Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada, T2N 1N4.
Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada, T2N 1N4
J Exp Biol. 2019 Nov 27;222(Pt 22):jeb209924. doi: 10.1242/jeb.209924.
Sarcomere length (SL) instability and SL non-uniformity have been used to explain fundamental properties of skeletal muscles, such as creep, force depression following active muscle shortening and residual force enhancement following active stretching of muscles. Regarding residual force enhancement, it has been argued that active muscle stretching causes SL instability, thereby increasing SL non-uniformity. However, we recently showed that SL non-uniformity is not increased by active muscle stretching, but it remains unclear if SL stability is affected by active stretching. Here, we used single myofibrils of rabbit psoas muscle and measured SL non-uniformity and SL instability during isometric contractions and for isometric contractions following active stretching at average SLs corresponding to the descending limb of the force-length relationship. We defined isometric contractions as contractions during which mean SL remained constant. SL instability was quantified by the rate of change of individual SLs over the course of steady-state isometric force and SL non-uniformity was defined as deviations of SLs from the mean SL at an instant of time. We found that whereas the mean SL remained constant during isometric contraction, by definition, individual SLs did not. SLs were more stable in the force-enhanced, isometric state following active stretching compared with the isometric reference state. We also found that SL instability was not correlated with the rate of change of SL non-uniformity. Also, SL non-uniformity was not different in the isometric and the post-stretch isometric contractions. We conclude that since SL is more stable but similarly non-uniform in the force-enhanced compared with the corresponding isometric reference contraction, it appears unlikely that either SL instability or SL non-uniformity contribute to the residual force enhancement property of skeletal muscle.
肌节长度（SL）不稳定和 SL 不均匀性已被用于解释骨骼肌的基本特性，例如蠕变、主动肌肉缩短后的力下降以及肌肉主动拉伸后的残余力增强。关于残余力增强，有人认为主动肌肉拉伸会导致 SL 不稳定，从而增加 SL 不均匀性。然而，我们最近表明，主动肌肉拉伸不会增加 SL 不均匀性，但 SL 稳定性是否受到主动拉伸的影响尚不清楚。在这里，我们使用兔腰大肌的单个肌原纤维，并在等长收缩期间以及在与力-长度关系下降支相对应的平均 SL 下进行主动肌肉拉伸后的等长收缩期间测量 SL 不均匀性和 SL 不稳定性。我们将等长收缩定义为平均 SL 保持不变的收缩。SL 不稳定性通过单个 SL 在稳态等力过程中的变化率来量化，SL 不均匀性定义为 SL 在瞬时与平均 SL 的偏差。我们发现，虽然在等长收缩期间，根据定义，平均 SL 保持不变，但单个 SL 并不稳定。与等长参考状态相比，在主动拉伸后的力增强等长状态下，SL 更稳定。我们还发现，SL 不稳定性与 SL 不均匀性的变化率无关。此外，在等长和拉伸后的等长收缩中，SL 不均匀性没有差异。我们得出的结论是，由于 SL 在力增强状态下比相应的等长参考收缩更稳定但同样不均匀，因此似乎 SL 不稳定性或 SL 不均匀性都不会导致骨骼肌的残余力增强特性。
