A study of lung volumes offers an overall index of thoracic mobility. The techniques of magnetometry and respiratory plethysmography by induction allow studies of thoracic wall movements to be separated into two compartments by measuring thoracic and abdominal movements. These techniques enable a quantitative evaluation of thoraco-abdominal incoordination seen notably in patients with chronic airflow obstruction and in cases of respiratory muscle fatigue. Similar information is obtained by measuring variations in inspiratory pleural and abdominal pressure, using oesophageal and gastric balloons. The maximal force of respiratory muscles is an important index of the functional reserve at a patient's disposal and may be assessed by measuring maximal inspiratory and expiratory pressures at the mouth against an occlusion. Diaphragmatic force is assessed by the maximal trans-diaphragmatic pressure. Respiratory muscle fatigue appears when their contraction is sustained above a certain threshold of tension and duration. This phenomenon may be detected by a diminution of maximal pressures. For example, the fatigue of inspiratory muscles is accompanied by a fall of maximal inspiratory and trans-diaphragmatic pressures. The fatigue process can also be detected early by changes in the pattern of the diaphragmatic electromyogram. The principles and limitations of different techniques of assessment of respiratory muscle fatigue are analysed. The use of one or several of these techniques accompanied by an awareness of the clinical signs of respiratory muscle fatigue ought to enable an early detection of this condition.