Comparing MyotonPRO-derived and tensile measures of Achilles tendon stiffness: a cross-sectional study in healthy...

Stiffness is a key mechanical property of tendon, relevant to both injury and athletic performance. Traditionally, stiffness is measured as tendon elongation in response to applied loading (tensile stiffness), requiring specialized equipment and methods. The MyotonPRO, a handheld device that delivers a small, rapid perturbation and measures dynamic stiffness from the resulting tissue oscillations, has emerged as a fast, user-friendly alternative. However, it assesses stiffness perpendicular to the tendon's primary axis and may not be appropriate as a surrogate for tensile stiffness. Tensile stiffness of the Achilles tendon was obtained from 18 healthy individuals using a combination of 3D motion analysis, passive ankle rotations performed on a dynamometer, and ultrasonography. Dynamic stiffness was obtained at a neutral ankle angle under resting conditions from a standardized location superficial to the Achilles tendon mid-portion using the MyotonPRO. Regression analysis showed a low correlation (R² = 0.17) between the two methods. Tensile stiffness was consistently greater than dynamic stiffness (114 ± 63 N/mm vs. 1.04 ± 0.27 N/mm). Bland-Altman analysis showed wide limits of agreement (-11.0 to 236.7 N/mm), and proportional bias (R² = 1) reflected a difference in scale. The poor agreement suggests the two methods are not interchangeable. The discrepancy likely reflects fundamental differences in measurement orientation and tissue behavior due to tendon anisotropy. Caution is warranted when interpreting MyotonPRO-derived stiffness, which may not reflect the same functional properties captured by tensile measurements.