Tensile properties of in vivo human tendinous tissue.

The biomechanical properties of tendinous structures have traditionally been studied using excised material. Limitations associated with displacement measurements and clamping, and uncertainties as to whether in vitro testing represents physiological function, necessitate developing a method for assessing the mechanical properties of tendinous tissue in the in vivo state. This paper reviews recent results taken with an in vivo and noninvasive protocol using ultrasound as a means of measuring tendon-aponeurosis elongation during tensile loading applied by contraction of the in-series muscle. The results obtained indicate that: (1) the Young's modulus and mechanical hysteresis of in vivo tendons is independent of physiological function and loading, (2) there is a strain variation along the tendon-aponeurosis, and (3) in vivo tendons may exhibit creep. These findings agree with reports from experiments on isolated material and have important biological implications for both the tendon and the in-series muscle. The method described here allows designing longitudinal studies on tendon adaptability, but it also has direct clinical applications.