FIBER-TYPE COMPOSITION OF RABBIT TIBIALIS ANTERIOR AND EXTENSOR DIGITORUM LONGUS MUSCLES

Rabbit tibialis anterior (TA) and extensor digitorum longus (EDL) musc les are used extensively in studies of stimulation-induced fibre type transformation, but the proportions and sizes of the 2 main fibre type s, and the way in which they are distributed within the muscles, have never been described in any detail. In this study, transverse sections were processed by enzyme histochemical and immunohistochemical techni ques and assessed morphometrically. The data were analysed by multivar iate methods. In both TA and EDL muscles, the proportion of type 1 fib res varied significantly, and to a similar extent, within a cross-sect ion, from lateral to medial and from superficial to deep parts. The fi bre density, an indirect estimate of the mean muscle fibre area, also varied significantly, but not systematically, within a cross-section. For the EDL muscle, the proportion of type 1 fibres was consistently h igher in the distal than in the proximal part of the muscle. The propo rtion of type 1 fibres was also significantly higher in the EDL than i n the TA muscle for each of the 6 rabbits. There was no systematic var iation between muscles from left and right limbs. The type proportions and fibre densities for both TA and EDL muscles differed significantl y between individual rabbits, but not between sexes. The study provide s a database that has hitherto been lacking on normal fibre type compo sition and its variation within and between these experimentally impor tant muscles.