Differential strain patterns of the human Achilles tendon determined in vivo with freehand three-dimensional ultrasound imaging

Abstract

Summary Often the human Achilles tendon (AT) has been considered to act as a single elastic structure in series with the muscles of the triceps-surae. As such it has been commonly modelled as a hookean spring of uniform stiffness. However, the free AT and the proximal AT have distinctly different structures which lend themselves to different elastic properties. This study aimed to use three-dimensional freehand ultrasound imaging to determine if the proximal AT and free AT exhibit different elastic behaviour during sub-maximal, fixed-end contractions of the triceps-surae. Six male and five female participants (mean ± sd age = 27 ± 5 years) performed fixed position contractions of the plantar-flexors on an isokinetic dynamometer at 50% of their maximum voluntary contraction in this position. Freehand 3D ultrasound imaging was used to reconstruct the free-tendon and proximal AT at rest and during contraction. The free-tendon exhibited significantly (P = 0.03) greater longitudinal strain (5.2 ± 1.7%) than the proximal AT (2.6 ± 2.0%). The lesser longitudinal strain of the proximal AT was linked to the fact that it exhibited considerable transverse (orthogonal to the longitudinal direction) strains (5.0% ± 4). The transverse strain of the proximal AT is likely due to the triceps-surae muscles bulging upon contraction and thus, the level of bulging may influence the elastic behaviour of the proximal AT. This might have implications for the understanding of triceps-surae muscle-tendon interaction during locomotion, tendon injury mechanics and previous measurements of AT elastic properties.