We have previously shown that muscle metabolic function measured during exercise is related to exercise performance and subsequent 1-yr weight gain. Because it is well 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 by (31)P magnetic resonance spectroscopy during 90-s 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 0.9 x TEE - sleeping energy expenditure from room calorimetry, and free-living physical activity (ARTE) was calculated by dividing AEE by energy cost of standard physical activities. 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 (r = 0.21, P < or = 0.05) during a maximal treadmill test. PME was not related to ARTE after inclusion of RPE in the multiple regression model, suggesting that PME may be obtaining its relationship with ARTE through an increased perception of effort during physical activity. In conclusion, physically inactive individuals tend to be more dependent on anaerobic glycolysis during exercise while relying on a glycolytic pathway that may not be functioning optimally.