Final answer:
Accessory muscles of inhalation are typically most active during forced breathing or hyperpnea, like during exercise or singing, when additional effort is required to increase lung volume beyond normal quiet breathing.
Step-by-step explanation:
Accessory muscles of inhalation are most active during situations that require forced breathing or hyperpnea, such as during exercise or while singing. While the diaphragm is the primary muscle for quiet breathing, accessory muscles like the scalenes and external intercostal muscles play a larger role when more intense respiratory effort is necessary. These muscles aid in expanding the thoracic cavity to accommodate a greater volume of air during inspiration. The neck muscles, including the scalenes, lift the thoracic wall to increase lung volume; meanwhile, the 11 pairs of superficial external intercostal muscles help in raising the rib cage to expand it further. Accessory muscles are also involved in forced expiration, where muscles such as the internal intercostals and abdominal muscles like the obliques help to push air out by forcing the rib cage to compress and the diaphragm to move upwards into the thorax. This concerted effort of various muscles ensures efficient ventilation during physically demanding activities where the body requires more oxygen.