Improvements to Hoang et al.'s method for measuring passive length-tension properties of human gastrocnemius muscle in vivo.

While the passive mechanical properties of a musculo-articular complex can be determined using the relationship between the articular angle and the passive torque developed in resistance to motion, the properties of different structures of the musculo-articular complex cannot be easily assessed. Recently, an elegant method has been proposed to estimate the passive length-tension properties of gastrocnemius muscle-tendon unit (Hoang et al., 2005). In the present paper, two improvements of this method are proposed to decrease the number of parameters required to assess the passive length-tension relationship from 9 to 2. Furthermore, these two parameters have physical meaning as they represent a passive muscle-tendon stiffness index (alpha) and the muscle-tendon slack length (l(0)). alpha and l(0) are relevant clinical parameters to study the chronic effects of aging, training protocols or neuromuscular pathologies on the passive mechanical properties of the muscle-tendon unit. Eight healthy subjects performed passive loading/unloading cycles at 5 degrees /s with knee angle at 6 knee angles to assess the torque-angle relationships and to apply the modified method. Numerical optimization was used to minimize the squared error between the experimental and the modeled relationships. The experiment was performed twice to assess the reliability of alpha and l(0) across days. The results showed that the reliability of the two parameters was good (alpha: ICC=0.82, SEM=6.1m(-1), CV=6.3% and l(0): ICC=0.83, SEM=0.29 cm, CV=0.9%). Using a sensitivity analysis, it was shown that the numerical solution was unique. Overall, the findings may provide increased interest in the method proposed by Hoang et al. (2005).