A muscle cramp during pregnancy may occur in any muscle or muscle group, but is most commonly experienced in the legs. Muscle spasms that occur in the back or abdomen are also possible. A muscle cramp or spasm is characterized by the following: 1  Involuntary contraction or tightening of a muscle. 2  Usually sudden.
During breathing, these muscles normally tighten and pull the rib cage up. Your chest expands and the lungs fill with air. Intercostal retractions are due to reduced air pressure inside your chest. This can happen if the upper airway (trachea) or small airways of the lungs (bronchioles) become partially blocked.
The external intercostal muscles are responsible for forced and quiet inhalation. They raise the ribs and expand the chest cavity, and originate from ribs one through 11, with insertion from ribs two to 12. The internal intercostal muscles are responsible for forced exhalation.he three layers are: external intercostal muscles, internal intercostal muscles, and the innermost intercostal muscles. These muscles are innervated and supplied with blood by the intercostal nerves, intercostal veins, and intercostal arteries.