Introduction: Ankylosing spondylitis (AS) is a chronic and progressive form of arthritis that primarily affects the axial skeleton. In Slovenia, the average time to diagnosis is seven years. Individuals with early-onset AS often present with impaired trunk and hip function; however, it remains unclear to what extent disease-related alterations in the contractility of trunk skeletal muscles—particularly lumbar spine and hip extensors—contribute to these deficits. Accurate assessment of skeletal muscle contractile properties is methodologically challenging, but tensiomyography (TMG) is a promising, non-invasive measurement technique suitable for clinical application. Aim: The aim of this study was to compare the contractile properties and endurance of the lumbar spine and hip extensors between individuals with AS and healthy controls. Methods: Thirty participants were included, comprising 15 individuals with AS (AS group) and 15 healthy controls aged 18–39 years. TMG measurements were performed once, followed by the Biering-Sørensen endurance test. To compare participant characteristics and TMG parameters (Dm, Tc, Vc, Td, and ILS) between groups, the Shapiro–Wilk test and Mann–Whitney U test for independent samples were applied. Results: The AS group demonstrated significantly (p < 0.05) lower contraction velocity (Vc = –0.03 mm/ms) and a reduced index of lateral symmetry (ILS = –14%) compared to controls. A trend toward lower displacement amplitude (Dm = –0.65 mm, p = 0.067) and longer contraction delay time (Td = 2.37 ms, p = 0.095) was also observed in the spinal extensors of the AS group. No significant differences in hip extensor TMG parameters were found between groups. The AS group showed significantly lower endurance of lumbar and hip extensors in the modified Biering-Sørensen test (BS test 45° = –45 s, p = 0.019). A trend toward reduced endurance was also evident in the standard Biering-Sørensen test (BS test = –29.2 s, p = 0.061). Discussion and Conclusion: To our knowledge, this is the first study worldwide to investigate alterations in spinal and hip extensor contractile properties using TMG in individuals with AS. Impairments in contractility and endurance were evident in the lumbar extensors, which likely contribute to the functional limitations experienced by patients.
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