The Winding Filament Hypothesis states that - upon calcium influx into the muscle sarcomere - titin binds to and winds upon actin thin filaments. The mechanical behavior of titin could serve as an explanatory mechanism for force enhancement with stretch, force depression with shortening, and dynamic force-velocity properties of active muscle.
The Winding Filament Model (WFM) is a two-parameter (resting length and peak isometric force) approximation of the molecular kinematics and kinetics of a muscle sarcomere. The model is comprised of a massless actin "pulley" of radius R, about which a damped contractile element (myosin head), a damped titin spring element, and a series elastic element interact. Current implementation in OpenSim is through the MATLAB scripting interface, with the goal of this project being a freely distributed WFM plug-in utilizing all features currently offered within the OpenSim 3.3 API.