Data on fibre size in thirty-six human muscles. An autopsy study.

Abstract The mean diameters of Type I and Type II fibres were calculated in 50 muscle sites in each of 6 previously normal male autopsy subjects. Type II fibres were found to be larger than Type I fibres in 44 out of the total of 50 sites, but in only 15 sites was the difference statistically significant. Of the 6 muscle sites in which the mean diameter of Type I fibres exceeded that of Type II fibres, only one statistically significant difference was recorded. Significant differences in fibre size did not seem to be related to muscle function in any way. Sampling multiple sites within one muscle revealed that the difference in the relative sizes of the two fibre types was greater in the deep areas than in the surface areas. The fibres of the medial head of gastrocnemius were found to be significantly larger than those of the lateral head; no such difference was found when the clavicular and sternal heads of pectoralis major and the lateral and medial heads of rectus femoris were compared. Both fibre types seemed equally variable in respect of fibre size and, because of the considerable differences between post-mortem subjects the normal ranges of fibre size calculated for individual muscles are correspondingly large. Since the majority of muscles showed no significant difference in the diameters of the two fibre types, it was thought probable that human muscles are adapted for their different physiological roles by means of their widely varying fibre type constitution and that fibre size is relatively unimportant in this respect.