A nonlinear eighth-order agonist-antagonist muscle model is identified, based on engineering analysis and design criteria, as the desired structure for the broad-range study of a variety of fundamental human joint movements. To complement this structure, systematic protocols, that combine material and geometrical information for each muscle, are developed to obtain the model parameter values needed for the various muscle constitutive equations. The parameters describing the four basic nonlinear relations are easy to visualize, representing the peak curve values and "shape" parameters. Elbow, knee, wrist, and ankle fiexion-extension and eye, wrist, and head rotation are simulated by this same model structure.

In each case, one set of model parameters is found to be adequate to simulate all fundamental movement tasks. Movement output task trajectories are due to the combination of the two neuro-input sequences and the external loadings to the model, and include any combination of isometric, isotonic, isokinetic, oscillatory, fast ballistic movements and fast-slow movement interaction, all under various cocontraction levels and feedback. Previous sensitivity analysis and optimization tools are extended, and the concepts of insight via task-specific sensitivity analysis and of sensitivity-based task-specific model reduction are developed.

An overview is presented of the various models' ability to simulate data in the literature, with the role of cocontraction, in particular, illuminated here. It is found that cocontraction dramatically influences the model response to external loading, while affecting neurally initiated movement to a lesser degree, and that the source of this phenomenon resides in the combination of series element and torque-velocity nonlinearities, and not in the static torque-angle relationship. Bias external torques work mechanically in a manner similar to cocontraction. Finally, it is seen that all four fundamental nonlinearities play major roles when the entire spectrum of tasks are considered, with only the torque-velocity relation of great importance for all tasks. The series element is of greater importance for tasks with transient external loadings, the torque-angle and parallel elastic nonlinearities dominate behavior near the movement operating-range extremes, and the excitation and activation processes are most significant for fast voluntary movemeats.