Elastic properties of tendon measured by two different approaches

Elastic properties of tendon were assessed by two different approaches. Six fresh bovine Achilles tendon specimens were used. The first approach directly measured Young's modulus along the transverse direction (E(perpendicular)) and the longitudinal direction (E(parallel)), using a cyclic compression-relaxation method. Young's moduli were derived based on the measured strain and stress values. The ratio of E(parallel): E(perpendicular) at smaller strains was around 4 and decreased to 0.6 approximately 1.1 at larger strains. The second approach assumed that tendons are transversely isotropic. Three observable second-order elastic stiffness constants (c(11), c(13) and c(33)) were obtained by sound speed measurements along various propagation directions. The measured elastic stiffness constants were also correlated with results from the first approach. It was shown that the transverse isotropy assumption was valid at small strains. However, a significant discrepancy existed between the two approaches. The discrepancy was primarily due to viscoelasticity associated with the first approach.