A method, apparatus, and system for measuring respiratory effort of a subject are provided. According to the method, a thoracic effort signal (T) is obtained, the thoracic effort signal (T) being an indicator of a thoracic component of the respiratory effort. An abdomen effort signal (A), the abdomen effort signal (A) being an indicator of an abdominal component of the respiratory effort. A respiratory flow (F) is obtained. The respiratory effort is determined by adjusting the components of a model of the respiratory system based on the thoracic effort signal (T), the abdomen effort signal (A), or the respiratory flow (F) or any combination of the thoracic effort signal (T), the abdomen effort signal (A), and the respiratory flow (F).
