In vivo determination of triceps surae muscle–tendon complex viscoelastic properties

Viscoelastic properties of muscles and tendons have an important influence on human motion performance. Proper determination of these properties is essential in the analysis and modelling of human motion dynamics. The purpose of our study was to develop a method for in vivo determination of the viscoelastic properties of the entire triceps surae muscle–tendon complex (MTC) including the gastrocnemius. Ten trained male subjects participated in this study. The measurement procedure consisted of two parts: soleus and Achilles tendon stiffness and viscosity were determined in the first part while the gastrocnemius stiffness and viscosity were determined in the second part. The measurement device and the procedure have been designed in such a manner that as few human body segments move as possible during the measurement. Thus, the measurement uncertainty due to the approximation of the properties of the human body segments was minimized. Triceps surae MTC viscoelastic properties of both legs were measured for each subject. There were no significant differences in viscoelastic coefficients for left and right lower extremities; however, there were noticeable differences between subjects. The soleus stiffness coefficient was greater than the gastrocnemius stiffness coefficient by 87.6 m⁻¹ in average. For all subjects, soleus viscosity was equal or greater than gastrocnemius viscosity. Values of viscoelastic parameters obtained by our method can be used in the analysis and modelling of human movement in situations where the knee joint is not necessarily flexed and there is coactivation of the soleus and the gastrocnemius.