Assessing the Test-Retest Reliability of MyotonPRO for Measuring Achilles Tendon Stiffness

Objectives: This study evaluates the test-retest reliability and inter-rater reliability of the MyotonPRO for measuring Achilles tendon stiffness at two standardized sites over various time frames and settings. Methods: Eight healthy participants underwent assessments by three raters over six visits. Tendon stiffness was measured at proximal (mid-portion) and distal (insertional) regions of the Achilles tendon at various time frames (10-15 s, 10-15 min, 24 h, and 14 days apart). Measurements included participant repositioning and two activity stimuli (daily living and sport). Reliability was calculated using the intraclass correlation coefficient (ICC), its 95% confidence interval, coefficient of variation, standard error of measurement, and minimal detectable change. Results: Short-term reliability (10-15 min) was excellent, with an ICC of 0.956 (0.929-0.974). Between days reliability (24 h) was good, with an ICC of 0.889 (0.802-0.938). Between weeks reliability (2 weeks) was good with an ICC of 0.886 (0.811-0.931). Short-term reliability with the simulation of activity of daily living was good, with an ICC of 0.917 (0.875-0.945). Short-term reliability with the simulation of sport was good with an ICC of 0.933 (0.891-0.96). Between days reliability with the simulation of sport was good, with an ICC of 0.920 (0.859-0.955). Conclusions: When used in a standardized position, the MyotonPRO demonstrates reliable repeated measurements of Achilles tendon stiffness. This protocol provides a foundation for clinical research and rehabilitation by clarifying expected reliability across minutes, days, and weeks, thus aiding clinicians and researchers in monitoring tendon adaptations and making evidence-based decisions.