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

Abstract

Fibre type composition is heterogeneous and distribution varies spatially in many muscles, indicating there may 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 to the lateral compartment. The medial compartment also had a lower optimum cycle frequency for maximum net power generation (11 Hz vs. 15 Hz; P < 0.05) due to slower isometric kinetics, resulting in a lower level of activation and reduced net work generation at higher cycle frequencies, compared to 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 a differential recruitment pattern is likely during locomotion, with the medial compartment being utilised during slow speed locomotion and the lateral compartment during burst activities.

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