We developed muscle-tendon models incorporating Hill-type structure and length-dependent coupling between activation and velocity. The models were evaluated in electrically stimulated cat soleus muscles. Dynamic model parameters were estimated by a nonlinear parameter estimation algorithm from input-output data obtained during simultaneous random stimulation and length changes. Static parameters were estimated from the length-tension curve. A model with length history-dependent activation-velocity coupling predicted the behavior of the muscle under a wide variety of conditions, including during random perturbations and during isovelocity movements, where it captured short range stiffness and length history-dependent postyielding behavior. Furthermore, the model predicted twitch responses. The generality of this fixed parameter model makes it especially suitable for simulation and feedforward control, where muscle responses are not available for on-line parameter adaptation.