Kissane Roger W P, Egginton Stuart, Askew Graham N
School of Biomedical Sciences, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
Exp Physiol. 2018 Jan 1;103(1):111-124. doi: 10.1113/EP086483. Epub 2017 Nov 26.
What is the central question of this study? Mammalian muscle is typically heterogeneous in fibre-type distribution, with distinct regional variation in composition. The effects this might have on mechanical performance are largely unknown. What is the main finding and its importance? Contractile properties vary regionally within a heterogeneous muscle. The mixed extensor digitorum longus muscle has phenotypically distinct compartments that differ in their isometric twitch kinetics, the optimal cycle frequency for maximal power generation and fatigue resistance. The mechanisms underpinning the decline in performance during fatigue differ between compartments. Regional variation in mechanical performance suggests that regions of the extensor digitorum longus muscle might be differentially recruited during locomotion, depending upon functional demand. Fibre-type composition is heterogeneous, and distribution varies spatially in many muscles, indicating that there might be regional variation in recruitment and mechanical output. The rat extensor digitorum longus muscle is composed of predominantly fast-twitch fibres and exhibits a gradient in phenotype, resulting in oxidative medial (areal composition 24.3% type I/IIa) and glycolytic lateral (92.4% type IIx/IIb) compartments. Here, we investigated the variation in mechanical performance between the medial and lateral compartments during isometric, isotonic and cyclical contractions. Isometric tetanic stress and force-velocity relationships were similar in both compartments, but isometric twitch kinetics were slower in the medial compared with the lateral compartment. The medial compartment also had a lower optimal cycle frequency for maximal net power generation (11 versus 15 Hz; P < 0.05) attributable to slower isometric kinetics, resulting in a lower level of activation and reduced net work generation at higher cycle frequencies, compared with the lateral compartment. The more oxidative, medial compartment had higher fatigue resistance, maintaining net power 26% longer than the lateral compartment. The predominant mechanisms underpinning the decrease in net power varied between the compartments, resulting from an increase in the work to extend the muscle and from a reduction in work during shortening in the medial and lateral compartments, respectively. Regional variation in mechanical performance and resistance to fatigue within a mixed muscle suggests that a differential recruitment pattern is likely during locomotion, with the medial compartment being used during slow-speed locomotion and the lateral compartment during burst activities.
本研究的核心问题是什么？哺乳动物的肌肉在纤维类型分布上通常是异质性的，其组成存在明显的区域差异。而这可能对机械性能产生的影响在很大程度上尚不清楚。主要发现及其重要性是什么？在异质性肌肉中，收缩特性存在区域差异。混合的趾长伸肌具有表型上不同的区域，这些区域在等长收缩抽搐动力学、产生最大功率的最佳周期频率和抗疲劳能力方面存在差异。疲劳期间性能下降的潜在机制在不同区域之间也有所不同。机械性能的区域差异表明，在运动过程中，趾长伸肌的不同区域可能会根据功能需求而被差异性地募集。纤维类型组成是异质性的，且在许多肌肉中其分布存在空间差异，这表明募集和机械输出可能存在区域差异。大鼠趾长伸肌主要由快肌纤维组成，并表现出表型梯度，形成了氧化型的内侧区域（面积组成：24.3%为I型/IIa型）和糖酵解型的外侧区域（92.4%为IIx型/IIb型）。在此，我们研究了在等长、等张和周期性收缩过程中，内侧和外侧区域之间机械性能的差异。两个区域的等长强直张力和力 - 速度关系相似，但内侧区域的等长收缩抽搐动力学比外侧区域慢。内侧区域产生最大净功率的最佳周期频率也较低（11赫兹对15赫兹；P < 0.05），这归因于较慢的等长动力学，与外侧区域相比，导致在较高周期频率下激活水平较低且净功产生减少。氧化程度更高的内侧区域具有更高的抗疲劳能力，其维持净功率的时间比外侧区域长26%。内侧和外侧区域净功率下降的主要机制各不相同，内侧区域是由于肌肉伸展时功的增加，外侧区域则是由于缩短时功的减少。混合肌肉中机械性能和抗疲劳能力的区域差异表明，在运动过程中可能存在差异性的募集模式，内侧区域在低速运动时被使用，外侧区域在爆发性活动时被使用。
