Chapter 1: Introduction to Kinesiology ( Biomechanics) for physical therapy students. Reference: JOINT STRUCTURE AND FUNCTION - by Pamela K. Levangie. Easy to understand and with lot of examples.

1. INTRODUCTION TO KINESIOLOGY
PREPARED BY: DR. VANDANA PATEL
MPT (NEUROSCIENCES- PAEDIATRICS)
ASSISTANT PROFESSOR
SSAIP

2. • Kinesis = means movement or to move.
• Logos = means the science or to study.
• Kinesiology can be defined as the scientific study of
human motion.
• Kinesiology means the science of movement including
active and passive structures.
• Kinesiologists: Those who study movements.
• Anatomical kinesiology
• Mechanical kinesiology

3. • Bios = life. (Mechanical kinesiology)
• Mechanics = study the effect of forces on bodies during
static and dynamic situations.
•Mechanics = It is the branch of physics which deals with
the interrelations between force, matter and motion.
Biomechanics is the application of mechanical
principles to living structures either animals or human
being at rest and during movement

5. Chapter 1
Biomechanical Applications to Joint Structure and
Function

6. Variables:
(1) the type of displacement (motion)
(2) the location in space of the displacement,
(3) the direction of displacement of the segment
(4) the magnitude of the displacement, and
(5) the rate of displacement or rate of change of
displacement(velocity or acceleration).

7. 1. Types of displacement
1. Translatory motion ( Linear displacement)
2. Rotatory motion (Angular displacement)
3. General motion – Curvilinear motion

8. 1. Translatory motion ( Linear displacement)
• Movement of a segment in a
straight line
• Each point on the segment
moves through the same
distance, at the same time,
in parallel paths

9. 2. Rotatory motion (Angular displacement)
• Movement of a segment
around a fixed axis [CoR]
in a curved path
• Each point on the segment
moves through the same
angle, at the same time, at a
constant distance from the
CoR.

10. 3. General motion
• When non segmented
objects are moved,
combinations of
rotation and translation
(general motion) are
common
• Curvilinear (plane or
planar) motion designates a
combination of translation
and rotation of a segment in
two dimensions.
• When this type of motion
occurs, the axis about which
the segment moves is not
fixed but, rather, shifts in
space as the object moves –
Instantaneous axis of
rotation (ICOR)

11. 2. Location of displacement
• Cartesian coordinate system
• The x-axis runs side to side in the body -coronal axis;
• The y-axis runs up and down in the body - vertical axis;
• the z-axis runs front to back in the body -
anteroposterior (A-P) axis
• An unconstrained segment can either rotate or translate
around each of the three axes, which results in six potential
options for motion of that segment.
• The options for movement of a segment are also referred to
as degrees of freedom. A completely unconstrained
segment, therefore, always has six degrees of freedom.

13. • Rotation of a body segment
around the x-axis or coronal
axis occurs in the sagittal
plane
• Sagittal plane motions are
most easily visualized as
front to- back motions of a
segment
• Flexion/extension of the
upper extremity at the
glenohumeral joint).

14. • Rotation of a body segment
around the y-axis or vertical
axis occurs in the transverse
plane.
• Motions of a segment parallel to
the ground (medial/lateral
rotation of the lower extremity
at the hip joint).
• Through the length of long
bones that are not truly
vertically oriented-
longitudinal (or long) axis is
often used instead of
vertical axis.

15. • Rotation of a body segment
around the z-axis or A-P
axis occurs in the frontal
plane.
• Frontal plane motions are
most easily visualized as
side-to-side motions of the
segment
• Abduction/adduction of the
upper extremity at the
glenohumeral joint

16. 3. Direction of displacement
• Flexion/ extension
• Abduction/adduction
• Internal rotation/eternal rotation
• Clockwise/anteclockwise
• Up-down
• Right- left

17. 4. Magnitude of Displacement
• ROTATORY
DISPLACEMENT
SI UNIT: Radians
US UNIT: Degrees
• 1 radian = 57.3
• 1 degree = 0.01745
radians.
• 360 degrees = 6.28 radians
• LINEAR
DISPLACEMENT
SI UNIT: Meter/cm/mm
US UNIT: Foot/inch
•The magnitude of rotatory motion that a body segment moves
through or can move through is known as its range of motion
(ROM).

18. 5. Rate of Displacement
• SPEED: Displacement per unit time regardless of
direction
• VELOCITY: Displacement per unit time in a given
direction (m/s)(degrees/s)
• ACCELERATION: Change in velocity per unit time
is acceleration (m/s²) (degrees/s²)

19. KINETICS
It is the area of biomechanics concerned with the
forces producing the motion.

20. Forces
• It is a push or a pull exerted by one material
object or substance on another.
• The unit for a force in the SI system: Newton
(N); in the US system is the pound (lb).
• Force = (mass)(acceleration) or F= (m)(a)
• Kg.m /sec²

21. • External forces: Pushes or pulls on the body that arise
from sources outside the body.
• GRAVITY is an external force that under normal
conditions constantly affects all objects.
• The attraction of the Earth’s mass to another mass, is an
external force that under normal conditions constantly
affects all objects.
• Weight = (mass)(gravity) or W= (m)(g)
• wind, water, other person

22. • Internal forces: forces that act on the body but arise from
sources within the human body.
• Muscle pull, ligaments and bones
• Essential for human function
• Serve to counteract external forces
• Some friction and atmospheric pressure can act both
external to and within the body.
• External forces can either facilitate or restrict
movement.
• Internal forces - essential for initiation of movement

23. Force vectors
• Vector can be defined by:
1. A point of application on the object being acted
on
2. An action line and direction
3. A magnitude – the quantity of force being
exerted

24. • The length of a vector is usually drawn proportional to
the magnitude of the force according to a given scale.
For example, if the scale is specified as 5mm 20 N of
force, an arrow of 10 mm would represent 40 N of
force.
• The length of a vector, however, does not necessarily
need to be drawn to scale as long as its magnitude is
labeled.

25. • Primary Rule of Forces
■ All forces on a segment must come from something that
is contacting that segment.
■ Anything that contacts a segment must create a force
on that segment
■ Gravity can be considered to be “touching” all objects.

26. Naming forces
• First part --- source of the force
• Second part---object being acted on
• Action line and direction:
• Push: away from the point of application
• Pull: close to the point of application
• Gravity exerts force on all objects on the earth

27. Force of gravity
• The force of gravity acting on an object or segment is
considered to have its point of application at the CoM or
center of gravity(CoG) of that object or segment—
• The hypothetical point at which all the mass of the object
or segment appear to be concentrated.
• In a symmetrical object--- the CoM at center of the object
• In an asymmetrical object, the CoM will be located toward
the heavier end.

28. Segmental Centers of Mass and Composition
of Gravitational Forces

29. LOG (Line Of Gravity)
• The force of gravity acting on an object is always vertically
downward toward the center of the earth. The gravitational
vector is commonly referred to as the line of gravity
(LoG).
• A string with a weight on the end - a plumb line

30. Center of Mass of the Human Body
• Anterior to the second sacral vertebra (S2)

31. • The CoM does not change its location in the rigid body
as the body moves in space, the LoG changes its relative
position or alignment within the body.(In case of rigid
body)
• With each rearrangement of body segments, the location
of the individual’s CoM will potentially change.
• COG, LOG and stability

37. • The larger the BoS of an object is, the greater is the
stability of that object.
• The closer the CoM of the object is to the BoS, the
more stable is the object.
• An object cannot be stable unless its LoG is located
within its BoS.

38. Introduction to Statics and Dynamics

39. • Statics is the study of the conditions under which
objects remain at rest.
• Dynamics is the study of the conditions under which
objects move.
• If all the forces acting on a segment are “balanced”; the
state is known as equilibrium
• Isaac Newton’s first two laws will govern whether an
object is static or dynamic.

40. Newton’s Law of Inertia
• Inertia is the property of an object that resists both the
initiation of motion and a change in motion and is directly
proportional to its mass.
• The law of inertia states that an object will remain at rest or in
uniform (unchanging) motion unless acted on by an unbalanced
(net or resultant) force.
• For an object to be in equilibrium, the sum of all the forces
applied to that object must be zero.
ΣF = 0

41. Newton’s Law of Acceleration
• Newton’s second law states that the acceleration(a) of an
object is proportional to the net unbalanced (resultant)
forces acting on it and is inversely proportional to the mass
(m) of that object:
a = Funbal/m

42. Newton’s Law of Reaction
• For every action, there is an equal and opposite reaction.
• When one object applies a force to the second object, the
second object must simultaneously apply a force equal in
magnitude and opposite in direction to that of the first
object. These two forces that are applied to the two
contacting objects are an interaction pair and can also
be called action-reaction (or simply reaction) forces.

45. Linear Force System
• Whenever two or more forces act on the same segment,
in the same plane, and in the same line
• Forces in a linear force system are assigned positive or
negative signs.
• The magnitudes of vectors in opposite directions should
always be assigned opposite signs.

46. • The net effect, or resultant, of all forces that are part
of the same linear force system is determined by
finding the arithmetic sum of the magnitudes of each
of the forces in that force system.
• A net force that moves a bony segment away from its
adjacent bony segment is known as a distraction
force.

50. Concurrent Force System
• forces applied to an object to have action lines that
lie at angles to each other
• A common point of application may mean that the
forces are literally applied to the same point on the
object or that forces applied to the same object
have vectors that intersect when extended in
length.

54. Parallel force systems
• It exists when two or more forces act on the
same lever but at the same distance from each
other and at the same distance from the axis
about which the lever will rotate.

56. Tensile Forces
• Opposite pulls on the object works as a tensile force.
 Tensile forces (or the resultants of tensile forces) on
an object are always equal in magnitude, opposite in
direction, and applied parallel to the long axis of the
object.
 Tensile forces are co-linear, coplanar, and applied to
the same object; therefore, tensile vectors are part of
the same linear force system.

57. • Tensile forces applied to a flexible or rigid
structure of homogenous composition create the
same tension at all points along the long axis of
the structure in the absence of friction; that is,
tensile forces are transmitted along the length
(long axis) of the object.

61. Joint Distraction and Distraction Forces
• Distraction forces create separation of joint surfaces.
• There must be a minimum of one distraction force on
each joint segment, with each distraction force
perpendicular to the joint surfaces, opposite in
direction to the distraction force on the adjacent
segment, and directed away from its joint surface.

62. • Joint distraction can be dynamic (through unequal
or opposite site acceleration of segments) or static
(when the tensile forces in the tissues that join the
segments are balanced by distraction forces of
equal or greater magnitude).

63. • Joint reaction forces are contact forces that result
whenever two or more forces cause contact
between contiguous joint surfaces.
• The two forces that cause joint reactions forces
are known as compression forces.
• Compression forces are required to push joint
surfaces together to produce joint reaction forces.

64. Shear and friction force
• A force (regardless of its source) that moves or
attempts to move one object on another is known as
a shear force (FS).
• A shear force is any force (or the component
of a force) that is parallel to contacting surfaces and
has an action line in the direction of attempted
movement.

65. • A friction force (Fr) potentially exists on an
object whenever there is a contact force on that
object.
• Friction forces are always parallel to contacting
surfaces and have a direction that is opposite to
potential movement.
• For friction to have magnitude, some other force (a
shear force) must be moving or attempting to move
one or both of the contacting objects on each other.

66. • The force of friction can be considered a special
case of a shear force because both are forces
parallel to contacting surfaces, but friction is a
shear force that is always in the direction opposite
to movement or potential movement.
• Shear and friction forces potentially exist whenever
two objects touch.

67. • If the two objects are not moving (objects are static), the
maximum magnitude of the force of static friction
(Frs) on each object is the product of a constant value
known as the coefficient of static friction (S) and the
magnitude of the contact force (FC) on each object;
Frs ≤ µsFc
• The coefficient of static friction µs is a constant value for
given materials.
Eg. Wood (0.25)

70. • Once an object is moving, the magnitude of the
force of kinetic friction (FrK) on the
contacting objects is a constant value, equal
to the product of the contact force (FC) and the
coefficient of kinetic friction (K):
FrK =(µK)(FC)
• The coefficient of kinetic friction

71. Torque/moment of force
• Two forces that are equal in magnitude, opposite in
direction, and applied to the same object at
different points are known as a force couple.
• A force couple will always produce pure rotatory
motion of an object.

72. • The strength of rotation produced by a force
couple is known as torque (T), or moment of
force.
• It is a product of the magnitude of one of
the forces and the shortest distance between the
forces.
T = (F)(d)
• The perpendicular distance between forces that
produce a torque, or moment of force, is also
known as the moment arm (MA).
T= (F)(MA)

73. • The greater the magnitude of the force couple is, the
greater the strength of rotation is. The farther apart
the forces of a force couple are (the greater the MA),
the greater the strength of rotation is.

74. Angular Acceleration and
Angular Equilibrium
• Angular acceleration (α)is given in deg/sec2 and is
a function of net unbalanced torque and the mass
(m) of the object:
α = Tunbal ÷ m
• When the torques on an object are balanced (ΣT
=0), the object must be in angular equilibrium.

77. Bending Moments and Torsional
Moments
• When parallel forces are applied to an un-
segmented object in a way that results in
equilibrium.
• The torques, or moments of force, applied to a
particular point on the object are considered to be
bending moments.

79. • Bending moments on a segment that is not rotating
are also known as three-point bending because
three parallel forces are required.
• A torsional moment is when a torsional force
which creates a rotation of a segment around its
long axis.

80. A force applied to the periphery of a long segment produces
a “torsional moment” that is directly proportional to the
magnitude of the force and its distance from the
longitudinal axis.

81. Identifying the joint axis about
which body segments rotate

82. • The force of gravity (GLf) would translate the
leg-foot segment down until tension in the
capsule (CLf) reached an equivalent magnitude,
at which point GLf and CLf would form a force
couple to rotate the leg-foot segment around the
point of application of CLf.

83. Three conditions for equilibrium
• ΣFH = (+FLf) + (-FpLf) = 0
• ΣFV = (+CLf) + (-GLf) +
(+FrLf) = 0
• ΣT= (+CLf )(2MA) +
(-GLf)(MA) + (+FrLf)(0) = 0
(Where, CLf= 48 N/2 and as CLf
is 24N FrLf should also be =
24N)

84. Anatomic Pulleys
• When the direction of pull of a muscle is altered,
the bone or bony prominence causing the deflection
forms an anatomic pulley.
• The function of any pulley is to redirect a force to
make a task easier.
• The “task” in human movement is to rotate a body
segment.
• More the MA- easier the task performance

85. • The MA will always be the shortest distance between
the vector and the axis of rotation.
• By increasing the MA for a muscle force, a force of
the same magnitude (with no extra energy
expenditure) produces greater torque.
• The MA for any force vector will always be the length
of a line that is perpendicular to the force vector and
intersects the joint axis.
• MA will be parallel to and lie along the lever ---
LEVER ARM (LA)

87. Patella as Anatomical pulley

88. SAM’S EXAMPLE OF TORQUE
• CoM for his leg-foot segment lies 0.25 m from his knee joint axis
• The weight boot (at the end of his leg-foot segment) is 0.5 m from his
knee joint axis
• GLf= 48N
• WbLf= 40N
• TGLf = (48 N)(0.25 m) =–12 Nm
• TWbLf = (40 N)(0.5 m) = –20 Nm
• GWbLf= 88N and torque = -32Nm
• So, MA= T/F = -32/-88 =0.36

89. Qlf 0.03m
Qlf 0.05m
Gwblf 0.27
Gwblf
0.36m
Qlf 0.01m

90. • As the angle of application of a force increases, the
MA of the force increases.
• As the MA of a force increases, its potential to
produce torque increases.
• The MA of a force is maximal when the force is
applied at 90 to its segment.
• The MA of a force is minimal (0.0) when the
action line of the force passes through the CoR of
the segment to which the force is applied.

93. LEVER SYSTEMS
• A lever is any rigid segment that rotates around a
fulcrum.
• A lever system exists whenever two forces are
applied to a lever in a way that produces opposing
torques.
• Effort force (EF)- the force that is producing
the resultant torque
• Resistance force (RF)- the other force which is
creating an opposing torque

94. • The MA for the EF is referred to as the effort arm
(EA), whereas the MA for the RF is referred to as
the resistance arm (RA).

95. First class lever
• It is a lever system in which the axis lies between
the point of application of the effort force and the
point of application of the resistance force.
• EA can be bigger than RA , smaller than RA, or the
same size as RA.

97. Example:
• Pull of the supraspinatus on the humerus

98. Second-class lever
• It is a lever system in which the resistance force
has a point of application between the axis and the
point of application of the effort force, which always
results in EA being larger than RA.

99. Example :
• When the gravity pulls leg segment downward
quadriceps work as resistive force and forms
second order lever.
• This produces lengthening of muscle which is
working as resistive force and that is termed as
eccentric contraction of muscle.

101. Third-class lever
• It is a lever system in which the effort force has
a point of application between the axis and the
point of application of the resistance force, which
always results in RA being larger than EA.

102. Example:
• Qlf Force against Gwblf in case of sam
alexander’s foot.
• That pull of muscle in third order produces
Concentric contraction.

104. Mechanical Advantage
• Mechanical advantage (M Ad) is a measure
of the mechanical efficiency of the lever
• Mechanical advantage of a lever is the ratio of the
effort arm to the resistance arm
• M Ad = EA/RA
• When EA is larger than RA, the M Ad will be greater
than 1.

105. • The torque of the effort force is always greater
than the torque of the resistance force; that is,
(EF)(EA) > (RF)(RA)
• SECOND ORDER LEVER--- EFFICIENT
• THIRD-CLASS LEVER IS--- MECHANICALLY
INEFFICIENT
• FIRST ORDER LEVERS IN BODY-
MECHANICALLY INEFFICIENT

106. Force components

107. Determining Magnitudes of
Component Forces
• Vector QLf is the hypotenuse (side opposite the 90
angle) and has a magnitude of 1000 N. The angle of
application θ is, in this example, presumed to be 25.
• Vector Fx is the side adjacent to angle θ, and vector Fy
is the side opposite to angle θ.

114. Force Components and the Angle of
Application of the Force
Changes in angle from 35 degree to 145 degree---
change in direction and magnitude of Fx

115. • With joint movement--- MA changes--- change
in torque--- torque is produced by Fy --- Fy
changes
• Fy is directly proportional to MA
• FX And Fy inversely proportional.

116. • The angle of pull of the majority of muscles is
small, with an action line more parallel to the
lever than perpendicular to the lever.
• The parallel (Fx) component of a muscle force
most often is larger than the perpendicular (Fy)
component.
• The parallel component of most muscle forces
contributes to joint compression, making muscles
important joint stabilizers

117. Manipulating External Forces to
Maximize Torque

118. • The torque of an external force can be increased
by increasing the magnitude of the applied force.
• The torque of an external force can be increased
by applying the force perpendicular to the lever.
• The torque of an external force can be increased
by increasing the distance of the point of
application of the force from the joint axis.

119. Translatory Effects of Force
Components

120. Rotatory Effects of Force
Components

121. Rotation Produced by
Perpendicular Force Components
When translation
is controlled
CMsLf and FyQLf
can produce
rotation ..
FyQLf MA=
0.06m
Torque produced
=
T FyQLf

122. Rotation Produced by Parallel Force
Components
Vectors FLf and
GWbLf, if extended, will
pass approximately through
the knee joint axis
So they will not produce
torque more than the
torque of FxQLf.
TFxQLf = (FxQLf)(MA)
FxQLf= 910 N
MA= 0.005m

123. • Rotation around a joint axis requires that ΣFx= 0.
If ΣFx ≠ 0 initially, translatory motion of the
segment will continue (alone or in combination
with rotation) until checked either by a
capsuloligamentous force or by a joint reaction
force (depending on the articular configuration), if
the effects of external and muscular forces have
already been accounted for.

124. • The majority of torque on a segment will be
produced by forces or force components (Fy) that
are applied at 90 to the segment and at some
distance from the joint axis.
• Parallel force components (Fx) will produce
limited amounts of torque if the action line does
not pass through the CoR of the segment.

125. • Whenever the goal is rotation of a joint, a net
unbalanced torque in the direction of movement
(ΣT≠0) is necessary to reach the goal.
• The greater the net unbalanced torque, the
greater the angular acceleration of the segment.

126. Total Rotation Produced
by a Force
• TTOT = [(FTOT)(MATOT)]
= [(Fy)(MAFY)] [(Fx)(MAFx)].

127. Multisegment (Closed-Chain)
Force Analysis
• Whenever one end of a segment or set of segments is
free to move in space, this is referred to as an open
chain.
• When both ends of a segment or set of segments are
constrained in some way (and not free to move in
space), this is referred to as a closed chain.