The purpose of this study was to determine whether muscle metabolic capacity was inversely related to age after adjusting for physical activity in sedentary premenopausal women. Eighty-three women (ages 23-47 yr) had their free-living, activity-related energy expenditure evaluated with doubly labeled water procedures, and room calorimeter determined sleeping energy expenditure. Maximum O₂ uptake and strength were evaluated in all subjects, whereas ³¹P-magnetic resonance spectroscopy determined metabolic economy during maximal exercise, and muscle biopsy maximal enzyme activity was evaluated in subsets of the sample (48 and 18 subjects, respectively). Age was significantly related to whole body treadmill endurance time (r = 0.32), plantar flexion strength (r = 0.29), maximum O₂ uptake (r = 0.27), ³¹P-magnetic resonance spectroscopy ADP recovery rate (r = 0.44), and anaerobic glycolytic capacity (r = 0.37), and muscle biopsy citrate synthase activity (r = 0.48), glyceraldehyde-3-phosphate dehydrogenase (r = 0.54), phosphofructokinase (r = 0.62), and phosphorylase (r = 0.58) activity even after adjusting for activity-related energy expenditure. These data suggest that, in sedentary premenopausal women, both oxidative and glycolytic muscle capacity decrease with age even when physical activity is taken into account.