It is well-recognised that steady-state isometric muscle force is decreased following active shortening (force depression, FD) and increased following active stretch (force enhancement, FE). It has also been demonstrated that passive muscle force is increased following active stretch (passive FE). Several studies have reported that FD increases with shortening amplitude and that FE and passive FE increase with stretch amplitude. Here, we investigate whether these trends continue with further increases in shortening or stretch amplitude. Experiments were performed using in situ cat soleus muscles (n=8 for FD; n=7 for FE and passive FE). FD, FE and passive FE were measured after shortening or stretch contractions that covered as wide a range of amplitudes as practically possible without damaging the muscles. FD increased approximately linearly with shortening amplitude, over the full range of amplitudes investigated. This is consistent with the hypothesis that FD arises from a stress-induced inhibition of crossbridges. FE increased with stretch amplitude only up to a point, and then levelled off. Passive FE, and the transient increase in force at the end of stretch, showed relationships to stretch amplitude that were qualitatively very similar to the relationship for FE, increasing only until the same critical stretch amplitude had been reached. We conclude that FE and passive FE do not increase with stretch amplitude under all circumstances. This finding has important consequences for determining the mechanisms underlying FE and passive FE because any mechanism that is proposed to explain them must be able to predict it.

译文

众所周知,主动缩短 (力降低,FD) 后,稳态等距肌肉力降低,主动拉伸 (力增强,FE) 后,稳态等距肌肉力增加。还已证明,主动拉伸 (被动FE) 后,被动肌肉力量会增加。一些研究报告说,FD随缩短幅度而增加,FE和被动FE随拉伸幅度而增加。在这里,我们研究这些趋势是否随着缩短或拉伸幅度的进一步增加而继续。使用原位猫比目鱼肌进行实验 (FD为n = 8; FE和被动FE为n = 7)。在缩短或拉伸收缩后测量FD,FE和被动FE,这些收缩实际上覆盖了尽可能宽的振幅范围,而不会损坏肌肉。在所研究的整个振幅范围内,FD随振幅的缩短而近似线性增加。这与FD由应力诱导的交叉桥抑制引起的假设是一致的。FE仅随拉伸幅度增加到一个点,然后趋于平稳。被动FE和拉伸结束时的瞬时力增加显示出与拉伸幅度的关系,在质量上与FE的关系非常相似,仅在达到相同的临界拉伸幅度之前才增加。我们得出的结论是,在所有情况下,FE和被动FE都不会随拉伸幅度而增加。这一发现对于确定FE和被动FE的潜在机制具有重要的影响,因为提出的任何解释它们的机制都必须能够预测它。

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