ASB Pre-Doctoral Award 2021: model-based insights into the effects of complex structure, dynamics, and neuromechanics on soleus muscle stiffness

Muscle stiffness provides insight into both intrinsic properties and dynamic function as a descriptive metric of neuromechanical structure-function arising from non-linear passive and active properties. The soleus muscle structure-function is important to ankle mobility but characterizing soleus stiffness is complicated by its morphology and contraction dynamics. In this study, we used a 3D finite element model to investigate methodological approaches to measure soleus stiffness during passive and active lengthening. Performing a single passive lengthening simulation, we calculated a range of Young's modulus values for the soleus muscle based on different definitions of stress and strain that all overestimated stiffness of the prescribed muscle material. Stiffness measurements made from fully active lengthening simulations were dependent on the timing of activation relative to muscle length change, with greater stiffness estimated when activation increased concurrently with muscle length compared to when the muscle was fully activated before lengthening. Simulations with independently varied activation of the posterior and anterior compartments demonstrated how these different regions of muscle fascicles have minimal effects on each's kinematics while influencing soleus force production. While the anterior compartment is difficult to measure experimentally, model predictions suggest it interacts with the posterior compartment to modulate soleus stiffness. Different calculations of Young's modulus representing organ-level and tissue-level soleus muscle properties varied in sensitivity to activation changes in the muscle compartments. Simulation results demonstrate challenges in characterizing muscle stiffness and scaling mechanical properties from tissue to whole muscle, providing insight for improving experimental design to assess intrinsic properties and dynamic function.