Joumaa Venus, Fitzowich Alex, Herzog Walter
Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Alberta, Canada, T2N 1N4
Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Alberta, Canada, T2N 1N4.
J Exp Biol. 2017 Apr 15;220(Pt 8):1509-1515. doi: 10.1242/jeb.117622. Epub 2017 Feb 23.
The steady-state isometric force after active shortening of a skeletal muscle is lower than the purely isometric force at the corresponding length. This property of skeletal muscle is known as force depression. The purpose of this study was to investigate whether the energy cost of force production at the steady state after active shortening was reduced compared with the energy cost of force production for a purely isometric contraction performed at the corresponding length (same length, same activation). Experiments were performed in skinned fibres isolated from rabbit psoas muscle. Skinned fibres were actively shortened from an average sarcomere length of 3.0 µm to an average sarcomere length of 2.4 µm. Purely isometric reference contractions were performed at an average sarcomere length of 2.4 µm. Simultaneously with the force measurements, the ATP cost was measured during the last 30 s of isometric contractions using an enzyme-coupled assay. Stiffness was calculated during a quick stretch-release cycle of 0.2% fibre length performed once the steady state had been reached after active shortening and during the purely isometric reference contractions. Force and stiffness following active shortening were decreased by 10.0±1.8% and 11.0±2.2%, respectively, compared with the isometric reference contractions. Similarly, ATPase activity per second (not normalized to the force) showed a decrease of 15.6±3.0% in the force-depressed state compared with the purely isometric reference state. However, ATPase activity per second per unit of force was similar for the isometric contractions following active shortening (28.7±2.4 mmol l mN s mm) and the corresponding purely isometric reference contraction (30.9±2.8 mmol l mN s mm). Furthermore, the reduction in absolute ATPase activity per second was significantly correlated with force depression and stiffness depression. These results are in accordance with the idea that force depression following active shortening is primarily caused by a decrease in the proportion of attached cross-bridges. Furthermore, these findings, along with previously reported results showing a decrease in ATP consumption per unit of force after active muscle stretching, suggest that the mechanisms involved in the steady-state force after active muscle shortening and active muscle lengthening are of distinctly different origin.
骨骼肌主动缩短后的稳态等长力低于相应长度下的纯等长力。骨骼肌的这一特性被称为力下降。本研究的目的是探究与在相应长度下进行的纯等长收缩（相同长度、相同激活状态）产生力的能量消耗相比，主动缩短后稳态下产生力的能量消耗是否降低。实验在从兔腰大肌分离出的去膜纤维上进行。去膜纤维从平均肌节长度3.0 µm主动缩短至平均肌节长度2.4 µm。在平均肌节长度2.4 µm下进行纯等长对照收缩。在进行力测量的同时，使用酶联测定法在等长收缩的最后30 s测量ATP消耗。在主动缩短后达到稳态以及纯等长对照收缩过程中，通过0.2%纤维长度的快速拉伸 - 释放循环计算刚度。与等长对照收缩相比，主动缩短后的力和刚度分别降低了10.0±1.8%和11.0±2.2%。同样，与纯等长对照状态相比，力下降状态下每秒的ATP酶活性（未按力进行归一化）降低了15.6±3.0%。然而，主动缩短后的等长收缩每秒每单位力的ATP酶活性（28.7±2.4 mmol·l⁻¹·mN⁻¹·s⁻¹·mm）与相应的纯等长对照收缩（30.9±2.8 mmol·l⁻¹·mN⁻¹·s⁻¹·mm）相似。此外，每秒绝对ATP酶活性降低与力下降和刚度下降显著相关。这些结果符合主动缩短后力下降主要由附着横桥比例降低引起这一观点。此外，这些发现与先前报道的结果一致，即在主动肌肉拉伸后每单位力的ATP消耗减少，表明主动肌肉缩短和主动肌肉拉长后稳态力所涉及的机制来源明显不同。
