Biomechanics of thorax and chest wall

1. Thorax and the chest wall
Jincy Ashish

2. Thorax and the chest wall
The thorax, consisting of the thoracic vertebrae, the ribs, and the sternum , has
several important functions.
The thorax provides a base for the attachment of muscles of the upper extremities,
the head and neck, the vertebral column, and the pelvis. The thorax also forms
protection for the heart, lungs, and viscera.
The most important function of the chest wall is its role in ventilation. The process
of ventilation depends on the mobility of the bony rib thorax and the ability of the
muscles of ventilation to move it.

3. Scapula
Manubrium
Sternal angle
Costochondral joint
Costal cartilage
Body of sternum
Xiphoid process

4. Thoracic vertebrae
Acromion
Clavicle
Angle of ribs

5. Rib Cage
The rib cage is a closed chain that involves many joints and muscles. The anterior border
of the rib cage is the sternum, the lateral borders are the ribs, and the posterior border is
formed by the thoracic vertebrae.
The superior border is formed by jugular notch of the sternum, by the superior borders of
the first costocartilages, and by the first ribs and their contiguous first thoracic
vertebra.
The inferior border of the rib cage is formed by the xiphoid process, the shared
costocartilage of ribs 6 through 10, the inferior portions of the 11th and 12th ribs, and
the 12th thoracic vertebra.
The sternum is an osseous protective plate for the heart and is composed of the
manubrium, body, and xiphoid process

6. 1)Manubrium
2)2nd costal notch
3)4th costal notch
4)7th costal notch
5)Costal cartilage of 1st
rib
6)Manubriosternal joint
7)Body of sternum
8)Xyphoid process
9)Jugular notch
1
2
3
4
5
8
6
7

7. There are 12 thoracic vertebrae that make up the posterior aspect of the rib cage. One of
the unique aspects of the typical thoracic vertebra is that the vertebral body and transverse
processes have six costal articulating surfaces, four on the body (a superior and an
inferior costal facet, or demifacet, on each side) and one costal facet on each transverse
process.
The rib cage also includes 12 pairs of ribs. The ribs are curved flat bones that gradually
increase in length from rib 1 to rib 7 and then decrease in length again from rib 8 to
rib 12.
The posteriorly located head of each rib articulates with thoracic vertebral bodies;
and the costal tubercles of ribs 1 to 10 also articulate with the transverse processes of a
thoracic vertebra.
Anteriorly, ribs 1 to 10 have a costocartilage that join them either directly or indirectly
to the sternum through the costal cartilages

9. 1)Transverse costal facet
2)Superior costal facet
3)Inferior costal facet
2
1
3

10. The first through the seventh ribs are classified as vertebrosternal (or “true”) ribs
because each rib, through its costocartilage, attaches directly to the sternum.
The costocartilage of the 8th through 10th ribs articulates with the costocartilages of the
superior rib, indirectly articulating with the sternum through rib 7. These ribs are
classified as vertebrochondral (or “false”) ribs.
The 11th and 12th ribs are called vertebral (or “floating”) ribs because they have no
anterior attachment to the sternum.5

12. 1)Costal tubercle
2)Superior facet of
head
3)Inferior facet of
head
4)Site of articulation
with costal cartilage
5)Outer surface of rib
6)Inner surface of rib
1
2
3
6
4
5

13. Articulations of the Rib Cage
The articulations that join the bones of the rib cage include the manubriosternal (MS),
xiphisternal (XS), costovertebral (CV), costotransverse (CT), costochondral (CC),
chondrosternal (CS), and the Manubriosternal and Xiphisternal Joints
Manubriosternal Joint (MS)
The manubrium and the body of the sternum articulate at the MS joint. This joint is also
known as the sternal angle or the angle of Louis and is readily palpable.The MS joint is
a synchondrosis. The MS joint has a fibrocartilaginous disk between the hyaline
cartilage–covered articulating ends of the manubrium and sternum—structurally similar to
the symphysis pubis of the pelvis. Ossification of the MS joint occurs in elderly persons.
The xiphoid process joins the inferior aspect of the sternal body at the XS joint. The XS
joint is also a synchondrosis that tends to ossify by 40 to 50 years of age.

14. Costovertebral Joint
The 1st, 10th 11th, and 12th ribs are atypical ribs because they articulate with only one
vertebral body and are numbered by that body. The CV facets of T10 to T12 are located
more posteriorly on the pedicle of the vertebra.
The typical CV joint is divided into two cavities by the interosseous or intra-articular
ligament.This ligament extends from the crest of the head of the rib to attach to the
annulus fibrosus of the intervertebral disk.
The radiate ligament is located within the capsule, with firm attachments to the
anterolateral portion of the capsule. The radiate ligament has three bands: the superior
band, which attaches to the superior vertebra; the intermediate band, which attaches to
the intervertebral disk; and the inferior band, which attaches to the inferior vertebra.
A fibrous capsule surrounds the entire articulation of each CV joint.

16. The typical CV joint is a synovial joint formed by the head of the rib, two adjacent vertebral
bodies, and the interposed intervertebral disk.
Ribs 2 to 9 have typical CV joints, the heads of these ribs each have two articular facets, or
so-called demifacets. The demifacets are separated by a ridge called the crest of the head of
the rib.
The small, oval, and slightly convex demifacets of the ribs are called the superior and
inferior costovertebral facets.
Adjacent thoracic vertebrae have facets corresponding to those of the 9 ribs that articulates
with them.
The head of each of the second through ninth ribs articulates with an inferior facet on the
superior of the two adjacent vertebrae and with a superior facet on the inferior of the two
adjacent vertebrae. The inferior and superior facets on the adjacent vertebrae articulate,
respectively, with the superior and inferior facets on the head of the ribs.

17. The atypical CV joints of ribs 1 and 10 through 12 are more mobile than the typical
CV joints because the rib head articulates with only one vertebra.
The interosseous ligament is absent in these joints; therefore, they each have only
one cavity.
The radiate ligament is present in these joints, with the superior band still attaching to
the superior vertebra.
Both rotation and gliding motions occur at all of the CV joints.

18. Costotransverse Joints
The CT joint is a synovial joint formed by the articulation of the costal tubercle of the
rib with a costal facet on the transverse process of the corresponding vertebra.
There are 10 pairs of CT joints articulating vertebrae T1 through T10 with the rib of the
same number.
The CT joints on T1 through T6 have slightly concave costal facets on the transverse
processes of the vertebrae and slightly convex costal tubercles on the corresponding ribs.
This allows slight rotation movements between these segments.
At the CT joints of approximately T7 through T10, both articular surfaces are flat and
gliding motions predominate.
Ribs 11 and 12 do not articulate with their respective transverse processes of T11 or
T12.

21. Three major ligaments support the CT joint capsule. These are the lateral
costotransverse ligament, the costotransverse ligament, and the superior
costotransverse ligament.
The lateral costotransverse ligament is a short, stout band located between
the lateral portion of the costal tubercle and the tip of the corresponding
transverse process.
The costotransverse ligament is composed of short fibers that run within the
costotransverse foramen between the neck of the rib posteriorly and the
transverse process at the same level.
The superior costotransverse ligament runs from the crest of the neck of the
rib to the inferior border of the cranial transverse process.

22. Costochondral and Chondrosternal Joints
The CC joints are formed by the articulation of the 1st through 10th ribs
anterolaterally with the costal cartilages. The CC joints are synchondroses. The
CC joints have no ligamentous support.
The CS joints are formed by the articulation of the costal cartilages of ribs 1 to 7
anteriorly with the sternum.
Rib 1 attaches to the lateral facet of the manubrium, rib 2 is attached via two
demifacets at the manubriosternal junction, and ribs 3 through 7 articulate with
the lateral facets of the sternal body.
The CS joints of the first, sixth, and seventh ribs are synchondroses. The CS joints
of ribs 2 to 5 are synovial joint

23. Costochondral Joint
Chondrosternal Joint

24. The CS joints of the first through seventh ribs have capsules that are
continuous with the periosteum and support the connection of the cartilage as a
whole.
Ligamentous support for the capsule includes anterior and posterior radiate
costosternal ligaments.
The sternocostal ligament is an intra-articular ligament.
The costoxiphoid ligament connects the anterior and posterior surfaces of the
seventh costal cartilage to the front and back of the xiphoid process.

25. Interchondral Joints
The 7th through the 10th costal cartilages each articulate with
the cartilage immediately above them.
For the 8th through 10th ribs, this articulation forms the only
connection to the sternum.
The interchondral joints are synovial joints and are supported by a
capsule and interchondral ligaments.
The interchondral articulations, like the CS joints, tend to become
fibrous and fuse with age.

26. Kinematics of the Ribs and Manubriosternum
The movement of the rib cage is a combination of complex geometrics governed by the
types and angles of the articulations, the movement of manubriosternum, and the
contribution of the elasticity of the costal cartilages.
The anterior articulation of rib 1 is larger and thicker than that of any other rib.The first
costal cartilage is stiffer than the other costocartilages. Also, the first CS joint is
cartilaginous (synchondrosis), not synovial, and therefore is firmly attached to the
manubrium. Finally, the first CS joint is just inferior and posterior to the
sternoclavicular joint. For these reasons, there is very little movement of the first rib
at the anterior CS joint.
Posteriorly, the CV joint of the first rib has a single facet, which increases the
mobility at that joint. During inspiration, the CV joint moves superiorly and
posteriorly, elevating the first rib.

27. There is a single axis of motion for the 1st to 10th ribs through the centre of the CV
and CT joints.
This axis for the upper ribs lies close to the frontal plane, allowing thoracic motion
predominantly in the sagittal plane (pump handle movement).
The axis of motion for the lower ribs is nearly in the sagittal plane, allowing for
thoracic motion predominantly in the frontal plane.(Bucket handle movement).
The axis of motion for the 11th and 12th ribs passes through the CV joint only,
because there is no CT joint present. The axis of motion for these last two ribs also
lies close to the frontal plane.

28. During inspiration, the ribs elevate.
In the upper ribs, most of the movement occurs at the anterior aspect of the rib, given the
nearly coronal axis at the vertebrae.
The costocartilage become more horizontal.
The movement of the ribs pushes the sternum ventrally and superiorly.
The excursion of the manubrium is less than that of the body of the sternum because the first rib is
the shortest, with the caudal ribs increasing in length until rib 7. The discrepancy in length causes
movement at the MS joint.The motion of the upper ribs and sternum has its greatest effect by
increasing the anteroposterior (A-P) diameter of the thorax. This combined rib and sternal
motion that occurs in a pre- dominantly sagittal plane has been termed the “pump- handle”
motion of the thorax.

30. Elevation of the lower ribs occurs about the axis of motion lying nearly in the sagittal plane. The
common axis of motion for the upper ribs passes through the centres of the CV and CT joints and
lies nearly in the frontal plane.
The axis through the CV and CT joints for the lower ribs lies closer to the sagittal plane obliquity
increases from1 to rib 10 and an indirect attachment anteriorly to the sternum. These factors allow the
lower ribs more motion at the lateral aspect of the rib cage.
The elevation of the lower ribs has its greatest effect by increasing the transverse diameter of the
lower thorax. This motion that occurs in a nearly frontal plane has been termed the “bucket-handle”
motion of the thorax.
There is a gradual shift in the orientation of the axes of motion from cephalad to caudal; therefore, the
intermediate ribs demonstrate qualities of both types of motion. The 11th and 12th ribs each have
only one posterior articulation with a single vertebra and no anterior articulation to the sternum;
therefore, they do not participate in the closed-chain motion of the thorax.

31. Bucket Handle Movement

33. Muscles Associated with the Rib Cage
The muscles that act on the rib cage are generally referred to as the ventilatory
muscles.
The ventilatory muscles are striated skeletal muscles that differ from other skeletal
muscles in a number of ways:
(1) the muscles of ventilation have increased fatigue resistance and greater
oxidative capacity.
(2) these muscles contract rhythmically throughout life rather than
episodically.(3) the ventilatory muscles work primarily against the elastic
properties of the lungs and airway resistance rather than against gravitational
forces.
(4) neurologic control of these muscles is both voluntary and involuntary.
(5) the actions of these muscles are life sustaining.

34. Any muscle that attaches to the chest wall has the potential to contribute to
ventilation. The recruitment of muscles for ventilation is related to the type
of breathing being performed.
In quiet breathing that occurs at rest, only the primary inspiratory muscles
are needed for ventilation.
During active or forced breathing that occurs with increased activity or with
pulmonary pathologies, accessory muscles of both inspiration and expiration
are recruited to perform the increased demand for ventilation.
The ventilatory muscles are most accurately classified as either primary or
accessory muscles of ventilation.

35. Primary muscles of ventilation
The primary muscles are those recruited for quiet ventilation. These include the
diaphragm, the intercostal muscles (particularly the parasternal muscles),
and the scalene muscles.
These muscles all act on the rib cage to promote inspiration. There are no
primary muscles for expiration, expiration at rest is passive.

36. Diaphragm
The diaphragm is the primary muscle of ventilation, accounting for
approximately 70% to 80% of inspiration force during quiet breathing. The
diaphragm is a circular set of muscle fibers that arise from the sternum,
costocartilages, ribs, and vertebral bodies. The fibers travel cephalad
(superiorly) to insert into a central tendon. The lateral leaflets of the
boomerang- shaped central tendon form the tops of the domes of the right and
left hemidiaphragms. Functionally, the muscular portion of the diaphragm is
divided into the costal portion, which arises from the sternum, costo- cartilage
and ribs, and the crural portion, which arises from the vertebral bodies.

37. The costal portion of the diaphragm attaches by muscular slips to the
posterior aspect of the xiphoid process and inner surfaces of the lower six
ribs and their costal cartilages. The costal fibers of the diaphragm run
vertically from their origin, in close apposition to the rib cage, and then
curve to become more horizontal before inserting into the central tendon. The
vertical fibers of the diaphragm, which lie close to the inner wall of the
lower rib cage, are termed the zone of apposition.
The crural portion of the diaphragm arises from the anterolateral surfaces of
the bodies and disks of L1 to L3 and from the aponeurotic arcuate
ligaments. The medial arcuate ligament arches over the upper anterior part
of the psoas muscles and extends from the L1 or L2 vertebral body to the
transverse process of L1, L2, L3. The lateral arcuate ligament covers the
quadratus lumborum muscles and extends from the transverse process of L1,
L2, or L3 to the 12th rib.

38. During tidal breathing, the fibers of the zone of apposition of the
diaphragm contract, causing a descent of the diaphragm but only a slight
change in the contour of the dome. As the dome descends, the abdominal
contents compress, increasing intra-abdominal pressure.
With a deeper breath, the abdomen, now compressed, acts to stabilize the
central tendon of the diaphragm, With a continued contraction of the costal
fibers of the diaphragm against the central tendon that is stabilized by
abdominal pressure, the lower ribs are now lifted and rotated outwardly in
the bucket-handle motion. As the diaphragm reaches the end of its
contraction, the fibers become more horizontally aligned, and further
contraction no longer lifts the lower rib cage.

39. The action of the crural portion results in a descending of the central tendon,
increasing intra-abdominal pressure. This increased pressure is transmitted
across the apposed diaphragm to help expand the lower rib cage.
The thoracoabdominal movement during quiet inspiration is a result of the
pressures that are generated by the contraction of the diaphragm. When the
diaphragm contracts and the central tendon descends, the increase in
abdominal pressure causes the abdominal contents to be displaced
anteriorly and laterally.
The resultant increase in thoracic size with descent of the diaphragm results
in the decreased intrapulmonary pressure that is responsible for
inspiration .
Exhalation shows a decrease in thoracic size. As the diaphragm returns to its
domed shape, the abdominal contents return to their starting position.

40. Crural Fibers
Costal Fibers
CentralTendon
Zone of apposition

43. Thoracic expansion
Abdominal expansion
anteriolateral
Diaphragmatic descent

44. Compliance
Compliance is a measurement of the distensibility of a structure or system. During
diaphragm contraction, the abdomen becomes the fulcrum for lateral expansion of the rib
cage. Therefore, compliance of the abdomen is a factor in the inspiratory movement of
the thorax.
Compliance =▲volume/pressure. Compliance is the change in volume per unit of
pressure
Increased compliance of the abdomen, as in spinal cord injury in which the abdominal
musculature may not be innervated, decreases lateral rib cage expansion as a result of the
inability to stabilize the central tendon. Without stabilization of the central tendon, the costal
fibers of the diaphragm cannot lift the lower ribs.
Decreased compliance of the abdomen, as in pregnancy, limits caudal diaphragmatic
excursion and causes lateral and upward motion of the rib cage to occur earlier in the
ventilatory cycle.

45. Intercostal Muscles
The external and internal intercostal muscles are categorised as ventilatory muscles.
However, only the parasternal muscles (or portions of the internal intercostals adjacent
to the sternum) are considered primary muscles of ventilation. The internal and external
intercostal and the subcostales muscles connect adjacent ribs to one another and are
named according to their anatomic orientation and location.
The internal intercostal muscles arise from a ridge on the inner surfaces of the 1st
through 11th ribs, and each inserts into the superior border of the rib below. The
fibers of the internal intercostal muscles lie deep to the external intercostal muscles and
run caudally and posteriorly. The internal intercostals begin anteriorly at the
chondrosternal junctions and continue posteriorly to the angles of the ribs, where they
become an aponeurotic layer called the posterior intercostal membrane.

46. External intercostal fibers
The external intercostal fibers run caudally and anteriorly, at an oblique angle to the
internal intercostal muscles. The external intercostal muscles originate on the inferior
borders of the 1st through 11th ribs, and each inserts into the superior border of
the rib below. The external intercostal muscles begin posteriorly at the tubercles of
the ribs and extend anteriorly to the costochondral junctions, where they form the
anterior intercostal membrane.

47. Given these attachments, only the internal intercostal muscles are present
anteriorly from the chondrosternal junctions to the costochondral joints. These are
the segments of the internal intercostal muscles that are referred to as the
parasternal muscle There are only external intercostal muscles present posteriorly
from the tubercle of the ribs to the angle of the ribs. Laterally, both internal intercostal
and external intercostal muscle layers are present and may be referred to in this
location as the interosseous or lateral intercostal muscles.
The subcostal muscles are also intercostal muscles but are generally found only in the
lower rib cage. The subcostal muscles are found at the rib angles and may span more
than one intercostal space before inserting into the inner surface of a caudal rib. Their
fiber direction and action are similar to those of the internal intercostal muscles.

48. Hamberger proposed the simplistic theory that the external intercostal
muscles tend to raise the lower rib up to the higher rib, which is an
inspiratory motion, and the internal intercostal muscles tend to lower
the higher rib onto the lower rib, which is an expiratory motion.
The activation of the intercostal muscles during the ventilatory cycle is
from cranial to caudal, meaning that the recruitment of fibers begins
in the higher intercostal spaces early in inspiration and moves
downward as inspiration progresses. Activation of the lower
intercostal muscles appears to occur only during deep inhalation.30

50. Parasternal muscles
The parasternal muscles, the most anterior portion of the internal
intercostal muscles, are considered primary inspiratory muscles during
quiet breathing. The action of the parasternal muscles appears to be a
rotation of the CS junctions, resulting in elevation of the ribs and
anterior movement of the sternum. The primary function of the
parasternal muscles, however, appears to be stabilization of the rib cage.
This stabilizing action of the parasternal muscles opposes the decreased
intrapulmonary pressure generated during diaphragmatic contraction,
preventing a paradoxical, or inward, movement of the upper chest wall
during inspiration.

51. Subcostal s
External intercostal muscle
Internal intercostal
muscle
Parasternal portion of internal IC

52. Lateral (internal and external) intercostal muscles
The function of the lateral (internal and external) intercostal muscles
involves both ventilation and trunk rotation. The lateral intercostal
muscles, although active during the respiratory cycle, have a relatively
small amount of activity in comparison with the parasternal muscles and
the diaphragm.
The major role of the lateral intercostal muscles is in axial rotation of the
thorax, with the contralateral internal and external intercostal muscles
working synergistically to produce trunk rotation (e.g., right external and
left internal intercostal muscles are active during trunk rotation to the left).

53. Scalene Muscles
The scalene muscles are also primary muscles of quiet ventilation.The scalene muscles
attach on the transverse processes of C3 to C7 and descend to the upper borders of the
first rib (scalenus anterior and scalenus medius) and second rib (scalenus posterior).
Their action lifts the sternum and the first two ribs in the pump-handle motion of the
upper rib cage. Activity of the scalene muscles begins at the onset of inspiration and
increases as inspiration gets closer to total lung capacity. The length-tension relationship
of the scalene muscles allows them to generate a greater force late into the respiratory
cycle, when the force from the diaphragm is decreasing. The scalene muscles also
function as stabilizers of the rib cage. The scalene muscles, along with the parasternal
muscles, counteract the paradoxical movement of the upper chest caused by the
decreased intrapulmonary pressure created by the diaphragm’s contraction.

54. Scalenus anterior
Scalenus medius
Scalenus posterior

56. Accessory Muscles of Ventilation
The muscles that attach the rib cage to the shoulder girdle, head, vertebral
column, or pelvis may be classified as accessory muscles of ventilation. These
muscles assist with inspiration or expiration in situations of stress, such as
increased activity or disease.
When the trunk is stabilized, the accessory muscles of ventilation move the
vertebral column, arm, head, or pelvis on the trunk. During times of increased
ventilatory demand, the rib cage can become the mobile segment. The accessory
muscles of inspiration, therefore, increase the thoracic diameter by moving the
rib cage upward and outward. The accessory muscles of expiration move the
diaphragm upward and the thorax downward and inward.

57. The sternocleidomastoid runs from the manubrium and superior medial
aspect of the clavicle to the mastoid process of the temporal bone. The
usual bilateral action of the sternocleidomastoid is flexion of the cervical
vertebrae. With the help of the trapezius muscle stabilizing the head, the
bilateral action of the sternocleidomastoid muscles moves the rib cage
superiorly, which expands the upper rib cage in the pump-handle motion.
The recruitment of this muscle seems to occur toward the end of a maximal
inspiration.

59. The sternocostal portion of the pectoralis major muscle can elevate the upper rib cage
when the shoulders and the humerus are stabilized. The clavicular head of the
pectoralis major can be either inspiratory or expiratory in action, depending on the
position of the upper extremity. When the arm is positioned so that the humeral
attachment of the pectoralis major is below the level of the clavicle, the clavicular
portion acts as an expiratory muscle by pulling the manubrium and upper ribs
down. With the humeral attachment of the pectoralis major above the level of the
clavicle, such as when the arm is raised, the muscle becomes an inspiratory
muscle, pulling the manubrium and upper ribs up and out.
The pectoralis minor can help elevate the third, fourth, and fifth ribs during a forced
inspiration. The subclavius, a muscle between the clavicle and the first rib, can also
assist in raising the upper chest for inspiration.

61. Posteriorly, the fibers of the levatores costarum run from the transverse processes
of vertebrae C7 through T11 to the posterior external surface of the next lower rib
between the tubercle and the angle and can assist with elevation of the upper ribs.
The serratus posterior superior (SPS) has its superior attachment at the spinous
processes of the lower cervical and upper thoracic vertebrae, and attaches caudally
via four thin bands just lateral to the angles of the second through fifth ribs. The SPS
and the serratus posterior inferior (SPI) have been assumed to be accessory
muscles of respiration based in large part on their anatomical origins and insertions.
The presumed actions would be elevation the ribs by the SPS, and lowering of the
ribs and stabilizing the diaphragm by the SPI.

63. The abdominal muscles (transversus abdominis, internal oblique abdominis,
external oblique abdominis, and rectus abdominis) are expiratory muscles,
as well as trunk flexors and rotators. The major function of the abdominal
muscles with regard to ventilation is to assist with forced expiration. The
muscle fibers pull the ribs and costocartilage caudally, into a motion of
exhalation. By increasing intra-abdominal pressure, the abdominal
muscles can push the diaphragm upward into the thoracic cage, increasing
both the volume and speed of exhalation.

65. Although considered accessory muscles of exhalation, the abdominal muscles play
two significant roles during inspiration.
First, the increased intra-abdominal pressure created by the active abdominal
muscles during forced exhalation pushes the diaphragm cranially and exerts a
passive stretch on the costal fibers of the diaphragm. These changes prepare the
respiratory system for the next inspiration by optimizing the length- tension
relationship of the muscle fibers of the diaphragm.
Second, the increased abdominal pressure created by lowering of the diaphragm in
inspiration must be countered by tension in the abdominal musculature. Without
sufficient compliance in the abdominal muscles, the central tendon of the
diaphragm cannot be effectively stabilized so that lateral chest wall expansion
occurs. During periods of increased ventilatory needs, the increased muscular
activity of the abdominal muscles assists in both exhalation and inhalation.

66. The transversus thoracis (triangularis sterni) muscles are a flat layer of muscle
that runs deep to the parasternal muscles. The transversus thoracis muscles
originate from the posterior surface of the caudal half of the sternum and run
cranially and laterally, inserting into the inner surface of the costal cartilages of the
third through seventh ribs.These muscles are recruited for ventilation along with
the abdominal muscles to pull the rib cage caudally. Studies have shown that
these muscles are primarily expiratory muscles, especially when expiration is
active, as in talking, coughing, or laughing, or in exhalation into functional
residual capacity.

67. Gravity acts as an accessory to ventilation in the supine position.
Gravity, acting on the abdominal viscera, performs the same
function as the abdominal musculature in stabilizing the central
tendon of the diaphragm. In fact, in the supine position, the
abdominal muscles and the trangularis sterni are silent on the
EMG monitoring during quiet breathing.

68. Ventilatory Sequence During Breathing
Although the coordinated function and sequence of breathing are complex
when activities are combined, the following sequence of motions and
muscle actions is typical of a healthy person at rest during quiet breathing.
The diaphragm contracts, and the central tendon moves caudally.
The parasternal and scalene muscles stabilize the anterior upper chest
wall to prevent a paradoxical inward movement caused by the decreasing
intrapulmonary pressure.
As intra-abdominal pressure increases, the abdominal contents are
displaced in such a way that the anterior epigastric abdominal wall is
pushed ventrally.

69. Further outward motion of the abdominal wall is countered by the abdominal
musculature, which allows the central tendon to stabilize on the abdominal viscera.
The appositional (costal) fibers of the diaphragm now pull the lower ribs cephalad and
laterally, which results in the bucket-handle movement of the lower ribs.
With continued inspiration, the parasternal, scalene, and levatores costarum muscles
actively rotate the upper ribs and elevate the manubriosternum, which results in an
anterior motion of the upper ribs and sternum (Pump-handle movement).
The lateral motion of the lower ribs and anterior motion of the upper ribs and sternum
can occur simultaneously.
Expiration during quiet breathing is passive, involving the use of the recoil of the
elastic components of the lungs and chest wall.

70. Developmental Aspects of Structure and Function
The compliance, configuration, and muscle action of the chest wall changes significantly from the infant to the elderly person. The newborn has a cartilaginous, and therefore extremely compliant, chest wall that allows the distortion necessary for the infant’s thorax to travel through the birth canal. The increased compli- ance of the rib cage is at the expense of thoracic stabil- ity. The infant’s chest wall muscles must act as stabilizers, rather than mobilizers, of the thorax to counteract the reduced intrapulmonary pressure cre- ated by the lowered diaphragm during inspiration. Complete ossification of the ribs does not occur for several months after birth.
Differences Associated with the Neonate
The compliance, configuration, and muscle action of the chest wall
changes significantly from the infant to the elderly person. The
newborn has a cartilaginous, and therefore extremely compliant,
chest wall that allows the distortion necessary for the infant’s thorax
to travel through the birth canal. The increased compliance of the rib
cage is at the expense of thoracic stability. The infant’s chest wall
muscles must act as stabilizers, rather than mobilizers, of the thorax
to counteract the reduced intrapulmonary pressure created by the
lowered diaphragm during inspiration. Complete ossification of the ribs
does not occur for several months after birth.

71. Whereas the ribs in the adult thorax slope down- ward and the diaphragm
is elliptically shaped, the rib cage of an infant shows a more horizontal
alignment of the ribs (circular shape), with the angle of insertion of the
costal fibers of the diaphragm also more horizontal than those of the adult .
There is an increased tendency for these fibers to pull the lower ribs
inward, thereby decreasing efficiency of ventilation and increasing
distortion of the chest wall. There is very little motion of the rib cage
during tidal breathing of an infant.Only 20% of the muscle fibers of the
diaphragm are fatigue-resistant fibers in the healthy newborn, in comparison
with 50% in the adult. This discrepancy predisposes infants to earlier
diaphragmatic fatigue.

72. Accessory muscles of ventilation are also at a disadvantage in the infant. Until
infants can stabilize their upper extremities, head, and spine, it is difficult for
the accessory muscles of ventilation to produce the action needed to be
helpful during increased ventilatory demands.
As the infant ages and the rib cage ossifies, muscles can begin to
mobilize rather than stabilize the thorax. As the infant gains head
control, he is also gaining accessory muscle use for increased
ventilation. As the toddler assumes the upright position of sitting and
standing, gravitation forces and postural changes allow for the
anterior rib cage to angle obliquely downward.

73. The elliptical thorax allows for a greater bucket- handle
motion of the rib cage. The attachments for the muscles of
ventilation move with the increasingly angled ribs, improving
their action on the thorax. Throughout childhood, the numbers
of alveoli and airways continue to increase. In early
adolescence, the sizes of the alveoli and airways continue to
expand, as demonstrated by increases in pulmonary function
test results

74. Differences Associated with the Elderly
Skeletal changes that occur with aging affect pulmonary function. Many of the
articulations of the chest wall undergo fibrosis with advancing age.
The inter- chondral and costochondral joints can fibrose, and the chondrosternal
joints may be obliterated.
The xiphosternal junction usually ossifies after age 40.
The chest wall articulations that are true synovial joints may undergo
morphologic changes associated with aging, which results in reduced mobility.
The costal cartilages ossify, which interferes with their axial rotation..

75. Overall, chest wall compliance is significantly reduced with age.
Reduction in diaphragm-abdomen compliance has also been reported and is at
least partially related to the decreased rib cage compliance, especially in the
lower ribs that are part of the zone of apposition.
Aging also brings anatomical changes to the lung tissue that affect the function of
the lungs.
The airways narrow, the alveolar duct diameters increase, and there are
shallower alveolar sacs.
There is a reorientation and decrease of the elastic fibers, Overall, there is a
decrease in the elastic recoil and an increase in pulmonary compliance.54

76. If the lungs retain more air at the end of exhalation, there will be a decrease in
inspiratory capacity of the thorax.
Functionally, the changes result in a decrease in the ventilatory reserve
available during times of need, such as during an illness or increased activity.
Because the resting position of the thorax depends on the balance between the
elastic recoil properties of the lungs pulling the ribs inwardly and the
outward pull of the bones, cartilage, and muscles, the reduced recoil
property of the lung tissue allows the thorax to rest with an increased A-P
diameter (a relatively increased inspiratory position).

77. An increased kyphosis is often observed in older individuals, which decreases
the mobility not only of the thoracic spine but also of the rib cage.
The result of these skeletal and tissue changes is an increase in the amount of air
remaining in the lungs after a normal exhalation (i.e., an increase in functional
residual capacity).
If the lungs retain more air at the end of exhalation, there will be a
decrease in inspiratory capacity of the thorax.
Functionally, the changes result in a decrease in the ventilatory
reserve available during times of need, such as during an illness or
increased activity.

78. Muscles of ventilation of the elderly person have a documented loss of strength,
fewer muscle fibers, a lower oxidative capacity, a decrease in the number or
the size of fast-twitch type II fibers, and a lengthening of the time to peak
tension.
The resting position of the diaphragm becomes less domed, with a decrease in
abdominal tone in aging.There is an early recruitment pattern for accessory
muscles of ventilation. For example, the transverse thoracic muscles are active
during quiet expiration in older subjects in the standing position.41

79. Pathological Changes in Structure and Function
In scoliosis, a change in the musculoskeletal structure renders a change to ventilation.
Changes in the pulmonary system can affect the biomechanics of the thorax Eg:Chronic Obstructive
Pulmonary Disease
CHRONIC OBSTRUCTIVE PULMONARY DISEASE
The major manifestation of COPD is damage to the airways and destruction of the alveolar
walls.
As tissue destruction occurs with disease, the elastic recoil property of the lung tissue is
diminished.
Passive exhalation that depends upon this elastic recoil property becomes ineffective in
removing air from the thorax.
Air trapping and hyperinflation occur.
The static position of the thorax changes as more air is now housed within the lungs at the
end of exhalation. This affects the lung volume and ventilatory capacities.

80. The static resting position of the thorax is a function of the balance between the
elastic recoil properties of the lungs pulling inward and the normal outward
spring of the rib cage.
In COPD, there is an imbalance in these two opposing forces. As elasticity decreases,
an increase in the A-P diameter (more of a barrel shape) of the hyper-inflated
thorax is apparent, along with flattening of the diaphragm at rest. The range of
motion, or excursion, of the thorax is limited. Although the basic problem in COPD
is an inability to exhale, it is clear that inspiratory reserve is compromised.

81. Hyperinflation affects not only the bony components of the chest wall but also the
muscles of the thorax.
The fibers of the diaphragm are shortened, decreasing the available range of
contraction.
The angle of pull of the flattened diaphragm fibers becomes more horizontal with a
decreased zone of apposition. In severe cases of hyperinflation, the fibers of the
diaphragm will be aligned horizontally. Contraction of this very flattened diaphragm
will pull the lower rib cage inward, actually working against lung inflation.

82. With compromise of the diaphragm in COPD, the majority of inspiration is
now performed by other inspiratory muscles that are not as efficient as the
diaphragm.
The barrel-shaped and elevated thorax puts the sternocleidomastoid muscles
in a shortened position, making them much less efficient. The parasternal and
scalene muscles are able to generate a greater force as the lungs approach total
lung capacity; consequently, hyperinflation has a less dramatic effect on them.

86. The diaphragm has a limited ability to laterally expand the rib cage, and so inspiratory
motion must occur within the upper rib cage.
In a forceful contraction of the functioning inspiratory muscles of the upper rib cage, the
diaphragm and the abdominal contents actually may be pulled upward.
This is a paradoxical thoracoabdominal breathing pattern because the abdomen is
pulled inward and upward during inspiration, and is pushed back out and down during
exhalation.
The paradoxical pattern is a reflection of the maintained effectiveness of the upper
inspiratory ribcage musculature and the reduced effectiveness of the diaphragm.
The disadvantages of these biomechanical alterations of hyperinflation are compounded by
the increased demand for ventilation in COPD. More work is required of a less effective
system.
The energy cost of ventilation, or the work of breathing, in COPD is markedly increased.

87. Scoliosis
Scoliosis is a pathologic lateral curvature of the spine, frequently associated
with rotation of the vertebrae. A right thoracic scoliosis (named by the side of
the convexity of the curve) results in left lateral flexion of the thoracic spine.
The coupled rotation in a typical right thoracic scoliosis causes the bodies of
the vertebrae to rotate to the right and the spinous processes to rotate left.
The right transverse processes of the vertebrae rotate posteriorly, carrying the
ribs with them. This is the mechanism causing the classic posterior rib hump
of scoliosis. On the concave side of the scoliotic curve, the effects are just the
opposite. The left transverse processes of the vertebrae move anteriorly,
bringing the articulated ribs forward. The rib distortion that results from the
vertebral rotation is evident bilaterally .

88. These musculoskeletal abnormalities limit range of motion of the chest cage and
the spine and, therefore, decrease ventilation abilities.Not only do the anatomical
changes that occur in scoliosis alter the alignment and motion of the thorax, but also
there is a consequence to the length- tension relationship and the angle of pull of the
muscles of ventilation. On the side of the convexity, with sufficient curvature, the
intercostal space is widened and the intercostal muscles are elongated. On the
side of the concavity, the ribs are approximated and the intercostal muscles are
adaptively shortened. Lung volumes and capacities are reduced from those in
someone without thoracic deformity, as a result of the altered biomechanics of the
scoliotic thorax. Uneven shoulder, one shoulder blade that sticks out more than the
other rib hump, uneven hips, a rotating spine problems breathing because of reduced
area in the chest for lungs to expand, back pain are some clinical findings.

89. Cobb Angle
The Cobb Angle is used as a standard measurement to determine and track the progression
of scoliosis. Dr John Cobb invented this method in 1948.
The angle of curvature be measured by drawing lines parallel to the upper border of the
upper vertebral body and the lower border of the lowest vertebra of the structural curve,
then erecting perpendiculars from these lines to cross each other, the angle between these
perpendiculars being the ‘angle of curvature’.
Generally, it takes at least 10 degrees of deviation from straight before scoliosis is defined.
A Cobb angle of 20 degrees usually requires that a back brace is worn and that you or your
child undergo intensive physical therapy. Once the Cobb angle reaches 40 degrees,
surgery is considered. Often a spinal fusion is done to force the curve to stop developing.

91. • Other methods, scoliometer, scoliotic index, Risser-Ferguson method,
Radiography, plumb line examination.