Content area
Abstract
A nonlinear eighth-order agonist-antagonist muscle-joint 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. This one model structure is used to study movements of seven different generalized joints: elbow, knee, wrist and ankle flexion-extension; and eye, head and wrist rotation. Six fundamental properties of muscle are identified and modeled: (i) neural excitation; (ii) neuromuscular activation; (iii) static contraction torque-angle property; (iv) contractile element torque-velocity; (v) series compliance element; (vi) and the passive plant. The only difference between the various models are in the parameter values. Constitutive equations were developed that could be defined by sets of intuitive, easy-to-visualize parameters. For the flexion-extension models, many of the parameter values were obtained by using systematic protocols that combine tissue material properties with muscle-joint geometry.
A wide variety of simulations were performed for all seven joint models. In all cases the (mechanical) model parameters for the given joint model were never touched: each task is defined only by the combination of the neural and external loading signal sequences. Tasks included: (i) voluntary maximal contraction movements, under the condition of no external loading and under the isometric condition; (ii) responses to various transient external loadings while under a variety of neural conditions and added external loadings; (iii) responses to sinusoidal signals; (iv) maximal contraction isokinetic and isotonic testing; (v) fast, point-to-point movements; and (vi) the interaction between various types of movements.
Based on these simulations, it is concluded that the one general model structure that is used here is able to simulate any type of task for any of the seven joints quite adequately - and in most cases very well. It is also found that each of the basic nonlinear properties of muscle are of significance for some set of tasks, with only the contractile element torque-velocity relation always of great importance. An important, more specific contribution was the quantification of the effect of the ongoing co-contraction level for different types of tasks and the identification of the source of this phenomena.
