The document covers the basics of biomechanics, emphasizing its interdisciplinary nature and importance in understanding human movement, injury treatment, and sports performance. It outlines key concepts and principles, such as kinematics, kinetics, and Newton's laws, alongside the intricacies of muscular anatomy and muscle action. Additionally, it presents nine fundamental principles of biomechanics that inform movement analysis and contribute to the development of effective athletic techniques.

1. CHAPTER-4
BIOMECHANICS

2. COURSE OUTCOME
At the end of the unit, you would be able to understand
o Key mechanical concepts
o 9 fundamentals of biomechanics
o Biomechanics of passive muscle tendon unit
o Biomechanics of bone
o Biomechanics of ligaments

3. WHAT IS BIOMECHANICS
 Video 0

4. NECESSITY
5
4
3
2
1
Highly interdisciplinary
Treat injuries
Replicate human motion
Study human behaviour
Posture and form

5. INTRODUCTION
 Video 1

6. WHY STUDY BIOMECHANICS
 Highly interdisciplinary field with people from mechanical,
computer science, electronics all come together
 Helps to understand human movements
 Useful for treating sports injury (physiotherapy)
 Useful to know the right postures when doing exercises to
remove stress and strain
 Useful to replicate human motion in AI and robotics
 Study human behavior by studying their movement (gait
analysis)
 Useful if one wants to work on material sciences and human
implants
 Work in collaboration with other departments for medical device
design

7. APPLICATION BIOMECHANICS
 Video 1.1 games
 Video 1.2 Sports
 Video 1.3 Movies

8. WHY STUDY BIOMECHANICS?
 The purpose of studying Biomechanics is;
 To understand the forces acting on the human body.
 To manipulate these forces in treatment procedures so that human
performance may be improved and further injury may be prevented.
 Better understanding of both joint function and dysfunction
 To understand how the musculoskeletal system functions.
 Useful in patient evaluations and treatments.
 Important for clinicians such as orthopaedic surgeons and physical
and occupational therapists

9. BIOMECHANICS
• Kinesiology is the scholarly study of human movement.
• Kinesiology = biomechanics + anatomy+ physiology+neuroscience
• “Biomechanics has been defined as the study of the movement of living
things using the science of mechanics (Hatze, 1974)”.
• Biomechanics is the science of movement of a living body, including how
muscles, bones, tendons, and ligaments work together to produce
movement.
• Involves the principles of anatomy and physics in the descriptions and
analysis of movement.
• How system and structures react to external force

10. Biomechanics
Kinematics
study of motion
(change in position) of
a body or object
without reference to
causes of motion
(forces)
Linear motion Angular motion General motion
Kinetics
Effect of forces
involved in the
movement of an object
or body
Linear force Angular torque

11. FOUNDATIONS OF MOVEMENT
 To move, the human body goes through a complex series of interactions
that involve different body systems. Communication between the
muscular, skeletal, and nervous systems all come into play.
 Skeletal system: The human skeleton is the
framework that supports the human body. Without
bones, you could not stand, sit, or walk.
 Muscular system: Muscles are connected to
bones. When a muscle contracts, it often produces
movement around a joint and also supports the
body and increases stability.
 Nervous system: Every movement you make is
controlled by the nervous system, or brain-body
connection.

12. BIOMECHANICS & NEWTON’S LAWS OF MOTION
 Law of inertia: An object at rest will stay at rest and an object in motion will stay in
constant linear motion unless acted on by an outside force. If a ball is rolling, it will
keep rolling forever unless something stops it. In the same way, if a ball is at rest, it
will stay at rest until a force pushes it to move.
 Law of acceleration: The acceleration of an object is directly proportional to the
force acting on it and inversely proportional to the mass of the object. As the force
acting upon an object increases, the acceleration of the object also increases. As
the mass of an object increases, the acceleration of the object decreases for a
fixed force.
 Law of action and reaction: For every action there is an equal and opposite
reaction. When one object exerts a force on a second object, the second object
reacts by exerting an equal force in the opposite direction on the first object. For
example, when you walk, your body pushes down on the ground to move yourself
forward. The ground pushes back against you with the same magnitude of force,
propelling you forward.

13. NINE FUNDAMENTALS OF BIOMECHANICS
 Biomechanists measure all kinds of linear and angular mechanical
variables to document and find the causes of human motion.
 There are nine principles of biomechanics constitute the minimum number
or core principles that can be applied to all human movements.
 These are used to develop generic biomechanical principles for all human
movements.
 They provide a simple paradigm or structure to apply biomechanical knowledge.
 Each can be directly linked to the concepts and laws of biomechanics.

14.  Human to outside world
 Video 1.4

15. B(alance) I(nertia) F(orce-Motion) F(orce-Time)
C R O S S
(oordination (ange) (ptimal) (egmental) (pin)
Continuum) projection interaction

16. 1. BALANCE
 The degree of control over stability/ instability.
 Balance is a person's ability to,
 control their body position relative to some base of
support.
 Stability and mobility of body postures are inversely
related, and
 several biomechanical factors are involved in
manipulating a person's stability and mobility.
 To increase stability increase base of support, lower center of gravity
increase mass of the body.
 Line of gravity should fall in the middle of your base of support for max
stability.

17. Factors that affect Balance
1. Mass (more mass, more is the force required to unstabilize it)
2. Base Of Support- larger base of support is more stable.
3. C.O.G- Closer the COG to the BOS, more is the stability
4. Line of gravity- If it crosses through BOS, more stable

18. BALANCE
 Video 2 COG
 Video 2.1 Fossbury jump

19. 2. INERTIA
 Inertia can be defined as the property of all objects to resist
changes in their state of motion (either reset or in motion).
 Defined by Newton's first law of motion.
 As we have linear motion and angular motion, we have linear
and angular measures of inertia.
 The heavier the mass, more force it requires to be moved.
 Moment of Inertia refers to the resistance of rotational
motion.

20. 3. FORCE-MOTION
 Force are required to change the state of motion.
 Unbalanced forces are acting on our bodies when we create or modify movement.
 An important thing to notice in this principle is the sequence of events.
 Forces must act first, before changes in motion can occur.
 Relates to newtons 3 laws.
Example
Standing still- forces acting on a person are equal and because of this there is no movement.

21. 2. FORCE-TIME
 Substantial changes in motion do not instantly occur
but are created over time, which leads to the
principle of Force–Time.
 Impulse=Force X Time.
 The greater the time of which force is applied the
greater the resulting motion.
 The impulse–momentum relationship, the original
language of Newton’s second law, is the
mathematical explanation of this important principle.
Example of Force-Time
 Using the sweep shot in hockey. More force and time
is applied giving it much more power then a hit.

23. 4. COORDINATION CONTINUUM
 Organization between simultaneous sequential actions.
 Involves sequencing and timing bodies actions to create movement.
 High levels of force are effectively created through simultaneous segmental movements while
lower force and high speed movements require a sequential pattern of movement to be effective.
 The continuum contains simultaneous at one end and sequential at another. Most actions fall
somewhere in between.

24. 6. RANGE OF MOTION
 Body motion used in a
movement
 Relates to the type of motion
(linear or angular) of body
segments used to create
movement.
 Reduced R.O.M is best for low
force and high accuracy actions
while increased R.O.M is
required for speed and force
production.
 Example of Range Of Motion
 Reduced R.O.M= Throwing a
dart.
 Increased R.O.M= Throwing a
javelin

25. 8. OPTIMAL PROJECTION
 Impact of release conditions that optimize performance.
 There is an optimal angle of projection to achieve a
specific goal.
 Biomechanical research shows that optimal angles of
projection provide the right compromise between vertical
velocity and horizontal velocity within the typical
conditions encountered in many sports.
 Maximum speed/distance of an optimal angle=45
degrees.
 Parameters that needs to be considered:
 Velocity (horizontal and vertical)
 Angle of release
 Height of release

26. PROJECTILE
 Matlab

27. 5. SEGMENTAL INTERACTIONS
 The forces acting in a system of linked rigid bodies can be transferred through
the links and joints .
 Using your body parts in order to create maximum power in a shot.
 Begins with the largest, slowest, strongest segments and works through to the
slowest and fastest segments.
 Example of Segmental Interactions
 Golf shot.
 Tennis shot

28. 9. SPIN
 Projectile rotation to stabilize flight and adjust trajectory
(particularly sport balls).
 Spin will cause an object to lift.
 Lift will cause a curve and counter the affects of gravity.
Example of Spin
 A tennis player putting a top spin on a ball to
make it drop quicker.
 A volleyball player performing a jump serve
should strike above the center of the ball to
impart topspin to the ball.

29. SPIN
 Video 3 cricket
 Video 3.1 football

30. MUSCULAR ANATOMY
 Muscular system consists of three muscle types: cardiac, smooth, and skeletal.
 Skeletal muscle most abundant tissue in the human body (40-45% of total body weight).
 Human body has more than 430 pairs of skeletal muscle; most vigorous movement
produced by 80 pairs.
 Skeletal muscles provide strength and protection for the skeleton, enable bones to
move, provide the maintenance of body posture against gravity.
 Skeletal muscles perform both dynamic and static work
 Dynamic: locomotion & positioning of segments
 Static: maintains body posture

31. TYPES OF MUSCLES
 In the body, there are three types of muscle: skeletal (striated), smooth,
and cardiac.
 Skeletal Muscle
 Skeletal muscle, attached to bones, is responsible for skeletal
movements.
 These muscles are under conscious, or voluntary, control.
o Smooth Muscle
 Smooth muscle, found in the walls of the hollow internal organs
such as blood vessels, the gastrointestinal tract, bladder,
and uterus, is under control of the autonomic nervous system.
 Smooth muscle cannot be controlled consciously and thus acts
involuntarily.
 The non-striated (smooth) muscle cell is spindle-shaped and has
one central nucleus. Smooth muscle contracts slowly and
rhythmically.
o Cardiac Muscle
 Cardiac muscle, found in the walls of the heart, is also under
control of the autonomic nervous system.
 The cardiac muscle cell has one central nucleus, like smooth
muscle, but it also is striated, like skeletal muscle.
 The cardiac muscle cell is rectangular in shape.
The contraction of cardiac muscle is involuntary, strong, and
rhythmical.

32. MUSCLE ANATOMY
 Skeletal muscles
 Covered by protective layer of connective tissue –
epimysium (protects from friction)
 Muscles made of thousands of bundles called
muscle fibre bundles/fascicles
 Each bundle covered by another connective tissue
perimysium
 Each bundle contains thousands of muscle fibres,
which is covered by endomysium
 Endomysium is connected with blood vessels and
ion exchange channels.Mostly made up of
collagen.
 Allows automatic gliding during muscle contraction
 Each muscle fibre made of myofibrils which
contains repeating units of sarcomeres
 Each sarcomere contains thick (myosin) and thin
(actin) filaments.

33. REVIEW OF MUSCLE STRUCTURE
 Muscle fibers are some of the
largest cells in the body and are
long cylindrical structures with
multiple nuclei.
 Besides many nuclei there are
hundreds to thousands of smaller
protein filaments called
myofibrils in every muscle fiber.
 These small sections of a
myofibril between two Z lines
(thin dark band) are called
sarcomeres.
 Sarcomeres are the basic
contractile structures of muscle.
 These contain the sarcolemma

34. MOLECULAR STRUCTURE OF MYOFIBRIL
 Myosin composed of individual molecules each
has a globular head and tail
 Head binds with actin and moves along with it
 Cross-bridge: actin & myosin overlap (A band)
 Actin has double helix; two strands of beads
spiraling around each other
 troponin & tropomysin regulate making and
breaking contact between actin & myosin
 During resting stage, Tropomyosin covers
actin’s active sites so that actin-myosin
interaction does not take place.
 When impulses reach the muscle sarcolemma,
Ca2+ channels open and Ca2+ ions interact
with tropomyosin and allows the formation of
actin-myosin cross links.
 Movement of myosin heads over actin filaments
based on ATP activity causes muscle
contraction.

35. ATP –ADP CYCLE IN MUSCLE MOTION

36. MUSCLE CONTRACTION THEORY
 Video 4
 Video 4.1

37. MUSCLE ACTION
 Muscle forces are the main internal motors and brakes for human
movement.
 Gravity and other external forces can be used to help us move.
 Torques created by skeletal muscles that are coordinated with the
torques from external forces to obtain the human motion of interest.
 The activation of skeletal muscle has traditionally been called contraction
or muscle action.
 Muscle contraction involves the activation of muscle fibres and force
generation that facilitates body movements and posture maintenance.
“Muscle action is the neuromuscular activation of muscles that
contributes to movement or stabilization of the musculoskeletal system.”

38. MUSCLE ACTION
 4 major actions –
 Isometric – muscle tension increases
but muscle length remains same
 Isotonic – fibre length changes but the
tension remains the same
 Concentric – muscle length shortens.
Requires more energy investment.
 Eccentric – muscle length increases.
Greater forces at lower costs. Response
to a greater opposing force
 Isokinetic – velocity of muscle
contraction remains same but length
changes. Can be concentric or
eccentric.
* Low-velocity exercises generally
increase muscle strength, while high-
velocity exercises are mainly used for the
recovery of muscle endurance following
an injury

39. MUSCLE ACTION
 Video 5

40. TYPES OF MUSCLE CONTRACTION
Type of Contraction Definition Work
Concentric Force of muscle contraction 
resistance
Positive work; muscle moment and
angular velocity of joint in same
direction
Eccentric Force of muscle contraction 
resistance
Negative work; muscle moment
and angular velocity of joint in
opposite direction
Isokinetic Force of muscle contraction =
resistance; constant angular
velocity; special case is isometric
contraction
Positive work; muscle moment and
angular velocity of joint in same
direction
Isometric Force of muscle contraction 
resistance; series elastic component
stretch = shortening of contractile
element (few to 7% of resting
length of muscle)
No mechanical work; physiological
work

41. MUSCLE ACTION
 Tension in the muscles in the main stimulus to develop muscles –size and strength
 Result from both active and passive components of muscle tension.
 Passive tension- recoil tension/force. No energy
 Force that comes with the elongation of the connective tissue components of the muscle
tendon unit
 Ex. Tension felt in the muscles during stretching exercise is the passive resistance to the
elongation of the stretch.
 Represented by the Force-Length relationship of the muscles.
 Active tension-
 tension/force when muscle is stimulated to contract. Involves ATP
 Forces created between actin and myosin fibres in the sarcomeres
 Force created by the contractile proteins using ATP
 Represented by the Force-velocity relationship

42. MUSCLE ACTION
 Active tension ● Passive tension
https://www.youtube.com/watch?v=eTs9EnlbFew
Concentric contraction
F increases, V decreases
Eccentric contraction
F increases, V increases

43. FORCE LENGTH RELATIONSHIP
 Video 6

44. HILL MUSCLE MODEL
 One of the most widely used mechanical models of muscle that takes
into account both the active and passive components of muscle
tension is the three-component model developed by A. V. Hill in 1938 :
Hill model
 The Hill model of muscle describes the active and passive tension
created by the muscle tendon unit (MTU).
• Computational models of muscles serve
as important tools to understand the
musculoskeletal physiology and
biomechanics.
• Such models have been widely
implemented in a variety of simulation
platforms and incorporate varying
degrees of physiological details.

45. HILLS MODEL
 The Hill muscle model has two elements in
series and one element in parallel.
 The contractile component (CC) represents
the active tension of skeletal muscle.
 Parallel elastic component (PEC) and series
elastic component (SEC) represent two key
sources of passive tension in muscle.
 The Hill muscle model has been the dominant
theoretical model for understanding muscle
mechanics.
 Also used in biomechanical computer models
employed to simulate human movement.

46.  Generalizations about the mechanical behavior of muscle are based on Hills model.
 Elasticity (connective tissue) in the production of active muscle tension modeled by the series elastic
component.
 The source of this series elasticity is likely a mixture of the actin/myosin filaments, cross bridge stiffness,
sarcomere non-uniformity, and other sarcomere connective tissue components.
 Second, the passive tension of relaxed muscle that is easily felt in stretching exercises or in passive
insufficiency affects motion at the extremes of joint range of motion.
 The “p” in the parallel elastic component that forms the primary source of passive tension in the Hill muscle
model.
 This third point can be generalized beyond the simple Hill muscle model, that is focused on the
complex transmission of force within the connective tissue components of muscle.
 Muscles may not create equal forces at their attachments because of force transmitted to
extra muscular connective tissues.
 Stretch and recoil of elastic structures are an integral part of all muscle actions.
HILLS MODEL

47. HILLS MODEL
(v+b)(F+a)= b(FO+a)
Where,
F is the tension in the muscle
V is the velocity of contraction
Fo is the maximum isometric tension
a is the coefficient of shortening heat
b= a.vo/Fo
Where
vo is the maximum velocity at F=0

48. MECHANICS OF THE PASSIVE MUSCLE–TENDON UNIT (MTU)
 The mechanical response of the MTU to passive stretching is viscoelastic.
 Viscoelastic means that the stress and strain in a material are dependent on the rate of loading.
 So the response of the tissue depends on the time or rate of stretch.
 At a high rate of passive stretch, the MTU is stiffer than when it is slowly
stretched.
 This is the primary reason why slow, static stretching exercises are preferred over ballistic
stretching techniques.
 A slow stretch results in less passive tension in the muscle for a given amount
of elongation compared to a faster stretch.

49. MUSCLE TENDON UNIT
 Video 7 Stretching

50. TENDON
 Tendon is the connective tissue that links muscle to bone and
strongly affects how muscles are used or injured in movement.
 Tendon is a well-vascularized tissue whose mechanical response
is primarily related to the protein fiber collagen.
 The parallel arrangement of collagen fibers in tendon and cross-
links between fibers makes tendon about three times stronger in
tension than muscle.

51. TENDON
 Video 8.1 Complete muscle building

52. TENDON
 The ultimate strength of tendon is usually about 100 MPa
(megapascals).
 Even though the diameter of tendons is often smaller than the
associated muscle belly, their great tensile strength makes tendon
rupture injuries rare.
 Acute overloading of the MTU usually results in strains (sports
medicine term for overstretched muscle, not mechanical strain) and
failures at the muscle tendon junction or the tendon/bone interface.

53. TENDON
 Application Video 8

54. TENDON
 Tendonities Video 9
 Tendon Rupture Video 9.1, 9.2

55. Biomechanics of bone

56. PURPOSE OF THE SKELETAL SYSTEM
 To protect internal organs, provide rigid kinematic links and muscle attachment sites, and
facilitate muscle action and body movement.
 Bone has unique structure and mechanical properties that allow it to carry out these roles.
 Bones are living tissues with blood supplies, made of a high percentage of water (25% of bone mass)
 Among the body's hardest structures; only dentin and enamel in the teeth are harder.
 This highly vascular tissue has an excellent capacity for self-repair and can alter its properties and
configuration in response to changes in mechanical demand.
 Bones protect the vital organs and help to support the human body.

57. BONE COMPOSITION AND STRUCTURE
 Normal human bone is composed of:
 Mineral or inorganic portion:
 It consists primarily of calcium and phosphate, mainly in the form of small crystals
resembling synthetic hydroxyapatite crystals with the composition Ca10(PO4)6(OH)2.
 accounts for 60 to 70% of its dry weight.
 Water: 5-8%.
 Organic matrix: remainder of the tissue

58. BONE TISSUE
 Bone tissue, or osseous tissue, - a type of connective tissue used in
forming bones.
 Bone is composed mainly of collagen, or ossein, fibers, and bone cells
called osteocytes.
 There are two types of bone tissue, referred to as cortical bone and
cancellous bone.

59. Longitudinal section of human femur. The direction of principal stresses are shown in
the scheme on the right

60.  Compact bone or (Cortical bone).
 The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps
and spaces.
 This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total
bone mass of an adult skeleton.
 Trabecular bone
 It is an open cell porous network also called cancellous or spongy bone filling the interior of the organ.
 It is composed of a network of rod- and plate-like elements that make the overall organ lighter and
allowing room for blood vessels and marrow

61. TYPES OF BONES
 There are several types of bones in the body:
 short bones, such as those in the wrist;
 flat bones, such as the shoulder blade or scapula,
 irregular bones, such as the malleus inside the ear; and
 Long Bone- eg: thigh bone

63. LONG BONES (FEMUR)

64. SHORT BONES (WRIST, ANKLE)

65. FLAT BONES (SKULL, SCAPULA)

66. IRREGULAR BONES (VERTEBRAE)

67. BIOMECHANICS ASPECT OF BONE
 Bone tissue is viscoelastic — it has stress-strain characteristics that are
dependent upon the applied strain rate.
 In other words, a specimen of bone tissue that is exposed to very rapid loading will
absorb more energy than a specimen that is loaded more slowly.
 Bone tissue is also anisotropic (its modulus is dependent upon the direction of
loading) means the bone tissue can bear higher loads in the longitudinal direction.
 Therefore, bone tissue is both anisotropic and viscoelastic. Because of these
characteristics one must specify the strain rate and the direction of applied
loading when discussing bone material behavior.

70. BIOMECHANICS ASPECT OF BONE
 The ability of bone to resist an applied load before failure or fracture is dependent on
multiple factors.
 Age, disease, hormone levels, too little load, too much load, or even the direction in
which a load is applied can all influence the biomechanical properties of bone.
 In particular, the potential of the bone to resist fracture is affected by these physiologic
and mechanical sources.
 A thorough understanding of structural and mechanical properties relationship allows
one to consider how the aforementioned factors change the ability of bone to withstand
an applied load.

71. BIOMECHANICS ASPECT OF BONE
 Unlike muscle, the primary loads experienced by most bones are
compressive.
 The mechanical response of bone to compression, tension, and other
complex loads depends on the complex structure of bones.
 Immobilization or inactivity will result in dramatic decreases in bone
density, stiffness, and mechanical strength.

72. BIOMECHANICS ASPECT OF BONE TISSUE
 A German scientist is credited with the discovery that bones remodel (lay
down greater mineral deposits) according to the mechanical stress in
that area of bone.
 The laying down of bone where it is stressed and reabsorption of bone in
the absence of stress is called Wolff's Law.
“Bone in a healthy person or animal will adapt to the loads under
which it is placed”

73. BIOMECHANICS OF LIGAMENTS

74.  Ligaments are soft collagenous tissues that connects bone to
bone.
 Plays a important role in maintaining the stability of a joint and
also hold joints together. It is also capable to restrict joint motion.
 Shares similar structure as that of tendon.
 Ligament microstructure can be visualized using polarized light
that reveals collagen bundles aligned along the long axis of the
ligament and displaying an underlying "waviness" or crimp along
the length.
 Crimp is thought to play a biomechanical role, possibly relating to
the ligaments loading state with increased loading likely resulting
in some areas of the ligament uncrimping, allowing the ligament
to elongate without sustaining damage

75. STRUCTURE OF LIGAMENTS
 Ligaments are dense connective tissues that contain collagen, elastin,
proteoglycans, water, and fibroblasts.
 Approximately 70 to 80 percent of the dry weight of ligament consists of Type I
collagen, which is a fibrous protein.
 The collagen fibril is the basic load bearing unit of ligament.
 The fibril consists of bundles of microfibrils held together by biochemical bonds
(called cross-links) between the collagen molecules.
 Because these cross-links bind the microfibrils together, the number and state of
the cross-links are thought to have a significant effect on the strength of the
connective tissue.

76. SCHEMATIC DIAGRAM OF LIGAMENT

77. BASIS FOR COMPARISON TENDONS LIGAMENTS
Definition Tendon connects muscles to
bone, and are present at the end
of skeletal muscles. These are
the fibrous connective tissue
non-elastic.
Ligaments connect one bone to
another bone and so are found
in joints. These are also the kind
of connective tissue which is
stronger and flexible and helps
in movements of the bones.
Nature Tendons are inelastic and tough. Ligaments are elastic and
strong.
Fibres Fibres are present as compact
parallel bundles.
Fibres are compactly packed
and not arranged in parallel
bundles.
Fibroblasts In tendon, fibroblasts lie in
continuous rows.
In ligaments, fibroblasts are
scattered.
Formed of Tendons are made of white
fibrous connective tissue.
Ligaments are made of yellow
fibrous connective tissue.
It joins Tendons connect end of the
muscles to any place of the
bone.
It connects bones to bones at a
joints.
Classification No classification. They are divided into three
categories-peritoneal ligaments,
fetal remnant ligaments, and
articular ligaments.
LIGAMENTS VS TENDON

79. BIOMECHANICAL ASPECT OF LIGAMENTS
 Ligaments are passive collagenous structures that act primarily as tensile
restraints to control the distance between their attachment points.
 Ligaments normally traverse joints, and so they act to control the relative
separation of the bones that they are attached to.
 The ligaments control the patterns of movement, or kinematics, of joints,
as well as ensuring the stability of joints.
 In addition to this simple mechanical description of the role of ligaments,
they provide more subtle control of joint motion and stability via
proprioceptive feedback to the muscles.

80. BIOMECHANICAL ASPECT OF LIGAMENTS
 Ligaments are viscoelastic structures with unique mechanical properties.
 The ligaments are pliant and flexible.
 It allows natural movement of the bones to which they attach, but are
strong and inextensible so as to offer suitable resistance to applied
forces.
 Sustain chiefly tensile loads during normal and excessive loading.
 When injury happens, the degree of damage is related to the rate of
loading as well as the amount of load.

81. BIOMECHANICAL ASPECT OF LIGAMENTS
 Ligaments are often evaluated by using mounted specimens
such as a bone-ligament-bone specimen.
 However, recent advances have allowed for some
instrumentation to be used in the measurement of in situ forces
in humans.
 These include the use of buckle transducers, instrumentation
at insertion sites, magnetic resonance imaging, kinematic
linkage measurements, and implantable transducers.

82. STRESS-STRAIN PROPERTY OF LIGAMENTS
 When a tensile force is applied to the ligament at its resting length, the
tissue stretches.
 Force-length curves can be normalized to subtract out the effects of
geometry.
 Thus, force can be normalized by dividing by the cross-sectional area of a
tissue, while length can be normalized by dividing by the initial length of
the tendon or ligament.
 The resulting stress-strain curve displays three characteristic regions: the
toe region, the linear region, and the failure region.

83. IF ONE NEGLECTS VISCOELASTIC BEHAVIOR, A TYPICAL STRESS STRAIN CURVE FOR
LIGAMENTS AND TENDONS CAN BE DRAWN AS:

84. Toe region:
 As the load increases so does the recruitment of collagen fibres
causing them to ‘uncrimp’.
 This occurs when collagen is stretched to approximately 2% of its
original length and returns to normal length when the force is
removed, thus it is within its physiological range.
 It is characterized by relatively low stiffness. There is a non-linear
relationship on the stress-strain curve at this stage.

85. Linear region (Elastic phase):
 As the collagen fibrils become gradually uncrimped, the fibril itself
is being stretched.
 There now becomes a linear relationship between deformation
and load, as the tissue becomes relatively stiffer.
 This occurs when collagen is stretched to 2-4% of its original
length and returns to its original geometric shape. The tissue is
said to be elastic.

86. Yield and Failure region (Plastic phase):
 The continued increase in load past 4% causes micro failure to the fibrils and damage
to cross-links.
 It results in a plateau effect on the curve: this point represents the ultimate tensile
strength of the tendon and is termed the ‘yield point’.
 The yielding of fibres occurs when the deformation is approximately 4-10% of the
resting length.
 Stiffness is reduced and the fibrils do not return to normal length on release, the tissue
then becomes ‘viscous’. This is known as ‘plastic’ deformation.

87.  Finally, complete failure occurs as the ligament/tendon ruptures.
 Obviously this is a non-physiological range and there would be an
inability to support load or function.
 Collagen demonstrates various mechanical and physical properties in
response to load and deformation to allow it to withstand high tensile
stresses.
 The point between the elastic and plastic region is where gross integrity
is disrupted.

88. FACTORS THAT AFFECT THE BIOMECHANICAL PROPERTIES OF LIGAMENTS
 Numerous factors affect the biomechanical properties of tendons and ligaments.
 The most common are :
 Aging,
 Pregnancy,
 Mobilization and immobilization,
 Diabetes mellitus, connective tissue disorders, renal disease etc.
 Pharmacologic agents (steroids, nonsteroidal Anti-inflammatory drugs or NSAIDs).

89. VIDEO TUTORIAL
https://www.youtube.com/watch?v=U7PE736mRVk
https://www.youtube.com/watch?v=LaIVBd_wJ_M