The ultrastruct of normal human muscle in relation to fibre type.

A quantitative analysis using a stereological point-counting method was made of the volume fractions of the components of healthy human muscle in an attempt to distinguish between Type I and Type II fibres ultrastructurally. The Z-line widths of each fibre were also measured using the 38.5 nm axial periodicity in the I-band as a reference measurement. There was a wide range of values for the parameters measured. The volume of mitochondria was 5.14 plus or minus 1.85% (mean plus or minus standard deviation, n=24 fibres), the volume of the membrane systems was 1.59 plus or minus 0.31%, the volume of the sarcoplasm was 9.37 plus or minus 2.50% and the volume of lipid was 0.63 plus or minus 0.43%. The width of the Z-lines was 89.0 plus or minus 18.65 nm. The results show that no single feature can be used to distinguish fibre types reliably. If 2 factors are measured and certain assumptions are made, it is possible to classify about 50% of the fibres examined. These conclusions are discussed in relation to other workers' findings on the effects of age, sex and athleticism on the ultrastructure of muscle.