Internal mechanics of whole muscle contraction

我们对肌肉功能的大部分了解来自于单纤维和孤立的肌肉实验，这些实验通常具有稳定的收缩条件和最大的激活，肌肉模型反映了这一点。然而，我们的肌肉通常用于非最大收缩水平的不稳定和动态运动。整个肌肉的收缩比其组成肌肉纤维的收缩要复杂得多。事实上，最近在活体内产生整体肌肉力量的模型有很大的误差，这表明我们仍然有很多关于整个肌肉如何收缩的知识需要了解。肌肉不仅会产生作用力和沿其作用线的长度变化，而且作用力和变形都发生在所有三个维度上。最近，肌肉的三维结构以及收缩和结缔组织之间的相互作用的重要性已经变得明显。例如，压缩肌肉会抑制其动作路线上形成的力(高达16%)，肌肉收缩会导致横向功的完成(坐着时拉紧臀部会因为横向肌肉功而抬高身体)。此外，如果你最大限度地激活一块肌肉，那么它永远不会像它的组成纤维所能做到的那样快，这种差异(高达35%)对于处于低激活水平(我们日常运动的典型条件)的大肌肉来说是最大的。不同的实验室，包括我自己的实验室，都提出了解释这些现象的因素，包括内部弹性、力传递和横向功、非活动组织的阻力以及肌肉组织的惯性属性。然而，我们仍然缺乏一个通用的框架来将所有这些影响与肌肉的收缩性能联系起来。识别整个肌肉功能的机制将增加我们对肌肉生物力学的基本理解，并提高我们研究和保持动物和人类的机动性、敏捷性和健康的能力。肌肉收缩的可用能量是有限的，因此我们提出，使用内部能量的机制，甚至在横向上做外部功的机制，将沿着其作用线减少功输出。不仅如此，在使用内部能量和做横向功产生新陈代谢成本的情况下，肌肉收缩的效率也会降低。这项拟议的研究的目标是使用理论技术，并通过尖端实验数据进行验证，将肌肉内部能量的分布与其3D形状和力联系起来。特别是，我们将完善和验证肌肉收缩的有限元模型，研究横向压缩和膨胀对纵向力的关系，以及肌肉组织质量对肌肉收缩动力学和效率的影响。我的研究计划的长期目标是发展我们对整个肌肉收缩机制的机械学理解。

Much of our understanding of muscle function comes from single fibre and isolated muscle experiments that commonly have steady contractile conditions and maximal activation, and muscle models reflect this. However, our muscles are normally used for unsteady and dynamic movements at submaximal levels of contraction. Contractions of whole muscles are more complex than that of their constituent muscle fibres. Indeed, recent models of whole muscle force production in vivo have large errors, indicating that we still have much to learn about how whole muscles contract. Not only do muscles develop force and change in length along their line of action, but forces and deformations occur across all three dimensions. Recently, the importance of the 3D structure of muscle, and the interactions between contractile and connective tissues have become evident. For example, compressing a muscle supresses the force developed in its line of action (up to 16%), and muscle contractions result in transverse work being done (tensing your glutes whilst you are sitting will raise your body due to transverse muscle work). Additionally, if you maximally activate a muscle then it will never shorten as fast as its constituent fibres are capable of, with this discrepancy (up to 35%) being greatest for large muscles at low levels of activation (typical conditions for our everyday movements). Different labs, including my own, have proposed factors to explain these phenomena, including internal elasticity, force transmission and transverse work, drag of inactive tissues, and inertial properties of the muscle tissue. However, we still lack a general framework that relates all these effects to a muscle's contractile performance. Identifying the mechanisms that contribute to whole-muscle function will increase our basic understanding of muscle biomechanics and enhance our ability to study and maintain animal and human mobility, agility and health. There is finite energy available for muscle contractions and so we propose that mechanisms that use internal energy, or even do external work in a transverse direction, will diminish the work output along its line of action. Not only that, but the efficiency of muscle contractions will be reduced in situations that incur a metabolic cost for using internal energy and doing transverse work. The objective of the proposed research is to use theoretical techniques, validated with cutting-edge experimental data, to relate the distribution of internal energy within a muscle to its 3D shape and force. In particular, we will refine and validate a finite element model of muscle contractions, investigate the relation between transverse compression and bulging on longitudinal force, and investigate the effect of muscle tissue mass on the dynamics and efficiency of muscle contractions. The long-term goal of my research program is to develop our mechanistic understanding of the contractile mechanics of whole muscle.