Answer
In the process of breathing, the external intercostals are used more frequently: they function in both quiet and forced inhalation, while the internal intercostals function actively only in forced exhalation.
When the external intercostals contract, they raise the ribs and expand the chest. As the thoracic cavity expands and lung volume increases, intrapulmonary pressure falls, and air rushes. This is quiet inspiration.
In forced inspiration, other muscles besides diaphragm and external intercostals, are involved. Some of these accessory respiratory muscle are scalenes, pectoralis muscles (major and minor), sternocleidomastoid, and latissimus dorsi.
Quiet expiration is a passive process, and does not require the assistance of the internal intercostals. However, internal intercostals pay a role in forced expiration.
Work Step by Step
The internal intercostals (and innermost intercostals) play a role in forced exhalation. When they contract, they lower the ribs and compress the chest cavity. Consequently, intrathoracic and intra pulmonary pressures are increased, and air is forced out. This is forced exhalation. In this process ( forced expiration) the abdominal muscles are also involved: they contract and force the diaphragm upward; this increases pressure in the abdominal cavity, the thoracic ,cavity, and in the lungs. As a result, air is pushed out in forced exhalation.
