Tests of respiratory muscle strength are employed when weakness is suspected. In those with muscle weakness, expiratory volume is determined more by inspiratory muscle strength and lung and chest wall compliance than expiratory flow limitation. Therefore, forced expiratory manoeuvres such as FEV1 and FVC have less value in this patient group and may be difficult to perform. The loss of vital capacity (VC) is usually offset to some extent by an increase in residual volume (RV), particularly if there is weakness of expiratory muscles. The maximal expiratory pressure (MEP) measures the strength of the abdominal and other expiratory muscles. Maximal inspiratory pressure measures the global strength of the diaphragm and other inspiratory muscles. The cough peak flow is the velocity of air leaving the respiratory tract during a cough manoeuvre, usually from maximum inspiration. Cough peak flow is an important determinant of susceptibility to chest infections, which account for both morbidity and mortality in patients with severe muscle weakness.