Contractile Properties of the Rat Extensor Digitorum Longus Muscle in Relation to Regional Variation in Fibre Type Composition

Abstract

Muscle contractile characteristics are commonly investigated in relation to global phenotype. The extensor digitorum longus (EDL) muscle has frequently been used in physiological studies as an example of a typical fast muscle. However, the fibre type composition is not homogeneous. In the rat, the EDL contains a mixture of oxidative and glycolytic fibres, heterogeneously distributed across the muscle cross section to produce a gradient of aerobic capacity. The aim of this study was to characterize the contractile characteristics of this mixed muscle in relation to the regional variation in fibre type composition. It was hypothesized that isometric twitch characteristics and optimal frequency for power generation would not differ regionally, while the fatigability will correlate with aerobic capacity.The EDL muscle from male Wistar rats comprises four segments which insert on to phalanges 2–5; we used the medial and lateral segments, which insert on to phalanges 2 and 5, respectively. The contractile characteristics of each segment were determined under isometric conditions, in order to quantify twitch kinetics and tetanic stress. Power output of the muscle was measured at a range of cycle frequencies during cyclical contractions, using the work loop technique. The ability to sustain power was quantified using a series of repeated cyclical contractions. A range of monoclonal antibodies was used to determine fibre type composition.The two muscle segments had significantly different fibre type composition; the medial segment containing 41% Type IIb, 49% Type IIa and 10% Type I while the lateral segment contained 95% Type IIb and 5% Type IIa. The medial segment had slower twitch kinetics than the lateral segment, consistent with the presence of Type 1 fibres and lower proportion of Type 2 fibres. Isometric stress was lower in the medial segment producing 155.3 kN m−2 compared to 194.6 kN m−2 in the lateral segment; this may be related to the higher proportion of non‐contractile elements such as mitochondria in the medial segment, i.e. a lower proportion of myofibrils, compared with the lateral segment. Peak net power output and the frequency at which this was attained were similar between both muscle segments.Variation in the physiological properties of the lateral and medial segments of the rat EDL are consistent with the decrease in oxidative capacity and a decrease in the proportion of fast‐twitch fibres from medial to lateral sides. This suggests that there may be regional variation in mechanical function, with a shift from the medial segment providing work for ankle flexion at relatively low speeds, to the lateral segment increasingly fulfilling the role at higher speeds.