The effect of starvation-related malnutrition on muscle performance and on the energy cost of exercise remains unknown, as does the timing of improvement by refeeding. Indeed, in most diseases that induce malnutrition, muscle dysfunction is worsened by an inflammatory process. Thus, physical performance and the energy cost of exercise were studied in 15 semistarvated malnourished anorexia nervosa (AN) patients during exercise on an ergometric bicycle (3-min steps of 30 W) before and after 8, 30, and 45 d of refeeding. Results were compared with those of 15 normal-weight healthy subjects matched for age, sex, and physical activity. Before refeeding, the workload reached during the exercise was 49% lower in AN patients than in control subjects (P < 0.01). It was correlated with body weight, fat-free mass, and leg muscle circumference (P < 0.002). The performance improved dramatically during refeeding (P < 0.03), reaching normal values after 45 d of refeeding, despite fat-free mass and leg muscle circumference values that were still 20% lower in AN patients than in control subjects (P < 0.01). At this time, the exercise-related VO2 remained unchanged, being approximately 25% lower than that of the control subjects when corrected for muscle mass differences (P < 0.03). In conclusion, in AN patients muscle performance was restored by refeeding long before the patients achieved normal nutritional status. The economic cost of physical activity for these malnourished patients allows them to maintain a relatively high level of physical activity. This relative overactivity has two goals in AN: it reinforces anorexia and contributes to the excess of energy expenditure needed for weight loss.