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GravityGravit

y An external force under normal circumstances

affects all objects It should be the first external force to be

considered acting on human body

Force of gravity is the pull of earth on a bodyor its segments

Described as mutual attraction between the

earth and an object Gravitational force is always directed towards

the centre of earth which is towards theground

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Gravity is the most consistent force encountered

by the human body and behaves in a predictable

manner

Gravity being a force is a vector quantity

Point of application: COG

Line of application:Centre of mass of object

Direction :Towards centre of the earth

Magnitude:Equal to Gravitational Force

Magnitude of gravitational force is directly

proportional to the product of the two masses andinversely proportional to square of the distance

between them

Fg=gm1m2/r

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Although gravity acts at all points on an

object or segment of an object,its

application is given as Centre of Gravity(COG)

Center of Gravity is also referred as centre

of Mass

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In a asymmetrical object the COG will belocated towards the heavier end where all

the mass is evenly distributed around thatpoint

COG is only a hypothetical point and maylie outside the object but still is the point atwhich gravity acts

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Segmental COG:

Each segment in body is acted on by the

force of gravity and has its own COG

When segments are combined,gravity

acting on combined segments can be

represented by a single COG

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COG of Human Body:

When all segments are combined and body

is taken as a single rigid object in anatomic

position,the COG lies approximately

anterior to the second sacral vertebra

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When the trunk is inclined forward,locationof new COG lies outside the body

Location of COG varies with each of themany and varied postures the body assumes

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Line of Gravity (LOG):

Action line and direction of the force of

gravity are always vertically downwardtoward the centre of the earth regardless of

the orientation in space of the object

Gravity vector is commonly referred to asthe Line of Gravity (LOG)

LOG can best be visualized as a string with

a weight on the end (a plumb line) with thestring attached to the COG of an object

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On an average when the posture is good the

LOG passes through the mid cervical and

mid lumbar vertebrae and in front ofthoracic vertebrae

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Base Of Support:

Base as applied to a rigid body is the area by

which it is supported Base of Support is that part of the body that

is in contact with the supporting surface

In lying position posterior aspect of thewhole body forms the base

In stride standing BOS is an area as wide asthe feet and as long as the distance betweentheir outer borders

Different types of walking aids are used toincrease BOS of patients

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Equilibrium:

State in which a body is at rest with

neither translatory nor rotatory motion(static equilibrium) or in which a body is

in constant motion with no acceleration

(dynamic equilibrium) Equilibrium results when the forces acting

upon a body are perfectly balanced

There are three states of equilibrium

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Stable equilibrium:

If the forces acting upon the body at rest

tend to restore it to its original positionafter it has been displaced,the body is said

to be in stable equilibrium

The condition of equilibrium is moststable when COG is as low as possible

and the LOG falls near the center of an

extensive base Object becomes progressively less stable

as the COG raises and LOG falls nearer to

the margin of the base

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Unstable equilibrium:

If a body is given an initial displacement and

the forces acting upon it increase this initial

displacement,however small the latter may

be the body is said to be in unstable

equilibrium COG as high as possible and a small base

result in unstable equilibrium because even

very small displacement cause the LOG tofall outside the base and the body will fall

Eg.,standing on toes

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1) Neutral equilibrium:

If,in spite of displacement of a body,the

height and position of its COG remain thesame in relation to base,the body is said tobe in neutral equilibrium

Eg.,ball rolling on a plane surface

* Stability of human body is greatest inlying position

* Human body becomes progressively less

stable as the COG is raised and base isreduced as in sitting and standing

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Equilibrium Board

Body has proprioceptors all around it whichsense where parts of the body are evenwithout looking

Proprioceptors are present in varyingcompositions in different structures of humanjoints, if these structures are damaged/injuredthen the proprioceptors get damaged

Rocker Board-allows for one plane ofinstability at a time

Wobble Board-provides multiple planes of

instability

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Rocker Board

Wobble Board

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In injury these receptors do not function

efficiently, increasing the chance of re-injury

These boards offer an unstable and sensory-

stimulating surface, improve n facilitate

balance, coordination, posture, proprioceptive

training while reducing the risk of injuriesthrough enhanced neuro-muscular

awareness,reaction time and automatic

postural reactions

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1. Sitting down place the wobble board

under the feet and slowly rotate it a

number of times in all directions2. Stand on the wobble board, feet apart.

Hold on to a chair for support if needed

and rock the board forwards andbackwards, then side to side. Do this for 2

to 3 minutes

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Initiation of Training

Balance sandals

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1. Stand on the wobble board, feet apart. Rotate

the wobble board round so that the edge of

the board is in contact with the floor at alltimes. Again try this for 2 to 3 minutes

2. Stand on the board,again feet apart - no chair

allowed! Rock the board front to back for 1minute then side to side for one minute

3. Balance on the wobble board for as long as

you can without the edges touching the floor.Aim for over 2 minutes without touching the

floor

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1. Rotate the wobble board in

clockwise/anticlockwise but do not allow

edge of the board to touch the floor. Aimfor 2 minutes

2. Stand on the wobble board with one leg.

Rock the board from front to back for 1minute and then side to side for 1 minute

3. Again stand on the wobble board with one

leg only. Rotate the board in a circularmotion in one direction for 1 minute then

repeat in the other direction

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Stability and COG:

For an object to be stable the LOG must fall

with in the Base of Support(BOS)

When LOG falls outside the BOS the object

will fall(less stable)

When BOS of an object is large,

the LOG has more freedom

to move with out passingbeyond the limits of the base

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When a man stands with his legs spread apart,the

base is larger side to side and the trunk can move a

good deal in that plane with out displacing the

LOG from COG

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When a person grasps or leans on another

object that object can become part of BOS

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When COG is low,movement of the object

in space is less likely to cause the COG &

LOG to fall outside the BOS

The longer the LOG,the higher the COG,the

less stable the object

The shorter the LOG,the lower the COG,the

more stable the object

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Relocation of Centre of Gravity:

Location of COG of a body depends not only

on the arrangement of segments in space butalso on the distribution of mass of the object

Most common way to functionallyredistribute mass in the body is to addexternal mass

Every time we add an object to the body bywearing it,carrying it or using it the new

COG for the combined body & the externalmass will shift toward the additionalweight;the shift will be proportional to theweight added

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When a cast is applied to right lower limb it

results in shifting the COG down and to the

right

Because his COG is now lower,he is

theoretically more stable

Though he is standing only on one leg

addition of crutches enlarges the base

of support and thus improves

the stability

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Holding a heavy suit case in right handresults in shift of the COG up and to the

right Because the LOG would move toward the

right foot the man leans to the left tocompensate

Man leans laterally to the

left not to relocate the COG

but to bring the LOG back

to the middle of base of

support