Relationship between compressive stiffness and tensile stiffness in the human Achilles tendon in vivo

Tendon elongation is measured using ultrasound images during isometric ramp contractions in traditional measurement techniques for human tendon mechanical properties, and tendon tensile stiffness (TS) is calculated from the slope of specific force intervals in the tendon force-elongation curve. Recently, the compressive stiffness (CS) of tendons, measured using a myotonometer, has attracted attention as a new technique for investigating tendon mechanical properties in vivo. This study aimed to clarify the relationship between tensile and compressive Achilles tendon stiffness in vivo.

Thirty young healthy adults (20.7 ± 18 years) participated in this study. TS was measured using B-mode ultrasonography and a dynamometer and calculated from the slope of the specific force intervals (25-45%, 30-70%, 50-100%, and 90-100% maximum voluntary contraction) in the tendon force-elongation curve. CS in the resting condition was measured at 3, 4, 5, 6, and 7 cm proximal to the calcaneus Achilles tendon insertion. The data obtained from each measurement region, as well as the average of those data, were used for further correlation analysis. The relationship between TS and CS was evaluated using Spearman rank correlation coefficient.

Correlation analysis demonstrated no relationship between TS and CS regardless of the force intervals in TS (r = - 0.019-0.251, P = 0.181-0.980).

The current results suggest that each measurement technique for the TS and CS evaluates different tendon mechanical properties. Researchers and clinicians should consider the characteristics of these two measurement techniques and select the appropriate method depending on the research purpose.