Real time ultrasound has become popular for in vivo assessment of human tendon stressstrain relationship. The same general principles of in vivo tendon testing have often been applied with the aim of characterizing the mechanical behavior of the human tendon in different in-vivo situations and conditions. The results obtained vary greatly (Arampatzis et al. 2005; Bojsen-Møller et al. 2004; Kubo et al. 2002, 2004; Maganaris and Paul 2002; Muramatsu et al. 2001; Reeves et al. 2005). In young sedentary adults, for example, the tendon stiffness, Young’s modulus and mechanical hysteresis values are ~ 17-760 Nm/mm, GPa, and 11-19%, respectively (for a review see Maganaris et al
