<P> During quiet breathing, there is little or no muscle contraction involved in exhalation; this process is simply driven by the elastic recoil of the thoracic wall . When forceful exhalation is required, or when the elasticity of the lungs is reduced (as in emphysema), active exhalation can be achieved by contraction of the abdominal wall muscles (rectus abdominis, transverse abdominis, external oblique muscle and internal oblique muscle). These press the abdominal organs cranially (upward) into the diaphragm, reducing the volume of the thoracic cavity . </P> <P> The internal intercostal muscles have fibres that are angled obliquely downward and backward from rib to rib . These muscles can therefore assist in lowering the rib cage, adding force to exhalation . </P>

Which of the following muscles is the main muscle of respiration