During what circumstances do accessory muscles become involved in respiration?

Accessory muscles become involved in respiration primarily during heavy exertion or respiratory distress. When the body requires increased ventilation, such as during intense physical activities, the primary muscles of respiration (the diaphragm and intercostal muscles) may need assistance to meet the higher demand for oxygen. In these situations, the body recruits accessory muscles located in the neck, shoulders, and abdomen to augment breathing efforts. These muscles include the sternocleidomastoid, scalene muscles, and pectoralis major.

Under normal resting conditions, the primary respiratory muscles usually suffice for adequate ventilation, and during sleep, accessory muscles are generally not required due to the decreased demand for oxygen. Furthermore, when lung function is impaired, as in the case of various respiratory conditions, the body often relies on the accessory muscles as a compensatory mechanism rather than during normal functioning. Thus, the involvement of accessory muscles is specifically linked to scenarios that escalate the need for respiratory effort, highlighting their role in ensuring sufficient ventilation during stress on the body's respiratory system.