From the Guidelines
The correct answer is a. The scalenes assist quiet and deep inspiration by elevating ribs 1 and 2. The scalene muscles are important accessory muscles of respiration that attach to the first two ribs and help expand the upper thorax during breathing [ 1 ]. During inspiration, they contract concentrically to lift these ribs, which increases thoracic volume. Some key points to consider:
- Option b is incorrect because quiet inspiration primarily involves the diaphragm, with minimal contribution from the sternocleidomastoid, which is mainly recruited during deep or labored breathing.
- Option c is incorrect because quiet inspiration is an active process requiring muscle contraction, while expiration is the passive phase that relies on elastic recoil [ 1 ].
- Option d is incorrect because the rectus abdominis is actually an expiratory muscle that helps force air out during active expiration, not inspiration [ 1 ]. It contracts during activities like coughing or sneezing to compress the abdominal contents and push the diaphragm upward. The use of accessory muscles such as the scalenes is critical in maintaining adequate ventilation, especially in conditions with low lung compliance [ 1 ].
From the Research
Respiratory Muscles
The following statements regarding respiratory muscles are correct:
Quiet Inspiration
Quiet inspiration involves:
- Concentric contraction of the thoracic diaphragm and other muscles, but the specific muscles involved are not limited to the diaphragm, scalenes, and sternocleidomastoid 2, 3
Deep Inspiration
Deep inspiration involves:
- Concentric contraction of various muscles, but the rectus abdominus is not typically involved in inspiration 3, 5
Key Points
Some key points to consider:
- The parasternal intercostal muscles play a predominant role during quiet breathing 2
- The external intercostal muscles are active mainly during inspiration, and the internal intercostal muscles during expiration 2
- The scalenes and sternocleidomastoid muscles are involved in inspiration, but their specific roles and contributions vary 3, 4