In vivo human tendon mechanical properties

1. The aim of the present study was to measure the mechanical properties of human tibialis anterior (TA) tendon in vivo. 2. Measurements were taken in five males at the neutral ankle position and involved: (a) isometric dynamometry upon increasing the voltage of percutan eous electrical stimulation of the TA muscle, (b) real-time ultrasonography for measurements of the TA tendon origin displacement during contraction a nd tendon cross-sectional area, and (c) magnetic resonance imaging for esti mation of the TA tendon length and moment arm. 3. From the measured joint moments and estimated moment arms, the values of tendon force were calculated and divided by cross-sectional area to obtain stress values. The displacements of the TA tendon origin from rest to all contraction intensities were normalized to tendon length to obtain strain v alues. From the data obtained, the tendon force-displacement and stress-str ain relationships were determined and the tendon stiffness and Young's modu lus were calculated. 4. Tendon force and stress increased curvilinearly as a function of displac ement and strain, respectively. The tendon force and displacement at maximu m isometric load were 530 N and 4.1 mm, and the corresponding stress and st rain values were 25 MPa and 2.5%, respectively. The tendon stiffness and Yo ung's modulus at maximum isometric load were 161 N mm(-1) and 1.2 GPa, resp ectively. These results are in agreement with previous reports on in vitro testing of isolated tendons and suggest that under physiological loading th e TA tendon operates within the elastic 'toe' region.