PURPOSE: The purpose of this work was to determine if racial differences in maximum oxygen uptake (VO(2) max) and difficulty in doing submaximum (sub) exercise tasks contribute to activity-related energy expenditure (AEE) and activity-related time equivalent (ARTE) index, an index of time spent in physical activity. METHODS: Subjects were 35 Black and 39 White sedentary premenopausal women group matched for age, weight and body composition. Percent fat was determined by dual energy X-ray absorptiometry, AEE by doubly labeled water and sleeping energy expenditure, ARTE index by AEE and energy cost above sleeping for five exercise tasks, and VO(2) max by a graded exercise test. Subexercise oxygen uptake was measured during walking at 3 mph on the flat and up a 2.5% grade, climbing stairs, and riding a bike ergometer at 50 W. Difficulty of exercise was assessed during the subexercise and taken as heart rate, ventilation, and ratings of perceived exertion (RPE) and expressed as a % of the individual's maximum. RESULTS: VO(2) max was related to AEE (r=0.29, P<0.04) and to ARTE index (r=0.37, P<0.01). All three difficulty measures were related to AEE (r=-0.35 to -0.42, P-values<0.02) and the ARTE index (r=-0.49 to -0.52, all P-values<0.01), with the exception of RPE with ARTE (r=-0.27, P=0.10). Compared to Black women, White women had significantly higher VO(2) max (12%), AEE (45%) and ARTE (50%), and significantly less physiologic difficulty of performing the subexercise tasks: heart rate (5%), ventilation (13%), and RPE (8%). Significant racial differences in AEE and ARTE disappeared after adjusting for any of the three difficulty measures. CONCLUSIONS: These results show that more participation in free-living physical activity is related to greater VO(2) max and less difficulty in being active. In addition, lower levels of physical activity found in Black women may be partially explained by lower VO(2) max and increased difficulty of engaging in physical activities.