Distribution of different fibre types in human skeletal muscles. I. Method for the preparation and analysis of cross-sections of whole tibialis anterior.

The aim of this study was to examine whether small biopsy specimens are representative of the whole human skeletal muscle or whether the different fibre types are unevenly distributed at different depths of the muscle. Ten micrometre thick cross-sections of whole human tibialis anterior were prepared using LKB PMV Cryo-Microtomes with a stroke length of 160 to 480 mm and the sections were stained for myofibrillar ATPase according to a modified procedure. The total and relative number of different fibres (Types 1 and 2) was determined in every 9th mm2 of the section. The data obtained were analysed by means of a computer program, which allowed assessment of bivariate data in the form of contour plots. The total number of fibres varied greatly between individuals (from 96 000 to 162 000; five individuals). The relative number of different fibres varied systematically in all individuals as a function of depth in the muscle. There was a gradual, often dramatic, relative increase in Type 2 fibre occurrence from the surface of the muscle (about 10--25%) towards the deeper regions (30--50%), the maximum being approximately along a line slightly posterior to the middle of the muscle. Additionally, superficial peaks were seen in places. In conclusion, the fibre type distribution in the tibialis anterior is not random. These results point to the importance of defining biopsy depth.