Chronic mucus hypersecretion (CMH) in, for example, chronic obstructive pulmonary disease (COPD) is associated with several poor outcomes such as reduced lung function and quality of life, increased frequencies of exacerbations and hospitalisations, and increased mortality . Moreover, subjects with COPD and CMH are more likely to die of pulmonary infections than subjects without CMH . Because of these poor associations, mucus clearance techniques have been of potential interest as a nonpharmacological treatment option . The rationale behind these techniques is based on the exertion of an optimal expiratory airflow velocity that applies shearing forces to sputum at the air– liquid interface . Airflow velocity could be manipulated by changes in airflow, lung volumes, pulmonary pressures and airway diameter [4, 5]. For example, autogenic drainage uses a balanced expiratory flow at different lung volumes to achieve the highest air velocity possible without causing airway compression to mobilise the secretions .