Human Performance Laboratory, University of Calgary, Alberta, Canada.
J Appl Physiol (1985). 2010 Feb;108(2):356-62. doi: 10.1152/japplphysiol.01108.2009. Epub 2009 Dec 10.
Force depression after active shortening has been observed in different muscle preparations. It has been assumed that force depression is caused by the development of sarcomere length nonuniformities after shortening. However, this hypothesis has never been investigated in a preparation where individual sarcomere lengths could be directly measured. Here, we investigated force depression in single myofibrils (n = 11) and tracked simultaneously the changes in individual sarcomere lengths (n = 60) before, during, and after shortening and after a purely isometric contraction performed at the final length. Shortening produced force depression in all myofibrils (mean +/- SE; 30.9 +/- 3.9%). During shortening, all sarcomeres shortened, but not by the same amount. Sarcomere lengths were nonuniform, with the same mean SD before (0.11 +/- 0.06 microm) and after shortening (0.11 +/- 0.06 microm) and after a purely isometric contraction at the final length (0.10 +/- 0.05 microm). Furthermore, greater shortening magnitudes were found for sarcomeres that were long in the initial isometric configuration. Nonuniformities of half-sarcomere lengths were also the same before (SD = 0.13 microm) and after (SD = 0.14 microm) shortening. We conclude from these results that the development of sarcomere (or half-sarcomere) length nonuniformities does not play a major role in force depression. Rather, force depression seems an intrinsic property of individual (half-) sarcomeres and muscle contraction.
在不同的肌肉标本中观察到主动缩短后力的下降。人们假设力的下降是由于缩短后肌节长度的不均匀性发展引起的。然而，这个假设从未在一个可以直接测量单个肌节长度的标本中得到过验证。在这里，我们研究了单个肌原纤维（n = 11）中的力下降，并同时跟踪了缩短前后以及在最终长度下进行纯等长收缩时的个体肌节长度（n = 60）的变化。所有肌原纤维都出现力下降（平均值 +/- SE；30.9 +/- 3.9%）。在缩短过程中，所有肌节都缩短了，但缩短的幅度不同。肌节长度不均匀，在缩短前（0.11 +/- 0.06 微米）和缩短后（0.11 +/- 0.06 微米）以及在最终长度下进行纯等长收缩后（0.10 +/- 0.05 微米）的平均值 +/- SD 相同。此外，在初始等长构象中较长的肌节会发生更大的缩短。半肌节长度的不均匀性在缩短前（SD = 0.13 微米）和缩短后（SD = 0.14 微米）也是相同的。从这些结果我们得出结论，肌节（或半肌节）长度不均匀性的发展并没有在力下降中发挥主要作用。相反，力下降似乎是单个（半）肌节和肌肉收缩的固有特性。
