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,稳态等距肌肉力量在主动缩短(力量压迫,FD)后降低,而在主动伸展(力量增强,FE)后增加。还已经证明,主动拉伸(被动FE)后,被动肌肉的力量会增加。几项研究报告说,FD随着幅度的减小而增加,而FE和被动FE随拉伸幅度而增加。在这里,我们调查这些趋势是否继续缩短或拉伸幅度的进一步增加。使用原位猫的比目鱼肌进行实验(FD为n = 8; FE和被动FE为n = 7)。 FD,FE和被动FE是在收缩或拉伸收缩后测量的,收缩或拉伸收缩实际上覆盖了尽可能宽的幅度范围,而不会损坏肌肉。在所研究的整个振幅范围内,FD随振幅的减小而近似线性地增加。这与FD是由应力引起的跨桥抑制作用引起的假设相一致的。 FE随拉伸幅度仅增加到一个点,然后趋于平稳。被动有限元,以及拉伸结束时力的瞬时增加,表明与拉伸幅度的关系在质量上与有限元的关系非常相似,仅在达到相同的临界拉伸幅度之前才增加。我们得出结论,在所有情况下,有限元和无源有限元都不会随拉伸幅度的增加而增加。这一发现对确定有限元和被动有限元的机制具有重要意义,因为提议用来解释它们的任何机制都必须能够对其进行预测。

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