A constitutive model for active-passive transition of muscle fibers

► New experimental data show the transition from the passive to the active state in a Manduca muscle under uniaxial extension. ► The data show that the active muscle, in the reference configuration, is no longer stress-free. ► The constitutive model accounts for multiple stress-free reference configurations, for the change in mechanical behavior under stimulus and for energy dissipation during unloading.