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.
1 The external intercostals are responsible for the elevation of the ribs and bending them more open, thus expanding the transverse dimensions of the thoracic cavity. 2  Internal intercostal muscles aid in forced expiration (quiet expiration is a passive process). External intercostal muscles aid in quiet and forced inhalation. 2  They originate on ribs 1-11 and have their insertion on ribs 2-12.