ABSTRACT In a population-based sample of 475 men the associations between muscle morphology, self-reported physical activity (PA) and insulin resistance (IR) syndrome were investigated. Also, we studied to what degree muscle morphology contributes to the association between PA and IR syndrome. Muscle morphology and the components of IR syndrome were compared in four groups categorized according to self-reported habitual PA data. We found a significantly higher percentage of type I fibres, fibre area and number of capillaries around the fibres and a lower proportion of type IIB fibres with higher level of PA. The relative distribution of type I fibres and capillarization were positively related to high density lipoprotein (HDL) cholesterol and negatively to serum triglycerides (TG) and plasminogen activator inhibitor-1 (PAI-1) activity. The percentage of type IIB fibres was were inversely related to HDL cholesterol and positively to serum TG, PAI-1 activity and resting heart rate. Insulin sensitivity was positively and independently related to PA level (P < 0.001). Regression analysis including all relevant variables regarding insulin sensitivity indicated that the significant explanatory variables left in the equation were body mass index (BMI), glucose intolerance, PAI-1 activity, serum free fatty acid concentration, proportion of type IIB fibres, HDL cholesterol level, drug treatment, PA level, and waist-to-hip ratio, which together explained 55% of the variation in the insulin sensitivity index. In conclusion, both fibre type distribution and muscle capillary density might contribute to the beneficial effect of PA on IR syndrome.