Distribution, total number and size of different types of fibres in male and female skeletal muscles : an enzyme histochemical study of whole muscle cross-sections

Abstract: In order to investigate the total number, relative proportions, size variations and distribution patterns of fibres of different types in human skeletal muscles, a technique was developed which allows whole muscle cross-sections to be collected and stained enzyme histochemically. Necropsy material was obtained from m. tibialis anterior and m. vastus lateralis of previously healthy young adults who had suffered sudden accidental deaths. The muscle specimens were extirpated within 72 hours post morten, frozen in liquid nitrogen, embedded in carboxymethylcellu- lose, sectioned and enzyme histochemically stained for myofibrillar ATPase in order to permit light microscopic identification of type 1 and type 2 muscle fibres.The present results show that there is a large variation in both the total number of fibres and the size of the whole muscle cross-sectional area between different individuals and, also, between different levels of the same muscle. The distribution of fibres of different types over the muscle cross-section was heterogeneous. Usually the relative proportion of type 2 fibres showed a peak along a medio- lateral line passing through the centre of the muscle cross-section. When comparing the left and right m. tibialis anterior of the same individual, one of the- muscles (usually the left one) was found to contain a larger number of fibres than the contralateral one. However, the pattern of distribution of fibres of different types was similar in the two muscles. The fibre sizes were also found to vary between different regions of the muscle cross-section. Both type 1 and type 2 fibres were significantly larger in the deep muscle region compared to the central or superficial sites. The mean fibre size as well as the total number of fibres correlated strongly with the whole muscle cross-sectional area. The female m. tibialis anterior contained fewer and smaller fibres than the corresponding male muscle, although the whole muscle cross-sectional area was of similar magnitude. The distribution of fibre types over the muscle cross-section differed somewhat between females and males. The variation in fibre sizes between different muscle sites was less pronounced in the females, but as in male muscles the type 2 fibres were always larger than the type 1 fibres.In conclusion, systematic variations in fibre type distribution and fibre size occurs over the muscle cross-section. The total number of fibres in the cross- section seems to vary with individual, sex, distance from muscle origin and left- right leg. The combination of fewer and smaller fibres in females compared to males leads to about 40 % smaller total muscle fibre cross-sectional area in female m. tibialis anterior. The results are discussed in relation to previous studies using muscle biopsy techniques. The possible contibution of variations in number, sizes or distribution patterns of muscle fibres to adaptation of muscle to varying functional demands is also discussed.