Relationships between muscle morphology and insulin sensitivity are improved after adjustment for intra-individual variability in 70-year-old men

The purpose of this investigation was to examine to what extent variability in the muscle morphology and insulin sensitivity influence the correlation between them. Reproducibility of muscle characteristics was estimated in d uplicate biopsies from the same thigh of 23 subjects from a cohort of 70-ye ar-old men. The coefficient of variation (CV) for different characteristics of muscle morphology was between 11 and 42% in duplicate biopsies. Coeffic ient of variation for markers of insulin sensitivity ranged between 12 and 39%. The variability reflected by intra-class correlation ranged from 0.23 to 0.60 for muscle morphology and from 0.68 to 0.96 for estimates of insuli n sensitivity. The correlation analysis between muscle morphology and insul in resistance was performed in a sample of 515 men from the cohort, correla tion coefficients were calculated with (r(true)) and without (r) adjustment for intra-individual variation. Insulin sensitivity showed a positive rela tionship with percentage of type I fibres (r(true) = 0.33, r = 0.21; P < 0. 0001) and capillary density (r(true) = 0.43, r = 0.21; P < 0.0001) and nega tive correlations with percentage of type IIB fibres (r(true) = 0.35, r = - 0.24; P < 0.0001). Capillary density was inversely correlated to insulin. T hus, an obvious improvement of the correlation was seen after correcting in tra-individual variation. in conclusion, owing to the low degree of reprodu cibility of muscle morphology variables and insulin sensitivity, implying a noticeable underestimation of correlations, the r-values should be adjuste d for within-subject variation in order to demonstrate a more accurate esti mate of the strength of the relationships studied.