Page 134 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

BIOMECHANICS OF THE RESPIRATORY MUSCLES 111

and the medial third of the clavicle. The scalene muscles comprise three bundles that run from the
transverse processes of the lower five cervical vertebrae to the upper surface of the first two ribs.
Contraction of these muscles raises the sternum and the first two ribs and thus assists in expanding
the rib cage (Legrand et al., 2003).

5.2.4 The Abdominal Muscles

The four abdominal muscle pairs forming the abdominal wall are the rectus abdominis, external
oblique, internal oblique, and transverse abdominis (Fig. 5.1). The rectus abdominis is the most ventral
one that runs caudally from the ventral aspect of the sternum and the fifth, sixth, and seventh costal
cartilages along the length of the abdominal wall to its insertion into the pubis (De Troyer, 1997).
The external oblique is the most superficial that originates from the external surface of the lower
eight ribs, well above the costal margin, and covers the lower ribs and intercostal muscles. Its fibers
radiate caudally to the iliac crest and inguinal ligament and medially to the linea alba. The internal
oblique lies deep to the external oblique. Its fibers arise from the inguinal ligament and iliac crest
and insert into the anterolateral surface of the cartilages of the last three ribs and into the linea alba.
The transverse abdominis is the deepest muscle of the lateral abdominal wall. Its fibers run circum-
ferentially around the abdominal visceral mass from the inner surface of the lower six ribs, lumbar
fascia, iliac crest, and inguinal ligament to the rectus sheath. Contraction of the abdominal muscles
pulls the abdominal wall inward, causing the diaphragm to move cranially into the thoracic cavity,
and pulls the lower ribs caudally to deflate the rib cage (De Troyer, 1997).

5.3 MECHANICS PERFORMANCE OF RESPIRATORY MUSCLES

All-inclusive function of the respiratory muscles is an important index in diagnosis and follow-up of
breathing problems due to respiratory muscle weakness. It can be assessed by employing different
techniques, which are based on different measurement protocols.

5.3.1 Global Assessment of Respiratory Muscles Strength

Measurements of maximal inspiratory or expiratory mouth pressures during quasi-static maneuvers
are widely used for assessment of the global strength of respiratory muscles (Black and Hyatt, 1969;
Chen and Kuo, 1989; Leech et al., 1983; McElvaney et al., 1989; Ratnovsky et al., 1999; Steier et al.,
2007; Wilson et al., 1984). The subject inspires or expires through a mouthpiece with an air leak
orifice (1.5 to 2 mm) to prevent the contribution of the facial muscles (Black and Hyatt, 1969).
Maximal static mouth pressure is measured while performing a maximal inspiratory or expiratory
effort against an obstructed airway for at least 1 second. Maximal expiratory mouth pressure is usu-
ally measured at lung volumes approaching total lung capacity (TLC), while maximal inspiratory
pressure is usually measured near functional residual capacity (FRC) or residual volume (RV).
A summary of published values of maximal inspiratory and expiratory mouth pressures is given
in Table 5.1.
The ability of respiratory muscles to generate force, like other skeletal muscles, depends on their
length and the velocity of contraction (Green and Moxham, 1985; Rochester, 1988). In the respira-
tory system, force is generally estimated as pressure and their length varies as lung volume changes
(Fauroux and Aubertin, 2007). Employment of the interrupter technique, in which the airflow inlet
is obstructed during forced expiration or force inspiration maneuvers, allowed measurement of
mouth pressures at different lung volumes during maximum inspiratory and expiratory efforts made
against different levels of resistance (Agostoni and Fenn, 1960; Cook et al., 1964; Ratnovsky et al.,
1999). This is particularly relevant when evaluating hyperinflated patients in whom the geometry of
the respiratory muscles changes, and thereby their ability to drive the respiratory pump decreases