SUMMARY. The respiratory system consists essentially of two parts, a gas-exchanging organ, the lung, and a pump to pump gas in and out the gas exchanging part, consisting of the respiratory muscles and the chest wall. The lung and its diseases have traditionally been the focal point of interest, whereas pump disorders have received comparatively little attention. Research on respiratory muscles has accelerated during the last two decades. Respiratory muscles are all skeletal muscles having similar fibre composition to the limb muscles. Fibre composition of respiratory muscles is an important factor for their endurance and contractile properties. There are two fibre types, the fast (FT) and slow (ST) twitch fibres. Of the FT fibres two subgroups named FOG and FG have been identified. Human intercostals muscles appear to have mostly ST fibres. The diaphragm has a high percentage of fatigue-resistant fibres. The main respiratory muscles are the diaphragm, intercostal muscles and muscles of the abdominal wall. The accessory muscles of respiration include the sternomastoid and other muscles of the neck, back and shoulder girdle. The intercostals muscles are subdivided into two groups: the external and internal intercostals. The contraction of the diaphragm which is the most important respiratory muscle, decreases the intrathoracic pressure and increases the abdominal pressure in normal man by lowering the diaphragmatic dome. Intercostal muscles move the rib cage and can be inspiratory or expiratory. The scaleni is now believed to be true muscles of inspiration and not "accessory". The most important accessory muscles of inspiration are probably the sternocleidomastoid muscles. The respiratory muscles are the motive power for breathing and are subject to weakness from a variety of processes that affect the motor nerves, neuromuscular junction and muscle cell. Chronic neuromuscular disorders result in altered lung volumes. The effectiveness of cough is reduced in expiratory muscle weakness. Patients with respiratory muscle weakness breath faster and with a smaller tidal volume compared to healthy subjects. The main change in blood gases in patients with respiratory muscle weakness is usually a fall in PaO2. Hypercapnia may be a late event. Muscle fatigue, which is a reversible event, can be defined as the inability to sustain the required or expected force with continued contractions. The respiratory muscles, particularly those of inspiration, can fatigue and precipitate or intensify ventilatory failure. Pneumon 2001, 14 (2): 91-108