Biomechanics of the Thorax

Original Editor - Sonal Joshi

Top Contributors - Sonal Joshi and Manisha Shrestha  

Introduction[edit | edit source]

Biomechanics is an area of science that uses principles of physics to measure and study, how forces interact with and affect living body[1]. These forces could be Internal i.e. within the body or External i.e. outside the body.

To understand the biomechanics of thorax we will need to study the basic anatomy, joint articulations, muscles associated in the thorax along with kinematics of the rib cage.

The thorax is formed by the thoracic vertebrae ,the ribs and the sternum.

Parts of Thorax

It has two main functions,

  • Providing anchor for muscle attachments
  • Major role of ventilation

Structure of Thorax[edit | edit source]

The main three structures mentioned above work in a coordinated motion in thorax to produce respiration.

Anatomy[edit | edit source]

  • The rib cage is a system of various bones and muscle. Bones involved are sternum, 12 pairs of Ribs, 12 Thoracic vertebrae.

It is like a compartment sealed by various structures from each side.

  • Refer to the diagram of anterior, posterior and lateral view of thorax to better understand the bones involved in the ribcage.
  • Thorax -Lateral View
    They can be described as[1],
Thorax - Posterior View
Thorax -Anterior View
Side of compartment Contents
Posterior-laterally - Thoracic vertebrae

- Ribs

- Intercoastal muscles and membrane

Anteriorly - Coastal cartilages

- Sternum

-Intercoastal muscles and membrane

Superiorly - Upper ribs and clavicle

- Cervical fascia surrounding esophagus and trachea

Inferiorly -Diaphragm muscle
  • The sternum consists of manubrium, body and xiphoid process. Its details can be referred to on page : sternum.
  • The ribs are from T1 to T12 region
  • Refer the Thoracic Anatomy for further details of Thoracic vertebrae & the joint which they form.

Articulations[2][edit | edit source]

Sternum -Articulations
Joint Bones involved in the joint articulation Special features
1. Manubriosternal Manubrium and superior part of body of Sternum - Synchondrosis type of joint

- Also called Angle of Louis

2. Xiphisternal Xiphoid process and inferior part of body of Sternum - Synchondrosis type of joint

- Ossifies by 40 to 50 yrs. of age

3. Typical Costovertebral (CV) Head of ribs 2 to 9, two adjacent vertebral bodies and

interposed intervertebral disk

- Synovial type of joint
4. Atypical Costovertebral Head of ribs 1, 10 to 12 and corresponding vertebral


- More mobile than

typical CV joint

5. Costotransverse Costal tubercle of rib 1 to 10 and costal facet on

transverse process of corresponding vertebrae

- Synovial type of joint
6. Costochondral 1 to 10th rib with costal cartilage - Synchondrosis type of joint

- Have no ligamentous support

7. Costosternal

(Refer diagram below)

Costal cartilage of ribs 1 to 7 with sternum -Joints with 1, 6 and 7 ribs

are synchondroses type.

-Joints with 2 to 5 ribs are

synovial type.

8. Interchondral 7 to 10 costal cartilage with cartilage above it - Synovial type of joint

Function of Thorax[edit | edit source]

Kinematics[edit | edit source]

The motion of the ribs in conjunction with sternum and thoracic vertebrae helps produce the movements of respiration. These consist of inspiration and expiration. Thoracic kinematics consists of understanding the changes in intrathoracic volume during ventilation. These occur due to complex, synchronized activity of the ribs along with sternum and primary muscles of respiration. It is determined by[2]

  • Type & angles of articulations
  • movement of manubrium sternum
  • elasticity of costal cartilage

The kinematics involved in the thorax can be further classified into[1],

  • Changes in vertical diameter
  • Changes in anterior-posterior diameter (Pump handle motion)
  • Changes in transverse diameter (Bucket handle motion)
Changes in Vertical diameter[1][edit | edit source]
  • This is mainly due to excursion of Diaphragm muscle.
  • During inspiration : Diaphragm contracts -> lowering of the dome of diaphragm -> increase in vertical diameter of thorax.
  • During expiration : Diaphragm relaxes -> dome recoils upward to resting position -> decrease in vertical diameter of thorax.
Changes in A-P and transverse diameter[2][edit | edit source]
  • There is a single axis of motion for 1 to 10 ribs through the center of their costovertebral(CV) and costotransverse(CT) joints. This influences the motion of these ribs during the process of respiration
  • The two main motions occurring in thorax during ventilation can be explained as below,
Refer to the video shown below to understand the kinematics of thorax better,
Changes in Anterior-Posterior diameter Changes in Transverse diameter
Mainly motion of Upper ribs Lower ribs
common Axis of motion oriented nearly in frontal plane nearly in sagittal plane
Thoracic motion occurs in sagittal plane in frontal plane
During Inspiration
  • Upper ribs elevate
  • Motion is of anterior part of the ribs
  • It pushes sternum ventrally and superiorly
  • Total motion of sternum and upper ribs together increases in anterior-posterior diameter of thorax
  • Changes produced mainly by muscles laterally placed in the ribcage[4]
  • Motion of sternum resembles pump handle movement, hence the name.
  • Lower ribs elevate
  • Motion is more in the lateral part of ribs due to its more angled shape and indirect attachment of the ribs
  • Total motion of lower ribs together increases the transverse diameter of thorax
  • Changes produced mainly by muscles anteriorly & posteriorly placed in the ribcage[4]
  • Motion of the lower ribs resembles bucket handle movement, hence the name.

Muscles involved in respiration[edit | edit source]

  • To bring about the intrathoracic volume changes during respiration, muscles of respiration play a crucial role. The muscles can be divided into primary and accessory depending on type of breathing i.e. quite or forced, in which they are used.
Forceful breathing.jpg
Quiet breathing.jpg
  • The detailed function of each muscle can be accessed by clicking on the muscle name mentioned below. Each has been linked to independent physiopedia pages dedicated to explaining them.
Primary Muscles[edit | edit source]
Accessory Muscles[edit | edit source]

References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 Neumann DA. Kinesiology of the musculoskeletal system-e-book: foundations for rehabilitation. Elsevier Health Sciences; 2016 Nov 3.
  2. 2.0 2.1 2.2 Pamela K. Levangie, Cynthia C. Norkin, 2005, Joint structure and function: A comprehensive analysis, 4th. Edn, Philadelphia, FA Davis publishers.
  3. MedicoPhysio Actors.Pump handle and Bucket handle motion.Available from:[last accessed 16/05/2022]
  4. 4.0 4.1 Loring SH. Action of human respiratory muscles inferred from finite element analysis of rib cage. Journal of Applied Physiology. 1992 Apr 1;72(4):1461-5.
Original Editor - [[User:Manisha Shrestha|Manisha Shrestha] Top Contributors - Sonal Joshi and Manisha Shrestha