Reliability of human Achilles tendon stiffness measures using freehand 3-d ultrasound

Abstract

Introduction: The Achilles tendon (AT) is the largest and strongest tendon in the human body and is subject to high forces during weight-bearing activities such as walking and running. Tendon stiffness, which is defined as resistance to elongation under load, is a key determinant of the efficiency of the triceps surae muscle output during human locomotion. Freehand three-dimensional ultrasound (3DUS) is a promising method for measuring stiffness of the Achilles tendon, and particularly the free AT (2-6 cm proximal to calcaneus), which is commonly injured. This study aimed to investigate the test-retest reliability and the smallest detectable change (SDC) of freehand 3DUS in measuring free AT stiffness in humans. Methods: The free Achilles tendon length of healthy participants (n=10) was scanned on the same day on two consecutive occasions (1 h apart) during rest and isometric planterflexion (PF) contractions at 20%, 40%, and 60% of maximum force. The slope of force-elongation curve over these force levels represented individual stiffness (N/mm). Relative reliability was assessed using the intra-class correlation coefficient (ICC) and absolute reliability was estimated with standard error of measurement (SEM), and smallest detectable change (SDC). Systematic bias in stiffness measures was explored by comparing test and retest distributions and Bland-Altman plots. Results: The test-retest reliability for free AT stiffness using freehand 3DUS was excellent [ICC= 0.994 (95% CI 0.978 to 0.999)]. The mean difference (SD) was -3.15 (± 26.6) N/mm, SEM was 18.82 N/mm, and SDC was 52.14 N/mm. The P-value for the difference between the means was = 0.72. The Bland and Altman plot showed no systematic bias within stiffness measures as data points were equally scattered around the mean difference (95% CI: -22.18-15.88) and within the 95% limits of agreement (LOA) = mean + (SD × 1.96). Discussion: The current study has established the reliability of freehand three-dimensional ultrasound for determining free AT stiffness in healthy humans. Freehand 3DUS could be implemented in research or clinical practice to measure and monitor changes in free AT stiffness that may occur with loading or as a consequence of tendon injury or pathology. Conflict of interest disclosure: The authors declare that they have no competing interests.