Physical exercise training is a powerful tool to maintain or improve mitochondrial density and function (mitochondrial capacity). This study aims to determine whether mitochondrial capacity is also associated with habitual physical activity in daily life (PADL). The capacity of classic markers for mitochondrial density, i.e. the capacity of citrate synthase (CS) and succinate dehydrogenase (SDH), as well the capacity of cytochrome c oxidase (COX) and beta-Hydroxyacyl-CoA dehydrogenase (HAD) was determined in homogenized muscle biopsy samples obtained from the M. Vastus Lateralis of non-exercising healthy, young (age 20 +/- 2 years) subjects (31 women, 7 men). PADL was measured during 2 periods of 14 days using a tri-axial accelerometer for movement registration. CS, SDH and COX were positively associated with PADL (P<0.05, R=0.36, 95% CI: 1.3.10(-4) to 2.2.10(-3); P<0.05, R=0.39, 95% CI: 1.1.10(-5) to 9.9.10(-5) and P<0.05, R=0.33, 95% CI: 7.5.10(-6) to 3.6.10(-4) respectively) and HAD tended to correlate positively with PADL (P=0.06, R=0.31, 95% CI: -2.2.10(-5) to 1.1.10(-3)). The population was subsequently stratified based on the intensity of the activities performed. CS was only associated with PADL in subjects spending more time on high intensity physical activity whereas HAD was only associated with PADL in subjects spending less time on low intensity physical activity. We are the first to report that even within the normal range of daily life activities, mitochondrial capacity is positively associated with the level of habitual physical activity in daily life. Thus, an active lifestyle may help to maintain or improve mitochondrial capacity. Key words: accelerometer, intensity, CS, SDH, COX.