<P> "Accessory muscles" refers to muscles that assist, but do not play a primary role, in breathing . There is some controversy concerning which muscles may be considered accessory muscles of inhalation . However, the sternocleidomastoid and the scalene muscles (anterior, middle and posterior scalene) are typically considered accessory muscles of breathing . Both assist in elevating the rib cage . The involvement of these muscles seems to depend on the degree of respiratory effort . During quiet breathing, the scalenes are consistently phasically active, while the sternocleidomastoids are quiet . With an increase in the respiratory volume, sternocleidomastoids also become active . Both muscles are simultaneously activated when one breathes in at the maximal flow rate . </P> <P> Apart from the above neck muscles, the following muscles have also been observed contributing to respiration: serratus anterior, pectoralis major and pectoralis minor, trapezius, latissimus dorsi, erector spinae, iliocostalis lumborum, quadratus lumborum, serratus posterior superior, serratus posterior inferior, levatores costarum, transversus thoracis, subclavius (Kendall et al., 2005). </P> <P> Use of the accessory muscles while at rest is often interpreted as a sign of respiratory distress . </P> <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>

Which of the following is true about the muscles active during breathing