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Boris I. Prilutsky

In this response, the major criticisms of the target article are addressed. Terminology from the target article that may have caused some confusion is clarified. In particular, the tasks that have the basic features of muscle coordination, as identified in the target article, have been limited in scope. Anew metabolic optimization criterion suggested by Alexander (2000) is examined for its ability to predict muscle coordination in walking. Issues concerning the validation of muscle force predictions, the rules of muscle coordination, and the role of directional constraints in coordination of two-joint muscles are discussed. It is shown in particular that even in one-joint systems, the forces predicted by the criterion of Crowninshield and Brand (1981) depend upon the muscle moment arms and the physiological cross-sectional areas in much more complex ways than either previously assumed in the target article, or incorrectly derived by Herzog and Ait-Haddou (2000). It is concluded that the criterion of Crowninshield and Brand qualitatively predicts the basic coordination features of the major one- and two-joint muscles in a number of highly skilled, repetitive motor tasks performed by humans under predictable conditions and little demands on stability and accuracy. A possible functional significance of such muscle coordination may be the minimization of perceived effort, muscle fatigue, and/or energy expenditure.

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Boris l. Prilutsky

The purpose of this paper is three-fold: (a) to summarize available data on coordination of major two- and one-joint muscles in multijoint tasks and identify basic features of muscle coordination, (b) to demonstrate that there may exist an optimization criterion that predicts essential features of electromyographic activity of individual muscles in a variety of tasks, and (c) to address the functional consequences of the observed muscle coordination and underlying mechanisms of its control. The analysis of the literature revealed that basic features of muscle coordination are similar among different voluntary motor tasks and reflex responses. It is demonstrated that these basic features of coordination of one- and two-joint muscles in two-dimensional tasks are qualitatively predicted by minimizing the sum of muscle stresses cubed. Functional consequences of the observed coordination of one- and two-joint muscles are (a) reduction of muscle force as well as stress, mechanical and metabolic energy expenditure, muscle fatigue, and perceived effort; (b) a spring-like behavior of a multi-joint limb during maintenance of an equilibrium posture; and (c) energy transfer between joints via two-joint muscles. A conceptual scheme of connections between motoneuron pools of one- and two-joint muscles, which accounts for the observed muscle coordination, is proposed. An important part of this scheme is the force-dependent inhibition and excitation from two-joint to one-joint synergists and antagonists, respectively.

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Antoine Falisse, Sam Van Rossom, Johannes Gijsbers, Frans Steenbrink, Ben J.H. van Basten, Ilse Jonkers, Antonie J. van den Bogert and Friedl De Groote

, piriformis muscle activations exceeded 1 (maximum 1.1) for 2 out of 14 trials during small intervals of the gait cycle (<7%) when scaling muscle activity by muscle volume in the static optimization objective function. This underlines the importance of the criterion used to solve the muscle redundancy problem