Muscular System

a combined contraction of the levator scapulae, rhomboid major and minor muscles and the latissimus dorsi muscle.

I would narrow this answer down to the Rhomboid Muscles

-Lucein
rhomboids

short laymen answer:

the diaphragm:a muscular partition separating the abdomin and thoracic cavities and the intercostal muscles.. they contract during inspiration. some neck muscles function in respiration and abdominal muscles assist to pull down the rib cage

long drawn out answer:

there are 7 muscles that assist in inhalation when running up the stairs and they are;

The lungs can be expanded and contracted in two ways:

by downward and upward movement of the diaphragm to lengthen or shorten the chest cavity, and by elevation and depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity.

Normal quiet breathing is accomplished almost entirely by the first method, that is, by movement of the diaphragm. During inspiration, contraction of the diaphragm pulls the lower surfaces of the lungs downward. Then, during expiration, the diaphragm simply relaxes, and the elastic recoil of the lungs, chest wall, and abdominal structures compresses the lungs and expels the air.

During heavy breathing, such that as in your question, however, the elastic forces are not powerful enough to cause the necessary rapid expiration, so that extra force is achieved mainly by contraction of the

Abdominal muscles, which pushes the abdominal contents upward against the bottom of the diaphragm, thereby compressing the lungs.

The second method for expanding the lungs is to raise the rib cage. This expands the lungs because, in the natural resting position, the ribs slant downward thus allowing the sternum to fall backward toward the vertebral column. But when the rib cage is elevated, the ribs project almost directly forward, so that the sternum also moves forward, away from the spine, making the anteroposterior thickness of the chest roughly 20 per cent greater during maximum inspiration than during expiration. Therefore, all the muscles that elevate the chest cage are classified as muscles of inspiration, and those muscles that depress the chest cage are classified as muscles of expiration.

The most important muscles that raise the rib cage are the

External intercostals, but others that help are the:

Sternocleidomastoid muscles, (promanant muscles of the neck) which lift upward on the sternum;

Anterior Serrati, muscles that in addition to other things elevate the rib cage; and

Scaleni, which lift the first two ribs.

The muscles that pull the rib cage downward during expiration are mainly

the

Abdominal recti, which have the powerful effect of pulling downward on

the lower ribs at the same time that they and other abdominal muscles also com-

press the abdominal contents upward against the diaphragm, and

Internal intercostals.

Fast twitch muscle fibers are Type II muscle fibers. These muscle fibers have high explosive power and are anaerobic. They have a fast contractile speed and can reach peak tension in 50ms. They have a high glycolytic capacity and have low to moderate resistance to fatigue. Their motor unit strength is high.

Slow twitch muscle fibers are Type I muscle fibers. These muscle fibers have steady power and are aerobic. The have a slow contractile speed and needs 110ms to reach peak tension. They have a low glycolytic capacity and a high resistance to fatigue. Their motor unit strength is low.