Department of Biomedical Engineering, Northwestern University, Evanston, Illinois.
Shirley Ryan AbilityLab, Chicago, Illinois.
J Appl Physiol (1985). 2020 Jan 1;128(1):8-16. doi: 10.1152/japplphysiol.00112.2019. Epub 2019 Sep 26.
Clinical assessments for many musculoskeletal disorders involve evaluation of muscle stiffness, although it is not yet possible to obtain quantitative estimates from individual muscles. Ultrasound elastography can be used to estimate the material properties of unstressed, homogeneous, and isotropic materials by tracking the speed of shear wave propagation; these waves propagate faster in stiffer materials. Although elastography has been applied to skeletal muscle, there is little evidence that shear wave velocity (SWV) can directly estimate muscle stiffness since this tissue violates many of the assumptions required for there to be a direct relationship between SWV and stiffness. The objective of this study was to evaluate the relationship between SWV and direct measurements of muscle force and stiffness in contracting muscle. Data were collected from six isoflurane-anesthetized cats. We measured the short-range stiffness in the soleus via direct mechanical testing in situ and SWV via ultrasound imaging. Measurements were taken during supramaximal activation at optimum muscle length, with muscle temperature varying between 26°C and 38°C. An increase in temperature causes a decrease in muscle stiffness at a given force, thus decoupling the tension-stiffness relationship normally present in muscle. We found that increasing muscle temperature decreased active stiffness from 4.0 ± 0.3 MPa to 3.3 ± 0.3 MPa and SWV from 16.9 ± 1.5 m/s to 15.9 ± 1.6 m/s while force remained unchanged (mean ± SD). These results demonstrate that SWV is sensitive to changes in muscle stiffness during active contractions. Future work is needed to determine how this relationship is influenced by changes in muscle structure and tension. Shear wave ultrasound elastography is a noninvasive tool for characterizing the material properties of muscle. This study is the first to compare direct measurements of stiffness with ultrasound measurements of shear wave velocity (SWV) in a contracting muscle. We found that SWV is sensitive to changes in muscle stiffness, even when controlling for muscle tension, another factor that influences SWV. These results are an important step toward developing noninvasive tools for characterizing muscle structure and function.
临床评估许多肌肉骨骼疾病都涉及肌肉僵硬的评估，尽管目前还无法从单个肌肉获得定量估计。超声弹性成像可用于通过跟踪剪切波传播速度来估计无应力、均匀和各向同性材料的材料特性；这些波在较硬的材料中传播得更快。尽管弹性成像已应用于骨骼肌，但几乎没有证据表明剪切波速度（SWV）可以直接估计肌肉僵硬，因为这种组织违反了 SWV 与僵硬之间存在直接关系所需的许多假设。本研究旨在评估 SWV 与收缩肌肉中肌肉力和刚度的直接测量之间的关系。数据来自 6 只异氟醚麻醉的猫。我们通过原位直接机械测试测量比目鱼肌的短程刚度，并通过超声成像测量 SWV。测量是在最佳肌肉长度下进行的最大刺激激活期间进行的，肌肉温度在 26°C 和 38°C 之间变化。随着温度的升高，给定力下肌肉刚度降低，从而解耦了肌肉中通常存在的张力-刚度关系。我们发现，随着肌肉温度从 4.0±0.3 MPa 增加到 3.3±0.3 MPa，SWV 从 16.9±1.5 m/s 增加到 15.9±1.6 m/s，而力保持不变（平均值±标准差）。这些结果表明，SWV 对主动收缩过程中肌肉刚度的变化敏感。需要进一步的研究来确定这种关系如何受到肌肉结构和张力变化的影响。剪切波超声弹性成像是非侵入性工具，用于描述肌肉的材料特性。本研究首次在收缩肌肉中比较了直接测量的刚度与超声测量的剪切波速度（SWV）。我们发现，SWV 对肌肉刚度的变化敏感，即使在控制肌肉张力的情况下也是如此，肌肉张力是另一个影响 SWV 的因素。这些结果是朝着开发用于描述肌肉结构和功能的非侵入性工具迈出的重要一步。
