A Constitutive Model for Preconditioining in Ligaments and Tendons

Collageneous tissues, such as ligaments and tendons, are viscoelastic materials: they creep under constant stress, relax under constant strain, and show hysteresis when cyclically loaded and unloaded [1]. Although these tissues are viscoelastic, researchers in biomechanics often study their “pseudo-elastic” properties after subjected them to “preconditioning.” Preconditioning is an experimental procedure consisting of loading and unloading biological tissues repeatedly until no further change between the loading and unloading stress-strain curves are observed. This procedure significantly affects the measurement of mechanical properties [2]. Unfortunately, because there is no standard preconditioning protocol used in biomechanical testing, comparison among different experimental studies that quantify the mechanical properties of biological tissues remains difficult [3].