School of Biomedical Sciences, The University of Queensland, Saint Lucia, Queensland, Australia.
School of Engineering and Science, University of the Sunshine Coast, Sippy Downs, Queensland, Australia.
J Appl Physiol (1985). 2023 Jun 1;134(6):1520-1529. doi: 10.1152/japplphysiol.00080.2023. Epub 2023 May 11.
Skeletal muscles bulge when they contract. These three-dimensional shape changes, coupled with fiber rotation, influence a muscle's mechanical performance by uncoupling fiber velocity from muscle belly velocity (i.e., gearing). Muscle shape change and gearing are likely mediated by the interaction between internal muscle properties and contractile forces. Muscles with greater stiffness and intermuscular fat, due to aging or disuse, may limit a muscle's ability to bulge in width, subsequently causing higher gearing. The aim of this study was to determine the influence of internal muscle properties on shape change, fiber rotation, and gearing in the medial (MG) and lateral gastrocnemii (LG) during isometric plantar flexion contractions. Multimodal imaging techniques were used to measure muscle shear modulus, intramuscular fat, and fat-corrected physiological cross-sectional area (PCSA) at rest, as well as synchronous muscle architecture changes during submaximal and maximal contractions in the MG and LG of 20 young (24 ± 3 yr) and 13 older (70 ± 4 yr) participants. Fat-corrected PCSA was positively associated with fiber rotation, gearing, and changes in thickness during submaximal contractions, but it was negatively associated with changes in thickness at maximal contractions. Muscle stiffness and intramuscular fat were related to muscle bulging and reduced fiber rotation, respectively, but only at high forces. Furthermore, the MG and LG had varied internal muscle properties, which may relate to the differing shape changes, fiber rotations, and gearing behaviors observed at each contraction level. These results indicate that internal muscle properties may play an important role in mediating in vivo muscle shape change and gearing, especially during high-force contractions. Here, we measured internal muscle properties in vivo to determine their influence on the varying shape change and gearing behaviors in the synergistic gastrocnemii muscles. These relationships have previously only been hypothesized or examined within isolated muscles during supramaximal contractions. Our results contribute to a more comprehensive understanding of the factors that influence a muscle's mechanical response to force with implications for preventing or treating muscle deficits associated with aging, disease, and disuse.
骨骼肌在收缩时会鼓起。这些三维形状变化，加上纤维旋转，通过将纤维速度与肌腹速度解耦（即齿轮传动）来影响肌肉的机械性能。肌肉形状变化和齿轮传动可能是由内部肌肉特性和收缩力之间的相互作用介导的。由于衰老或废用，肌肉僵硬和肌肉间脂肪增加，可能会限制肌肉在宽度上的鼓起能力，从而导致更高的齿轮传动。本研究的目的是确定内部肌肉特性对等长跖屈收缩时内侧（MG）和外侧比目鱼肌（LG）的形状变化、纤维旋转和齿轮传动的影响。多模态成像技术用于测量休息时的肌肉剪切模量、肌内脂肪和脂肪校正的生理横截面积（PCSA），以及 20 名年轻（24 ± 3 岁）和 13 名老年（70 ± 4 岁）参与者的 MG 和 LG 在亚最大和最大收缩期间同步的肌肉结构变化。脂肪校正后的 PCSA 与纤维旋转、齿轮传动和亚最大收缩期间厚度的变化呈正相关，但与最大收缩时厚度的变化呈负相关。肌肉僵硬和肌内脂肪分别与肌肉鼓起和纤维旋转减少有关，但仅在高力下。此外，MG 和 LG 具有不同的内部肌肉特性，这可能与在每个收缩水平观察到的不同形状变化、纤维旋转和齿轮传动行为有关。这些结果表明，内部肌肉特性可能在介导体内肌肉形状变化和齿轮传动中起重要作用，尤其是在高力收缩时。在这里，我们在体内测量了内部肌肉特性，以确定它们对协同比目鱼肌中不同形状变化和齿轮传动行为的影响。这些关系以前仅在超最大收缩时在孤立的肌肉中被假设或检查过。我们的研究结果有助于更全面地了解影响肌肉对力的机械反应的因素，对预防或治疗与衰老、疾病和废用相关的肌肉缺陷具有重要意义。
