Summary.In order to assess the variability in repeated determination of human muscle fibre type distribution, fibre area and enzyme activity measurements, two biopsies were taken within 10 days in the same vastus lateralis for 12 females and 13 males, and in the right and in the left muscles for 25 other subjects (13 females and 12 males). Within muscle, intraclass reliability coefficients were 0·88, 0·82 and 0·56 for type I, IIa and IIb per cent fibres, respectively, and ranged from 0·74 to 0·82 for fibre areas and from 0·71 to 0·90 for enzyme markers of different metabolic pathways. Correlations between right and left muscle measurements were also high for fibre areas (from 0·85 to 0·91) and enzyme activities (from 0·71 to 0·87), except for phosphofructokinase (r=0·63). In contrast, the right and left thigh muscle correlation reached 0·67, 0·40 and 0·64 for type I, IIa and IIb fibre distribution, respectively. Thus, the variation in muscle sampling and technical procedures reached about 15 of the total variation (i.e. total differences between subjects) for the proportion of fibre type I and IIa and about 20–25 for fibre areas and enzyme activities. On the other hand, the technical error for the proportion of fibre type I and IIa is about 6–7. This implies that differences brought about by any experimental treatment on these skeletal muscle characteristics in human studies have to be of a relatively large magnitude before being detectable. On the other hand, fibre areas and enzyme activities measured in single needle biopsy sample, from one of the vastus lateralis muscles, are quite representative of the other vastus lateralis. Similarity in fibre type proportion between right and left vastus lateralis cannot be postulated, however, without investig
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