Determinants of feelings of energy and physical function in middle-aged women are not well characterized. In addition, middle-aged breast cancer survivors (BCS) may have compromised feelings of energy and function due to the effects of disease and treatment. This study aimed to: 1) determine the contributions of adiposity and physical activity (PA) to feelings of energy in middle-aged postmenopausal women (N=74), 2) examine associations among PA, adiposity, and muscle quality (MQ) and physical function (N=64), and 3) determine if BCS differ from age and adiposity matched controls (CON) (N=13 per group) in feelings of energy and physical function. Body composition was measured via dual energy x-ray absorptiometry, PA via hybrid pedometer [stepsday-1, daily moderate to vigorous PA (MVPA)], vitality with the SF-36, leg strength and power via isokinetic dynamometry [60sec-1, (KN60)], and power rig. MQ was calculated as the ratio of: 1) KN60 to upper leg lean mass (MQ-KN60), and 2) power to lower body lean mass (MQ-Power). Physical function was evaluated with timed up and go (UPGO), 30-sec chair stand (CHR), and 6-minute walk (WALK).MVPA independently explained 8% of variance in vitality. Age and MQ-KN60 were independently related to CHR. Age and MQ-Power were significantly associated with UPGO. Total medical conditions, MQ-KN60, stepsday-1, and adiposity were predictors of WALK. Fatigue, energy, and PA were similar in BCS and CON; however, CON performed better than BCS on UPGO (15.8%, p=.05). In BCS and CON, group was a significant predictor of UPGO, group and MQ-KN60 were related to CHR, and adiposity explained 12% of WALK variance; however, when substituting MQ-Power into the regression analyses, stepsday-1 were related to UPGO and CHR, while adiposity explained 10% of WALK. Middle-aged women, including BCS, should engage in recommended amounts of PA to preserve feelings of energy, regardless of weight status, and maintain MQ for better physical function.