1. The Response of Biological Tissue to Stress
Chapter 4
The Response of Biological Tissue to Stress

2. Overview
A wide range of external and internal forces are either generated or resisted by the human body during the course of daily activities
Biological tissues must demonstrate the ability to withstand excessive or repetitive stresses if musculoskeletal health is to be maintained

3. Stress
The capacity of a tissue to withstand stress is dependent on a number of factors:
Age
The proteoglycan and collagen content of the tissue
The ability of the tissue to undergo adaptive change
The speed at which the adaptive change must occur

4. Terminology
Kinetics - the study of forces that arise as motions change
Mass - the quantity of matter composing a body
Inertia - the resistance to action or to change
Force - a vector quantity, with magnitude, direction and point of application to a body

5. Terminology Load - the type of force applied
Stress - the force per unit area that develops on the cross section of a structure in response to an externally applied load
Strain - the deformation that develops within a structure in response to externally applied loads
Hysteresis - the difference in the behavior of a tissue when it is being loaded versus unloaded

6. Load-deformation curve
The load-deformation curve, or stress-strain curve, of a structure depicts the relationship between the amount of force applied to a structure and the structure’s response in terms of deformation or acceleration

7. Load-deformation curve
The shape and position of the load-deformation curve depends on a number of factors:
Stiffness
Viscoelasticity
Age
Exercise

8. Levers
First Class: occurs when two forces are applied on either side of an axis in the fulcrum lies between the effort and the load. E.g. a seesaw
Second-class: occurs when the load is applied between the fulcrum and the point where the effort is exerted. E.g. the wheelbarrow
Third class: occurs when the load is located at the end of the lever. E.g. flexion at the elbow

9. Musculoskeletal stress
Macrotrauma - an acute stress (loading) that occurs when a single force is large enough to cause injury of biological tissues
Microtrauma - a repetitive stress that in of itself is insufficient to damage the tissue, causes injury when repeated over a period of time

10. Collagen
Collagen fibers have a wavy or folded appearance at rest (slack)
When a force lengthens the collagen fibers this slack is taken up
This slack is called the tissue’s crimp
Crimp is different for each type of connective tissue and this provides each of these tissues with different viscoelastic properties

11. Articular cartilage
Articular cartilage is a viscoelastic structure with a very high tensile strength and is resistant to compressive and shearing forces
Articular cartilage has the ability to undergo large deformations while still being able to return to its original shape and dimension

12. Articular cartilage
Damage to articular cartilage may result from microtrauma (degeneration), macrotrauma, or an inflammatory process
Degeneration: osteoarthritis
Primary and secondary
Inflammation: Rheumatoid arthritis

13. Ligament
Fibrous bands of dense connective tissue that connect bone to bone and which behave as a viscoelastic structures when exposed to stress
Ligament injuries are called sprains

14. Tendon Connects muscle to bone
The causes of a tendon injury center around microtrauma to the tendon tissue due to repetitive mechanical loading from external factors, or macrotrauma

15. Tendinitis
The term tendinitis implies an inflammatory reaction to a tendon injury - a microscopic tearing and inflammation of the tendon tissue, commonly resulting from tissue fatigue rather than direct trauma

16. Tenosynovitis
Tenosynovitis/tenovaginitis, peritendinitis, and paratenonitis, indicate an inflammatory disorder of tissues surrounding the tendon such as the tendon sheath – usually the result of a repetitive friction of the tendon and its sheath

17. Tendinosis
The term tendinosis refers to a degenerative process of the tendon.
Characterized by the presence of dense populations of fibroblasts, vascular hyperplasia, and disorganized collagen

18. Bone Bone is a solid with elastic properties
Bone is stiffer and stronger than other tissues at higher strain levels
Bone is better able to withstand compressive forces than tensile or torsional forces

19. Bone
Wolff’s law - forces applied to bone, including muscle contractions and weight bearing can alter bone the internal and external configuration of bone through adaptation to these stresses

20. Bone
If the adaptations of bone to stress do not occur fast enough, the bone is resorbed faster than it is replaced, and bone strength is compromised
Causes of decreased adaptation include:
An increase in the applied load
An increase in the number of applied stresses
A decrease in the size of the surface area over which the load is applied

21. Muscle tissue Muscle injury can result from: Excessive strain
Excessive tension
Contusions
Lacerations
Thermal stress
Myotoxic agents (local anesthetics, excessive use of corticosteroids, snake and bee venoms)

22. Hematoma Contusion to a muscle belly Two types:
Intramuscular: associated with a muscle strain or bruise. The size of the hematoma is limited by the muscle fascia
Intermuscular. This type of hematoma develops if the muscle fascia is ruptured and the extravasated blood spreads into the interfascial and interstitial spaces

23. Immobilization
Continuous immobilization of connective and skeletal muscle tissues can cause some undesirable consequences to the tissues of the musculoskeletal system

24. Immobilization The undesirable consequences include:
Cartilage degeneration
A decrease in the mechanical and structural properties of ligaments
A decrease in bone density
Weakness or atrophy of muscles