This structural model of the primary afferent incorporates fusimotor effects that arise from two intrafusal fiber submodels.  One submodel represents the bag1 intrafusal fiber and receives dynamic fusimotor input, while the other represents bag2 and chain fibers together and receives static fusimotor input (called bag2 submodel). Both submodels contain a sensory part (the central nuclear bag around which primary afferents terminate) and a muscular part (the contractile portion of a nuclear bag, containing sarcomeres). The sensory parts are modeled as identical linear elastic elements. The muscular part of the bag1 submodel is modelled as a slow twitch muscle fiber, and that of the bag2 as a fast twitch fiber. The forces generated by this part depend on fiber activation, fiber length and contraction velocity (similar to extrafusal muscle fibers). The two submodels differ in terms of the parameters that control the maximal isometric force, the passive elasticity, the maximal force for an eccentric contraction relative to the maximal isometric force, and the passive damping coefficient. The fusimotor input is linearly related to the active force, and scaled with respect to the fusimotor excitation for maximal effect. The model furthermore includes the force enhancement in response to stretch that has been observed in extrafusal muscle fibers.