Department of Biomedical Engineering, Northwestern University, Evanston, Illinois, United States.
Shirley Ryan Ability Lab, Chicago, Illinois, United States.
J Appl Physiol (1985). 2023 Apr 1;134(4):941-950. doi: 10.1152/japplphysiol.00125.2022. Epub 2023 Mar 2.
Ultrasound shear wave elastography can be used to characterize mechanical properties of unstressed tissue by measuring shear wave velocity (SWV), which increases with increasing tissue stiffness. Measurements of SWV have often been assumed to be directly related to the stiffness of muscle. Some have also used measures of SWV to estimate stress, since muscle stiffness and stress covary during active contractions, but few have considered the direct influence of muscle stress on SWV. Rather, it is often assumed that stress alters the material properties of muscle, and in turn, shear wave propagation. The objective of this study was to determine how well the theoretical dependency of SWV on stress can account for measured changes of SWV in passive and active muscles. Data were collected from six isoflurane-anesthetized cats; three soleus muscles and three medial gastrocnemius muscles. Muscle stress and stiffness were measured directly along with SWV. Measurements were made across a range of passively and actively generated stresses, obtained by varying muscle length and activation, which was controlled by stimulating the sciatic nerve. Our results show that SWV depends primarily on the stress in a passively stretched muscle. In contrast, the SWV in active muscle is higher than would be predicted by considering only stress, presumably due to activation-dependent changes in muscle stiffness. Our results demonstrate that while SWV is sensitive to changes in muscle stress and activation, there is not a unique relationship between SWV and either of these quantities when considered in isolation. Ultrasound shear wave elastography may be an inexpensive way to measure muscle stress in passive muscle. Here, using a cat model we directly measured shear wave velocity (SWV), muscle stress, and muscle stiffness. Our results show that SWV depends primarily on the stress in a passively stretched muscle. In contrast, the SWV in active muscle is higher than would be predicted by considering only stress, presumably due to activation-dependent changes in muscle stiffness.
超声剪切波弹性成像可用于通过测量剪切波速度（SWV）来描述无应力组织的机械特性，SWV 随组织硬度的增加而增加。SWV 的测量通常被认为与肌肉的硬度直接相关。一些人还使用 SWV 来估计肌肉的张力，因为在主动收缩期间肌肉硬度和张力是共变的，但很少有人考虑肌肉张力对 SWV 的直接影响。相反，通常认为张力改变了肌肉的材料特性，进而改变了剪切波的传播。本研究的目的是确定 SWV 对张力的理论依赖性在多大程度上可以解释被动和主动肌肉中 SWV 的测量变化。数据来自六只异氟烷麻醉的猫；三只比目鱼肌和三只内侧腓肠肌。同时测量 SWV 以及肌肉的张力和硬度。通过改变肌肉长度和激活来产生被动和主动产生的各种张力来进行测量，通过刺激坐骨神经来控制激活。我们的结果表明，SWV 主要取决于被动拉伸肌肉中的张力。相比之下，主动肌肉中的 SWV 高于仅考虑张力时的预测值，这可能是由于肌肉硬度的激活依赖性变化所致。我们的结果表明，尽管 SWV 对肌肉张力和激活的变化敏感，但当单独考虑时，SWV 与这两个量之间没有独特的关系。超声剪切波弹性成像可能是一种测量被动肌肉中肌肉张力的廉价方法。在这里，我们使用猫模型直接测量了剪切波速度（SWV）、肌肉张力和肌肉硬度。我们的结果表明，SWV 主要取决于被动拉伸肌肉中的张力。相比之下，主动肌肉中的 SWV 高于仅考虑张力时的预测值，这可能是由于肌肉硬度的激活依赖性变化所致。
