Muscle metabolic function measured during exercise is related to exercise performance and 1-year weight gain. Since it is established that physical activity is important in weight maintenance, we examined muscle function relationships with free-living energy expenditure and physical activity. Subjects were 71 premenopausal Black and White women. Muscle metabolism was evaluated using ³¹P magnetic resonance spectroscopy during 90 sec isometric plantar flexion contractions (45% maximum). Free-living energy expenditure (TEE) was measured using doubly labeled water, activity-related energy expenditure (AEE) was calculated as .9 X TEE - sleeping energy expenditure from room calorimetry, and free-living physical activity (ARTE) was calculated by dividing AEE by energy cost of standard activites. At the end of exercise, anaerobic glycolytic rate (ANGLY), and muscle concentration of phosphomonoesters (PME) were negatively related to TEE, AEE, and ARTE (p<0.05). Multiple regression analysis showed that both PME (partial r = -0.29, <0.02) and ANGLY (partial r = -0.24, p < 0.04) were independently related to ARTE. PME, primarily glucose-6-phosphate and fructose-6-phosphate, was significantly related to ratings of perceived exertion (RPE) (r = 0.21, p 0.05) during a maximal treadmill test. PME was no longer related to ARTE after inclusion of RPE in the regression model, suggesting that PME may be obtaining its relationship with ARTE through an increased perception of effort during activities of daily living. It is concluded that the physically inactive tend to be more dependent on anaerobic glycolysis during submaximal exercise while relying on a glycolytic pathway that may not be functioning optimally.