Proceedings of The Physiological Society

Future Physiology (Leeds, UK) (2017) Proc Physiol Soc 39, PC32

Poster Communications

Regional variation in the mechanical properties of the rat extensor digitorum longus muscle

R. W. Kissane1, S. Egginton1, G. Askew1

1. School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom.

  • Heterogeneity of the EDL.The heterogeneous oxygen supply capacity of the EDL is demonstrated by the rhodamine labelled capillaries (histology top row, a-d). The increased oxidative supply medially across the EDL is matched by an increasing oxidative demand, in the form of increasing oxidative fibre phenotype (histology bottom row, a-d). Type I; Red, Type IIa, Green; Type IIx/b, Unstained. Scale Bar = 200μm.

Many muscles are heterogeneous in fibre type composition and the distribution often varies spatially, indicating there may be regional variation in recruitment and mechanical output. The rat extensor digitorum longus muscle is composed of predominantly fast-twitch fibres but exhibits a gradient in phenotype, which results in an oxidative medial compartment (areal composition, 24.3% Type I or IIa) and a glycolytic lateral compartment (92.4% Type IIx/IIb). Here, we utilised this regional variation in composition to investigate differences in mechanical performance, using a more physiologically appropriate (work loop) technique than usually applied to estimate changes in functional capacity. Isometric tetanic stress (280 ± 50 vs. 300 ± 54 kN.m-2, P = 0.345) and maximum shortening velocity (expressed relative to muscle fibre length, 15.3 ± 1.5 vs. 14.8 ± 2.5 L0-F S-1, P = 0.736) were similar in the medial and lateral compartments, but isometric twitch kinetics (rise time and half relaxation time) were slower in the medial compared to the lateral compartment. The medial compartment also had a lower optimum frequency for maximum power generation (11Hz vs. 15Hz; P < 0.05) due to slower isometric kinetics, resulting in a lower level of activation and reduced work generation with increasing cycle frequencies, compared to the lateral compartment. The more oxidative, medial compartment had a higher resistance to fatigue, and maintained power significantly longer (time to half-initial value: medial 2.95 ± 0.13 s; lateral 2.34 ± 0.12 s; P = 0.004). Mechanisms underpinning the decrease in power generation varied among compartments, with the medial compartment showing a reduced capacity to relax, while the lateral compartment presented a decreased capacity to generate force. Our novel observations of regional variation in mechanical performance and response to fatigue within a mixed muscle suggests a differential recruitment pattern during locomotion, with the medial compartment utilised during slow speed locomotion and the lateral compartment during burst activities.

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