Department of Motion and Exercise Science, University of Stuttgart, Allmandring 28, 70569, Stuttgart, Germany.
Institute of Sport Science, Department of Biomechanics, University of Rostock, Rostock, Germany.
Pflugers Arch. 2024 Oct;476(10):1517-1527. doi: 10.1007/s00424-024-02991-4. Epub 2024 Jul 24.
After an initial increase, isovelocity elongation of a muscle fiber can lead to diminishing (referred to as Give in the literature) and subsequently increasing force. How the stretch velocity affects this behavior in slow-twitch fibers remains largely unexplored. Here, we stretched fully activated individual rat soleus muscle fibers from 0.85 to 1.3 optimal fiber length at stretch velocities of 0.01, 0.1, and 1 maximum shortening velocity, v, and compared the results with those of rat EDL fast-twitch fibers obtained in similar experimental conditions. In soleus muscle fibers, Give was 7%, 18%, and 44% of maximum isometric force for 0.01, 0.1, and 1 v, respectively. As in EDL fibers, the force increased nearly linearly in the second half of the stretch, although the number of crossbridges decreased, and its slope increased with stretch velocity. Our findings are consistent with the concept of a forceful detachment and subsequent crossbridge reattachment in the stretch's first phase and a strong viscoelastic titin contribution to fiber force in the second phase of the stretch. Interestingly, we found interaction effects of stretch velocity and fiber type on force parameters in both stretch phases, hinting at fiber type-specific differences in crossbridge and titin contributions to eccentric force. Whether fiber type-specific combined XB and non-XB models can explain these effects or if they hint at some not fully understood properties of muscle contraction remains to be shown. These results may stimulate new optimization perspectives in sports training and provide a better understanding of structure-function relations of muscle proteins.
在最初的增加之后，肌肉纤维的等速伸长可导致逐渐减少（文献中称为“屈服”），随后力增加。伸展速度如何影响慢肌纤维的这种行为在很大程度上仍未得到探索。在这里，我们以 0.01、0.1 和 1 最大缩短速度 v 的速度将完全激活的单个大鼠比目鱼肌纤维从 0.85 拉伸至 1.3 最佳纤维长度，并将结果与在类似实验条件下获得的大鼠 EDL 快肌纤维的结果进行了比较。在比目鱼肌纤维中，屈服为 0.01、0.1 和 1 v 时最大等长力的 7%、18%和 44%。与 EDL 纤维一样，在伸展的后半部分力几乎呈线性增加，尽管横桥数量减少，但其斜率随伸展速度增加。我们的发现与在伸展的第一阶段强力分离和随后的横桥再附着的概念以及在伸展的第二阶段强烈的粘弹性肌联蛋白对纤维力的贡献一致。有趣的是，我们发现伸展速度和纤维类型对两个伸展阶段的力参数都有相互作用效应，这暗示了横桥和肌联蛋白对离心力的贡献在纤维类型特异性上存在差异。是否纤维类型特异性的结合 XB 和非-XB 模型可以解释这些效应，或者它们是否暗示肌肉收缩的某些尚未完全理解的特性，仍有待证明。这些结果可能会激发运动训练的新优化视角，并更好地理解肌肉蛋白的结构-功能关系。
