The reliability of MyotonPro in assessing the stiffness in lumbar spinal erector muscles and the effect of different position

Objective: The aim of this study was to evaluate the inter-operator and intra-operator reliability of lumbar muscle stiffness assessment for the erector spinal muscle by MyotonPRO at different vertebral levels and at different trunk flexion positions on the left and right sides in healthy people. This study also aimed to explore the differences of muscle stiffness between different test points and between different trunk flexion positions.Method: This study recruited 54 participants aged from18 to 30 years old, including 22 men and 32 women. Two operators used MyotonPRO to evaluate the muscle stiffness of erector spinal muscles at three vertebrae levels (L1, L3 and L5). The assessment for each level was applied during participants keeping at 0° flexion position (natural standing), 30° flexion position, and 60° flexion position. The inter-operator reliability was considered as the assessment results from two operators in the same day, and the intra-operator reliability was calculated by the assessment results from the same operator in two different days with a 7-day interval. The differences of lumbar muscle stiffness between different test points and flexion positions were explore based on the assessment result from one operator. Moreover, the differences of muscle stiffness between difference teat points or different positions were detected by one-way analysis of variance (ANOVA) and post-hoc analysis. Results: Most ICC values for reliability of muscle stiffness assessment were higher than 0.75. In healthy people, the ICC values ranged from 0.719 to 0.940. Those of SEM ranged between 28.23 and 130.31 and those of MDC ranged between 78.24 and 361.20. Furthermore, the ICC values for inter-operator and intra-operator reliability on the right side were L1 (0.915, 0.846); L3 (0.865, 0.859) and L5 (0.908, 0.845) at neutral 0° flexion position; L1 (0.924, 0.926), L3 (0.925, 0.902) and L5 (0.916, 0.881) at 30° flexion position; L1 (0.897, 0.893); L3 (0.826, 0.835) and L5 (0.902, 0.785) at 60° flexion position. In terms of the effect of test points on the muscle stiffness, we found significant differences between L1, L3, and L5 in healthy people. The muscles stiffness of L1 was significantly lower than that of L3 and L5 at left and right sides at 0°, 30° and 60° flexion in two populations (p