Muscle input-output model: the static dependence of force on length, recruitment, and firing period

Steady-state isometric active muscle force has a bilinear dependence on length and firing period (inverse of firing rate). At a fixed length, force increases linearly with decreasing firing period. The slope decreases linearly with increasing muscle length, and the value of force extrapolated to zero firing period remains constant. This relationship can be used in Hill-type muscle models.