistency and variability between groups for strength tests, while higher variability was observed for endurance tests in PwMS. Trunk strength tests are reliable and present discriminant validity to distinguish mildly-disabled PwMS from HC. Conversely, the high measurement error and variability of the endurance tests may hinder their application in intervention programs.IMPACT: Determining the reliability and validity of the tests currently used to assess trunk function is of utter importance for PwMS (who not only show impaired trunk function but also wide fluctuations in performance) as it requires consistent and accurate measurements that are sensitive enough to detect minimal changes induced by rehabilitation.