Quantitative constitutive behaviour and viscoelastic properties of fresh flexor tendons.

The objective of this study was to obtain detailed high quality experimental data under well-controlled test conditions in order to quantify tendon viscoelastic behaviour and provide an experimental basis for large deformation mathematical modelling. Eighty-six fresh chicken flexor digitorum profundus (FDP) tendons were mechanically tested using an Instron 5565 universal testing machine and a Bioplus bath containing physiological saline solution. At low strain rates (<or=0.001s(-1)), no strain rate dependence was found and the value of the elastic modulus was 324+/-17 MPa. At medium strain rates (0.003s(-1) - 0.1s(-1)), the elastic stiffness increased with increasing strain rate. For example, the values of the elastic modulus were 427+/-10, 653+/-21 and 837+/-11 MPa for strain rates of 0.006s(-1), 0.012s(-1) and 0.05s(-1) respectively. A series of stress relaxation experiments at different levels of strain were conducted. The higher the initial stress level, the faster the stress relaxed. For all stress relaxation tests, the relationship of normalised relaxation function G(t) against log time (ln(t)) was approximately linear (R2 = 0.935 +/- 0.028) and this supports Fung's quasi-linear viscoelasticity (QLV) hypothesis. Some preliminary work on bovine flexor tendons using a video extensometer is also reported.