Regional variation in the mechanical properties and fibre‐type composition of the rat extensor digitorum longus muscle

• Published in: Experimental physiology 2018-01, Vol.103 (1), p.111-124
• Main Authors: Kissane, Roger W. P., Egginton, Stuart, Askew, Graham N.
• Format: Article
• Language: English
• Subjects: Animals; Biomechanical Phenomena - physiology; fatigue resistance; fibre type composition; Isometric Contraction - physiology; Isotonic Contraction - physiology; Knee; Male; Muscle Contraction - physiology; Muscle Fatigue - physiology; Muscle Fibers, Fast-Twitch - physiology; Muscle Fibers, Skeletal - physiology; Muscle, Skeletal - physiology; Rats; Rats, Wistar; Regional differences; Rodents; work loop
• ISSN: 0958-0670; 1469-445X
• DOI: 10.1113/EP086483

New Findings 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