This short review summarizes how lung and chest wall mechanics can be modelled and which are the mechanical constraints imposed on the ventilatory system and its components during exercise. In healthy humans the structural and functional characteristics of the ventilator pump are able to meet the increased demands of ventilation during exercise and it is rare that arterial blood gas is significantly altered up to maximal exercise. In contrast, exercise is frequently limited by the ventilator system in disease, especially when altered mechanical properties of the airway and lung make expiratory flow limitation (EFL) a common feature. EFL is a phenomenon that can be understood in terms of the viscous effects of gas flowing from the alveoli to the airway opening along a collapsible airway which leads during exercise to dynamic hyperflation and several abnormalities of the ventilatory pump. These, in turn, determine a series of secondary manifestations, namely dyspnoea, exercise limitation and hypercapnia that can cause serious morbidity.