Skeletal Muscle Properties and Force-Sharing Among Synergistic Muscles

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Arguably the most fundamental problem in biomechanics is the force sharing or distribution problem. The forces sharing problem deals with the accurate prediction of forces of individual muscle that cross joints. However, joints are typically crossed by more muscles than there are degrees of freedom. Therefore, we face a mathematically redundant system with an infinite number of possible solutions, and the question becomes: which is the correct solution? There are many reasons why the force sharing problem has not been solved with any reasonable success. A primary reason is the lack of understanding of the mechanical properties of individual muscles within an agonist group. In this discovery grant, we will focus on three sets of experiments that will provide novel information on (i) the mechanical properties of muscles when activated within their agonist group; (ii) the effect of series elasticity on muscle function; and (iii) the effect of sarcomere length non-uniformities on fundamental muscle properties. The purpose of the first set of experiments is to determine force-length and force-velocity properties of muscles in an agonist group when activated in isolation and when activated simultaneously with their agonist muscles. In order to achieve this aim, we use a newly developed technique that allows for isolated and simultaneous nerve stimulation of the rabbit quadriceps and triceps surae muscles, while simultaneously measuring muscle forces and deformations. The purpose of the second set of experiments is to determine the in vivo properties and the interaction of tendon series elasticity with muscle force and contractile conditions in selected sheep hind limb muscles during locomotion. In order to achieve this aim, we have complemented our established techniques of in vivo muscle force, deformation and activation measurements, with a wireless strain transducer for aponeuroses and tendon strain measurements in vivo. The purpose of the third set of experiments is to quantify sarcomere length non-uniformities in whole muscle preparations for different contractile conditions, and determine the effect of these non-uniformities on force-length, force-velocity and history-dependent properties. In order to achieve this aim, we have developed novel techniques that allow for measurement of sarcomere length in whole muscle preparations using multi-photon and second harmonic generation microscopy while simultaneously measuring muscle mechanical properties. Impact: It has been tacitly assumed that muscle properties remain unchanged when muscles work within their agonist group, that series elasticity is a major contributor to muscle efficiency, and that muscles can be treated like a uniform sarcomere. Our recent research indicates that these assumptions are incorrect and can lead to vastly misleading interpretations on muscle function. The experiments proposed here will challenge (possibly correct) current beliefs in muscle mechanics and function.