Reliability of human Achilles tendon stiffness measures using freehand 3D ultrasound

Eman Merza1, Dr Stephen Pearson2, Professor Glen Lichtwark3, Dr Alessandro Garofolini4, Associate Professor Peter Malliaras1

1Department of Physiotherapy, Faculty of Medicine, Nursing and Health Science, Monash University, Melbourne, Australia, 2Centre for Health, Sport and Rehabilitation Sciences Research, University of Salford, Greater Manchester, United Kingdom , 3Centre for Sensorimotor Performance, School of Human Movement and Nutrition Sciences, University of Queensland, St. Lucia, Brisbane, Australia, 4Institute for Health and Sport (IHES), Victoria University, Melbourne, Australia

Stiffness of the Achilles tendon (AT) 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 AT stiffness, and particularly the free AT (2-6 cm proximal to calcaneus), which is commonly injured. This study aimed to investigate the test-retest reliability of freehand 3DUS in measuring free AT stiffness in humans.

The free AT length of healthy participants (n=10) was scanned on the same day on two consecutive occasions (1 hour apart) during rest and isometric planterflexion contractions at 20%, 40%, and 60% of maximum force. 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.

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 Bland and Altman plot showed no systematic bias within stiffness measures.

This study has established the reliability of freehand 3DUS 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.


Eman Merza is a full-time PhD student at Monash University department of physiotherapy. She obtained her bachelor’s degree in physiotherapy from Kuwait University in 2008 and her master’s degree from the University of Nottingham (UK) in 2016. Eman research focus is Achilles tendinopathy. She is investigating the biomechanical adaptation of the Achilles tendon in response to load.

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