Agreement of MyotonPRO Measurements Across Standing and Prone Positions in Adolescents with Idiopathic Scoliosis

Background: Adolescent idiopathic scoliosis (AIS) is associated with alterations in muscle tone, stiffness, and viscoelastic properties that may affect musculoskeletal function and rehabilitation outcomes. Myotonometry offers a non-invasive means of quantifying these properties, but its agreement across testing positions in scoliotic populations remains insufficiently characterized. This study aimed to evaluate the agreement across positions of the MyotonPRO device for assessing superficial back muscle properties in adolescents with mild-to-moderate S-shaped AIS across standing and prone positions, and to examine positional and side-to-side differences. Methods: Nineteen adolescents (18 female, 1 male; mean age 15.3 ± 1.8 years) with mild-to-moderate dextroconvex thoracic and sinistroconvex lumbar idiopathic scoliosis were assessed bilaterally at the middle trapezius, lower trapezius, latissimus dorsi, and lumbar erector spinae. Muscle tone, stiffness, elasticity (logarithmic decrement), stress relaxation time, and creep were recorded by a single examiner in both standing and prone positions. Agreement across positions was quantified using intraclass correlation coefficients (ICCs), and between-position differences were tested with paired t-tests. Results: Agreement across positions was poor for most muscle and parameter combinations. The decrement of the lumbar erector spinae showed the highest agreement bilaterally (ICCs 0.829–0.844, good), and the left middle trapezius showed moderate agreement for tone, stiffness, stress relaxation time and creep (ICCs 0.567–0.649); all other muscle and parameter combinations showed predominantly poor agreement (ICCs < 0.50). Between-position differences were muscle- and side-specific: the middle trapezius differed significantly in all five parameters bilaterally (with higher tone and stiffness in standing), whereas both lumbar erectors showed no significant positional differences. The left lower trapezius and both latissimus dorsi showed lower tone in standing, in contrast to the middle trapezius pattern. Conclusions: Across standing and prone positions, MyotonPRO measurements showed predominantly poor absolute agreement for the superficial back muscles examined in adolescents with S-shaped AIS; only the decrement of the lumbar erector spinae reached good agreement.