Repeatability of Myotonometric Muscle Measurements in Infants Aged 0–3 Months: Toward an Objective Tool Supporting Early Motor Assessment

Background: Early detection of developmental disorders in infants is an important topic for scientists and clinicians, but above all for children and their families. Early detection of changes in the mechanical properties of muscles is crucial for starting therapeutic processes. This study aims to fill the research gap regarding the use of myotonometry in the infant patient group (aged 0–3 months). The study aimed to evaluate the test–retest repeatability of myotonometric measurements in infants at three time points, with particular attention to potential age-related differences. Methods: The study group consisted of healthy newborns born from physiological pregnancies with an Apgar score of 8–10 points. The studies began on the 1st–3rd day of life and lasted until the 12th week in cycles every 6 weeks. Results: In the study conducted on the 1st–3rd day of life, the repeatability of the studied parameters can be described as moderate (muscle tone, elasticity, relaxation, and creep) or poor (stiffness). Measurements conducted in the 6th week of life show high repeatability of muscle stiffness and elasticity or moderate repeatability of muscle tone, creep, and relaxation time. Measurements in the 12th week of life of the infants may be considered as high repeatability of muscle tone, stiffness, relaxation time, and the creep parameter, and moderate muscle elasticity. Conclusions: The study demonstrated the usefulness of using myotonometric measurement for infants from the 6th and 12th week of age in the studied parameters, confirmed by the high repeatability of measurements. Whereas in the group of newborns aged 1–3 days, the repeatability of all analyzed parameters can be described as moderate or low.