Introduction: The muscles of the spine are prone to overload and their weakness can significantly influence the development of degenerative changes. Measuring the contractile properties of the spinal erectors using tensiomyography (TMG) is non-invasive and does not require patient activation, so it may be useful when active testing of muscle function is not feasible due to pain. Aim: The aim of the study was to evaluate the reliability of TMG measurements of the contractile properties of the spinal erectors in subjects without low back pain and to compare them with the contractile properties of the same muscles in patients with segmental lumbar instability. Methods: 44 subjects were included in the study, of which 22 were healthy subjects with no history or presence of low back pain and 22 subjects with a diagnosis of segmental lumbar instability. In the healthy subjects, TMG of the spinal erectors was repeated over a period of 3 minutes and 72 hours. In subjects with segmental instability, TMG of the spinal erectors was performed only once. Student's t-test and Mann Whitney U-test for independent samples were used to compare baseline characteristics of subjects and values of parameters Dm, Tc, Vc, Td and ILS between groups. Inter-examiner reliability was assessed using the two-way ICC for mixed effects with 95% confidence intervals. Results: In healthy individuals, we found excellent intra-rater reliability (ICC > 0.97) of the parameters Dm, Tc, Vc and Td over a 3-minute period and good to excellent intra-rater reliability (ICC > 0.88) of Dm, Tc, Vc and Td over a 72-hour period. We found poor intra-rater reliability of ILS parameters at 3 minutes and 72 hours. The Dm, Tc, Vc, Td and ILS of the spinal erectors differed significantly (p < 0.05) between healthy subjects and patients with segmental instability. Discussion and conclusion: In healthy subjects, most TMG parameters of the spinal extensors show good to excellent intra-rater reliability over periods of 3 minutes and 72 hours. The values of the contractile muscle properties of the spinal erectors differ significantly between subjects with and without segmental lumbar instability, indicating a significant relationship between spinal instability and spinal erector function.
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