One of the main problems in motor-control research is the muscle load sharing problem, which originates from the fact that the number of muscles spanning a joint exceeds the number of degrees of freedom of the joint. As a consequence, many different possibilities exist for the activation of muscles in order to produce a desired joint torque. Several models describing muscle activation have been hypothesized over the last few decades to solve this problem. This study presents theoretical analyses of the various models and compares the predictions of these models with new data on muscle activation patterns for isometric contractions in various directions. None of the existing models fitted the experimental data in all aspects. The best fit was obtained by models based on minimization of the squared sum of muscle forces ( summation operator(m)φ(2)(m), which is almost equivalent to the Moore-Penrose pseudo-inverse solution), muscle stress sigma ( summation operator(m)sigma(m)(2)) or muscle activation alpha ( summation operator(m)alpha(m)(2)). Since muscle activation patterns are different for isometric contractions and for movements, it could well be that other models or optimization criteria are better suited to describe muscle activation patterns for movements. The results of our simulations demonstrate that the predicted muscle activation patterns do not depend critically on the parameters in the model. This may explain why muscle activation patterns are highly stereotyped for all subjects irrespective of differences between subjects in many neuro-anatomical aspects, such as, for example, in the physiological cross-sectional area of muscle.

In order to explain the task‐dependent activation of muscles, we have investigated the hypothesis that mono‐ and bi‐articular muscles have a different functional role in the control of multijoint movements. According to this hypothesis, bi‐articular muscles are activated in a way to control the direction of external force. The mono‐articular muscles are thought to be activated to contribute to joint torque mainly during shortening movements. To investigate this hypothesis, surface electromyographic (EMG) recordings were obtained from several mono‐ and bi‐articular arm muscles during voluntary slow movements of the wrist in a horizontal plane against an external force. The direction of force produced at the wrist and the direction of movement of the wrist were varied independently. The results revealed distinct differences between the activation patterns of mono‐ and bi‐articular muscles. The activation of the bi‐articular muscles was not affected by movement direction, but appeared to vary exclusively with the direction of force. The mono‐articular muscles showed significantly more EMG activity for movements in a specific direction, which equalled the movement direction corresponding to the largest shortening velocity of the muscle. The EMG activity decreased gradually for movements in other directions. This direction‐dependent activation appeared to be independent of the direction of the external force.

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One of the key problems in motor control concerns the apparent redundancy of muscles and joints. This biological and kinematic redundancy has been an object of study since long. In this paper we will give a review of the various approaches which have been proposed to solve this problem. We will give a comparison of the results of these approaches with special emphasis on recent models, which try to deal with this problem by eliminating the number of degrees of freedom. This reduction will be achieved by imposing constraints, which follow from the biomechanics of the system under study or from some plausible requirements on the behaviour of these systems in various motor tasks.Since this problem is a key issue both in the control of biological and artificial robot manipulators, we will also discuss models from the robotics community, as well as the possible relevance of biological models for robotics.