Which muscle contracts during forced expiration?
A) internal oblique
B) serratus anterior
C) sternocleidomastoid
D) pectoralis major
The correct answer and explanation is :
The correct answer is:
A) internal oblique
Explanation:
Forced expiration is an active process used to push air out of the lungs more forcefully than during normal, passive expiration. Normally, expiration at rest is passive and mainly due to the elastic recoil of the lungs and chest wall. However, during activities like vigorous exercise, coughing, or blowing out candles, forced expiration requires active muscle contraction to decrease thoracic volume more rapidly.
Role of the Internal Oblique in Forced Expiration:
The internal oblique muscles are part of the anterolateral abdominal wall muscles. During forced expiration, these muscles contract, increasing intra-abdominal pressure. This pressure pushes the abdominal organs upward against the diaphragm, which is relaxed and domed upwards during expiration. The upward movement of the diaphragm reduces the volume of the thoracic cavity, which in turn increases the pressure in the lungs, forcing air out.
Specifically, the internal obliques pull the lower ribs downward and compress the abdominal contents, aiding in decreasing the thoracic volume efficiently.
Why the other options are incorrect:
- B) Serratus anterior: This muscle primarily functions to protract and stabilize the scapula. It is involved in movements like pushing and punching, not expiration.
- C) Sternocleidomastoid: This muscle elevates the sternum and clavicle, helping in forced inspiration by increasing thoracic volume. It does not contract during expiration.
- D) Pectoralis major: Mainly involved in movements of the arm (adduction, medial rotation), and during deep inspiration, it can assist by elevating the ribs when the arms are fixed. It is not active during forced expiration.
Summary:
Forced expiration involves contraction of muscles that reduce thoracic volume beyond passive recoil. The internal oblique, along with other abdominal muscles like the external oblique and transversus abdominis, plays a key role in this process by increasing intra-abdominal pressure and pushing the diaphragm upward, thus expelling air forcefully from the lungs.