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 duplicate biopsies from the same thigh of 23 subjects from a cohort of 70-year-old men. The coefficient of variation (CV) for different characteristics of muscle morphology was between 11 and 42% in duplicate biopsies. Coefficient 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 insulin sensitivity. The correlation analysis between muscle morphology and insulin resistance was performed in a sample of 515 men from the cohort, correlation coefficients were calculated with (rtrue) and without (r) adjustment for intra-individual variation. Insulin sensitivity showed a positive relationship with percentage of type I fibres (rtrue=0.33, r=0.21; P < 0.0001) and capillary density (rtrue=0.43, r=0.21; P < 0. 0001) and negative correlations with percentage of type IIB fibres (rtrue=-0.35, r=-0.24; P < 0.0001). Capillary density was inversely correlated to insulin. Thus, an obvious improvement of the correlation was seen after correcting intra-individual variation. In conclusion, owing to the low degree of reproducibility of muscle morphology variables and insulin sensitivity, implying a noticeable underestimation of correlations, the r-values should be adjusted for within-subject variation in order to demonstrate a more accurate estimate of the strength of the relationships studied.