Harry J D, Ward A W, Heglund N C, Morgan D L, McMahon T A
Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138.
Biophys J. 1990 Feb;57(2):201-8. doi: 10.1016/S0006-3495(90)82523-6.
The Huxley 1957 model of cross-bridge cycling accounts for the shortening force-velocity curve of striated muscle with great precision. For forced lengthening, however, the model diverges from experimental results. This paper examines whether it is possible to bring the model into better agreement with experiments, and if so what must be assumed about the mechanical capabilities of cross-bridges. Of particular interest is how introduction of a maximum allowable cross-bridge strain, as has been suggested by some experiments, affects the predictions of the model. Because some differences in the models are apparent only at high stretch velocities, we acquired new force-velocity data to permit a comparison with experiment. Using whole, isolated frog sartorius muscles at 2 degrees C, we stretched active muscle at speeds up to and exceeding 2 Vmax. Force during stretch was always greater than the peak isometric level, even during the fastest stretches, and was approximately independent of velocity for stretches faster than 0.5 Vmax. Although certain modifications to the model brought it into closer correspondence with the experiments, the accompanying requirements on cross-bridge extensibility were unreasonable. We suggest (both in this paper and the one that follows) that sarcomere inhomogeneities, which have been implicated in such phenomena as "tension creep" and "permanent extra tension," may also play an important role in determining the basic force-velocity characteristics of muscle.
赫胥黎1957年的横桥循环模型极其精确地解释了横纹肌的缩短力-速度曲线。然而，对于强制拉长，该模型与实验结果存在偏差。本文探讨了是否有可能使该模型与实验结果更吻合，如果是这样，对于横桥的力学性能必须做出哪些假设。特别值得关注的是，如一些实验所建议的，引入最大允许横桥应变会如何影响模型的预测。由于模型中的一些差异仅在高拉伸速度下才明显，我们获取了新的力-速度数据以便与实验进行比较。在2摄氏度下使用完整的、分离的青蛙缝匠肌，我们以高达并超过2Vmax的速度拉伸活跃的肌肉。拉伸过程中的力始终大于等长收缩峰值水平，即使在最快的拉伸过程中也是如此，并且对于速度超过0.5Vmax的拉伸，力大致与速度无关。尽管对模型进行某些修改使其与实验结果更接近，但随之而来的对横桥伸展性的要求是不合理的。我们（在本文以及后续文章中）提出，肌节不均匀性可能在决定肌肉的基本力-速度特性方面也起着重要作用，这种不均匀性与“张力蠕变”和“永久性额外张力”等现象有关。
