The force-length characteristics of dense connective tissues
(DCTs) vary non-linearly as a function of strain rate. However, there is
no existing class of OpenSim® to incorporate strain rate effect into the
OpenSim® model. In this work, a new plugin for OpenSim® was developed to
incorporate the non-linear strain rate behaviour of dense connective
tissues (DCTs) of the human knee. Experimental force-length plots from
the literature was used to extract the shape factor, scale factor, the
coefficient of viscosity and elastic stiffness corresponding to specific
strain rates. A new class object termed as NonLinearLigament was
formulated using a custom plugin based on a structural constitutive
model. A test platform was created to evaluate the force-length patterns
at multiple strain rates ranging from 0.0001 s-1 to 100 s-1 for the DCT
bundles. Knee kinematics of 25 DCT bundles were subjected to forward
simulation at time frames corresponding to failure at dynamic activities.
To understand the significance, the force-length characteristics of each
of the DCTs were simulated as a function of strain rate for both existing
Ligament class of OpenSim® and the proposed NonLinearLigament class.
During simulation of tensile loading, higher forces were observed with an
increase of strain rate in DCTs. Existing Ligament class in OpenSim® was
devoid of the technique in calculating forces at different strain rates.
The developed plugin takes into account the short term viscoelastic
behaviour of DCTs and hence, would help in accurate simulation of tissue
behaviour understanding their injury mechanics.