Page 133 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

110 BIOMECHANICS OF THE HUMAN BODY

Sternomastoid
Scalene

External
intercostal
Internal Internal
intercostal intercostal

Diaphragm External
oblique

Internal oblique
Transverse abdominis
Rectus abdominis
FIGURE 5.1 Schematic description of the anatomy of human respiratory muscles. [From
www.concept2.co.uk/training/breathing.php, with permission.]

5.2.1 The Diaphragm
The diaphragm, the main muscle of inspiration, is a thin, flat, musculotendinous structure separating
the thoracic cavity from the abdominal wall. The muscle fibers of the diaphragm radiate from the
central tendon to either the three lumbar vertebral bodies (i.e., crural diaphragm) or the inner sur-
faces of the lower six ribs (i.e., costal diaphragm) (Fig. 5.1). The tension within the diaphragmatic
muscle fibers during contraction generates a caudal force on the central tendon that descends in order
to expand the thoracic cavity along its craniocaudal axis. In addition, the costal diaphragm fibers
apply a force on the lower six ribs which lifts and rotates them outward (De Troyer, 1997).

5.2.2 The Intercostal Muscles
The intercostal muscles are composed of two thin layers of muscle fibers occupying each of the
intercostal spaces. The external intercostal muscle fibers run obliquely downward and ventrally from
each rib to the neighboring rib below. The lower insertion of the external intercostal muscles is more
distant from the rib’s axis of rotation than the upper one (Fig. 5.1), and as a result, contraction of
these muscles exerts a larger torque acting on the lower rib which raises the lower rib with respect
to the upper one. The net effect of the contraction of these muscles raises the rib cage.
The internal intercostal muscle fibers, on the other hand, run obliquely downward and dorsally
from each rib to the neighboring rib below. The lower insertion of these muscles is less distant from
the rib’s axis of rotation than the upper one, and thus, during their contraction they lower the ribs
(De Troyer, 1997).

5.2.3 The Accessory Muscles
The accessory muscles of inspiration include the sternomastoid and scalene muscles. The ster-
nomastoid muscles descend from the mastoid process to the ventral surface of the manubrium sterni