A comparison of models explaining muscle activation patterns for isometriccontractions

One of the main problems in motor-control research is the muscle load shari ng problem, which originates from the fact that the number of muscles spann ing a joint exceeds the number of degrees of freedom of the joint. As a con sequence, many different possibilities exist for the activation of muscles in order to produce a desired joint torque. Several models describing muscl e activation have been hypothesized over the last few decades to solve this problem. This study presents theoretical analyses of the various models an d compares the predictions of these models with new data on muscle activati on patterns for isometric contractions in various directions. None of the e xisting models fitted the experimental data in all aspects. The best fit wa s obtained by models based on minimization of the squared sum of muscle for ces (Sigma(m)phi(m)(2), which is almost equivalent to the Moore-Penrose pse udo-inverse solution), muscle stress sigma (Sigma(m) sigma(m)(2)) Or muscle activation alpha (Sigma(m) alpha(m)(2)). Since muscle activation patterns are different for isometric contractions and for movements, it could well b e that other models or optimization criteria are better suited to describe muscle activation patterns for movements. The results of our simulations de monstrate that the predicted muscle activation patterns do not depend criti cally on the parameters in the model. This may explain why muscle activatio n patterns are highly stereotyped for all subjects irrespective of differen ces between subjects in many neuro-anatomical aspects, such as, for example , in the physiological cross-sectional area of muscle.