Skeletal Muscle Properties and Force-Sharing Among Synergistic Muscles

可以说，生物力学中最基本的问题是力的分配问题。力共享问题涉及对跨关节的单个肌肉的力的准确预测。然而，关节通常由更多的肌肉交叉，而不是自由度。因此，我们面对的是一个数学上冗余的系统，它有无数个可能的解决方案，问题就变成了：哪个是正确的解决方案？部队分担问题没有得到合理成功的解决，原因有很多。一个主要原因是缺乏对激动剂组中单个肌肉的机械特性的理解。在这项发现资助中，我们将重点关注三组实验，这些实验将提供以下方面的新信息：(i)在激动剂组内激活时肌肉的机械特性；（ii）系列弹性对肌肉功能的影响；（三）肌节长度不均匀性对基本肌肉特性的影响。第一组实验的目的是确定激动剂组中肌肉在单独激活和与激动剂肌肉同时激活时的力长度和力速度特性。为了实现这一目标，我们使用了一种新开发的技术，该技术允许对兔四头肌和三头肌进行孤立和同时的神经刺激，同时测量肌肉的力量和变形。第二组实验的目的是确定选定绵羊后肢肌肉在运动过程中肌腱系列弹性的体内特性及其与肌力和收缩条件的相互作用。为了实现这一目标，我们补充了我们已经建立的体内肌肉力、变形和激活测量技术，并使用了一种用于腱膜和肌腱应变测量的无线应变传感器。第三组实验的目的是量化不同收缩条件下全肌肉准备中肌节长度的不均匀性，并确定这些不均匀性对力长度、力速度和历史相关特性的影响。为了实现这一目标，我们开发了新的技术，允许使用多光子和二次谐波显微镜测量全肌肉制备中的肌节长度，同时测量肌肉的机械性能。影响：人们一直默认，当肌肉在激动剂组内工作时，肌肉特性保持不变，系列弹性是肌肉效率的主要贡献者，肌肉可以被视为统一的肌节。我们最近的研究表明，这些假设是不正确的，可能导致对肌肉功能的误解。这里提出的实验将挑战（可能是正确的）目前对肌肉力学和功能的看法。

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.