Data from: Incompressible fluid plays a mechanical role in the development of passive muscle tension

Over short time scales, muscle fibers maintain a nearly constant volume of intracellular fluid. This fluid is essential to normal biochemical function, but its role in determining the mechanical properties of muscle has been considered in only a few theoretical analyses. Here we investigate the mechanical role of fluid in a fundamental property of muscle, its development of passive tension in response to stretch. We test a model of muscle structure in which incompressible fluid directly influences passive tension by constraining the geometry of intramuscular connective tissues. This interaction is demonstrated using a simple physical model of muscle morphology comprising a fluid-filled bladder wrapped by helical fibers. The behavior of the model is compared to that of isolated bullfrog muscle subjected to an osmotic perturbation of intracellular fluid volume. Increasing muscle volume by 40% resulted in 69% increased passive tension, occurring in a manner consistent with the behavior of the model. These observations support the notion that the interaction of connective tissues with the muscle fibers they surround influences the mechanical behavior of whole muscles, and highlight the role of fluid as a mechanical component of muscle.

在短时间尺度下，肌纤维（muscle fibers）的细胞内液（intracellular fluid）体积几乎维持恒定。该液体对正常生化功能不可或缺，但其在决定肌肉机械特性方面的作用仅在少数理论分析中得到考量。本研究旨在探究液体在肌肉核心特性中的机械作用——即肌肉受牵张（stretch）时被动张力（passive tension）的生成过程。我们验证了一款肌肉结构模型：不可压缩流体（incompressible fluid）通过约束肌内结缔组织（intramuscular connective tissues）的几何形态，直接对被动张力产生影响。我们采用一种模拟肌肉形态的简易物理模型，证实了这一相互作用：该模型由螺旋纤维（helical fibers）包裹的充液囊（fluid-filled bladder）构成。随后将该模型的力学行为，与施加了细胞内液体积渗透压扰动（osmotic perturbation）的离体牛蛙肌组织（isolated bullfrog muscle）的行为进行对比。实验结果显示，将肌肉体积提升40%可使被动张力增加69%，该变化规律与该模型的行为高度相符。上述观测结果支持如下观点：肌内结缔组织与其包裹的肌纤维之间的相互作用会影响整块肌肉的力学行为，同时凸显了液体作为肌肉机械组分的关键作用。