This is similar to anorexia nervosa and is often a part of anorexic behavior. Activity disorder produces significant weight loss based on excessive exercise. There is an obsession and/or compulsion to visit the gym, do weight training, bicycling, long runs, and/or physical exercise such as push-ups, squats, or sit-ups at all hours of the day and night. The focus is on the elimination of caloric intake by exercise.

The defining characteristic of activity disorder is the harm caused to the individual's physical health. Symptoms include emotional fatigue, physical weakness, headaches, dizziness, faintness, chest pain, constipation, abdominal pain, low back pain, a feeling of fullness when eating, dry skin, and always feeling cold.