Variability in muscle fibre areas in whole human quadriceps muscle. How much and why?

To determine the variability in fibre areas in the human vastus lateralis muscle, cross-sections (15 microns) of whole autopsied muscles from eight young men have been prepared, and the cross-sectional area (CSA) of 375 type 1 and 375 type 2 fibres has been measured in five different regions throughout each muscle. The CSA of both fibre types varied significantly within all muscle cross-sections. Fibres in the deep parts of the muscle were larger than superficially. There was a significant correlation between the CSA of the two fibre types within each region: if a fibre of a given type was small, or large, the other fibre type was also small, or large. The CSA of type 2 fibres was larger than the CSA of type 1 fibres in 26 of the 40 regions: regions with type 1 fibres larger than type 2 fibres were mostly (71%) found deep in the muscle. The standard deviation of the CSA of type 1 fibres was significantly larger than for type 2 fibres in 35 of the 40 regions. In conclusion, the CSA of the different fibre types in the vastus lateralis of young men varies non-randomly. The pattern of variation, both throughout the muscle and in small sample regions, supports the general opinion that the functional demands placed on the fibre population are an important factor in the development of the fibre properties.