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19.4.12
 The
curvature of deltoid muscle is important.
If the force applied by the deltoid muscle
passed directly through the fulcrum, there
would be no torque
 With the deltoid muscle angled slightly
upward from the horizontal arm, the
direction of the force does not go through
the pivot point, giving a small nonzero lever
arm




If the point of insertion of deltoid muscle
is 15cm from the shoulder joint
Weight of the arm is 5% of an average
person’s weight. Say it is 36N and acting
at the center of gravity of the arm at a
distance of 25cm from the shoulder joint
The force in the deltoid muscle comes out
to be 356N which is about ten times the
weight of the arm. If an object weighing
10N is held at a distance of 64cm from the
joint the force in deltoid muscle goes upto
590N
The reason here again is that effort arm is
much smaller than the load arm

Let’s try to understand why it is advised to bend your
legs to lift a heavy object up rather than to bend over
Figure shows a person bending over with back
parallel to the ground
 The weight of the trunk, head, neck and arms
would tend to cause the person to rotate
counterclockwise. A bundle of muscles attached
to the spine and hip bones prevent him from
falling over by producing a clockwise torque
 The origin of these muscles is at the hip bones
with the insertion points located along the spine.
These erector spinae muscles are shown as a
single muscle attached at one point on the spine
at some angle (no torque possible if attached
along spine)




Let’ s give some numbers and calculate the force
exerted by spinae muscles.
For a person weighing 72kg, the weight of both arms
is 72N and the weight of trunk, head and neck is
400N. Suppose the weight of the arms act at a
distance of 61cm from the sacrum while that of
trunk, head and neck acts at 24cm. Muscle exert
force at a distance of 40cm from the sacrum making
an angle of 12° with spine
The force exerted by erector spinae muscles comes
out to be 1682N which is twice the total weight of
the person. If he is picking a weight of 200N, the
force would be four times the weight of the person
 Due
to such large forces exerted by
muscles (and hence on spine), you
can strain muscles in the back while
lifting a heavy load
 If we consider the person squatting
down to pick up the object, then the
majority of force is in legs. There is
still some force in the back, but it is
much less. It is the act of bending
over that causes large forces in the
muscles and on the spine
 Gravitational
force W applies at the center
of gravity CG of the body
 CG depends on body mass distribution! to
maintain stability
The key to stable equilibrium
is that the center of gravity
of the object must be over a
large enough base.
 A vertical line through the
center of gravity must fall
within its base of support.
This latter is formed by the
arch of the foot; its two ends
are the tuberosity of the
calcaneum posteriorly and
the head of the first
metatarsal bones anteriorly.



A person will be in stable equilibrium if he is
standing with his center of gravity lying over the
base formed by his feet. If he spreads his feet apart
then he has a wider base and will be more stable. If
he pulls his feet together he can be still stable but
less so
This works even if the person stands on one foot but
then he has to shift his body such that his center of
gravity is over his feet. He can do this if he thrusts
his hips in one direction and his shoulders in other
direction, changing the shape of his body and
location of his CG

Fig. A. - The body in the erect position; The vertical line a-b
through the centre of gravity c passes through the occipito-atloid
joint above, in front of the sacro-iliac joint g, the hip-joint h, the
knee-joint i and the ankle-joint j and falls between the points of
support d-e, passing through the astragaloscaphoid joint.



Fig. B. - When the trunk is inclined forward by bending at the
hip-joint, the increased projection of the head and upper
portion of the trunk in front of the centre of gravity is
counterbalanced by the increased projection of the hips and
lower portion of the trunk posteriorly. The vertical line through
the centre of gravity still cuts the base of support d-e and the
body remains in a state of equilibrium.
Fig. C. - When the body inclines backward, hyperextension at
the hip is prevented by ligaments; therefore, when the vertical
line a-b through the centre of gravity c falls beyond the base of
support d-e, the body is in unstable equilibrium and it falls.
Fig. D. - If the body, as occurs in some diseases and injuries, is
inclined so far forward as to bring the vertical line a-b through
the centre of gravity c, in front of the base of support d-e, then
it is in a state of unstable equilibrium and additional support is
used, in the form of a cane, to prevent falling forward.
Fig. A. - The body being erect, a vertical line a-b through
the centre of gravity c falls midway between the ankles
or base of support d-e and the body is in stable
equilibrium
Fig. B. - The trunk being inclined to the right, the centre
of gravity c is shifted to the right and a vertical line a-b
through it falls still within the line of support d-e and the
upright position can still be maintained.
 Fig. C. - If the relative length of the two legs is altered, as
by placing a block beneath one of them, the pelvis and
upper portion of the body inclines to the opposite side,
until a vertical line a-b through the centre of gravity C
falls beyond the extremity of the base of support d-e and
the body is in a position of unstable equilibrium.

 Air
is composed mainly of 79% nitrogen & 21%
oxygen
 Total pressure of air mixture is 760mmHg
1 atmosphere = 760mmHg
 Nitrogen partial pressure

79% of 760 mmHg = 600 mmHg
 Oxygen

partial pressure
21% of 760 mmHg = 160 mmHg
 Rate
of net diffusion is determined by
difference of partial pressures (pp)


If pp of gas in alveoli > blood then gas moves into
blood (Oxygen)
If pp of gas in blood > alveoli then gas moves into
alveoli (Carbon dioxide)
Though the partial pressure of oxygen in the
inspired air is 160mmHg, the process of warming
and mixing the air with saturated water vapor in
the bronchial tree reduces the partial pressure
by 47mmHg by the time it is in the alveolar sas
of the lung
 In addition, the increased fraction of CO2 in the
alveoli and the process of transfer of gas
between the blood and air reduces the partial
pressure of oxygen another 9mm Hg
 So that at atmospheric pressure the partial
pressure of oxygen in the alveoli is actually
about 104mm Hg

 In
the absence of red blood cells containing
hemoglobin the only mechanism for the
capture of oxygen would be the solubility of
oxygen in the blood plasma
 In fact the amount of oxygen carried by
blood in this way is represents only 1.5% of
the carrying capacity of oxygen saturated
blood containing normal amount of
hemoglobin
 The solubility of a gas in fluid such as water
or blood plasma is described by Henry’law
Henry's Law - solubility of a gas in a liquid
depends on temperature, the partial pressure of
the gas over the liquid, the nature of the solvent
and the nature of the gas
 It states that
“At a constant temperature, the amount of a
given gas that dissolves in a given type and
volume of liquid is directly proportional to the
partial pressure of that gas in equilibrium
with that liquid”

 The
attachment of oxygen to the hemoglobin
molecules in the red blood cells is governed
by the laws of statistical mechanics. Each
hemoglobin molecule when fully saturated
can hold four oxygen molecules. The
hemoglobin
molecules
release
these
molecules or reattach them reversibly
 The degree of saturation depends on the
partial pressure of oxygen surrounding the
blood





Figure shows how blood picks up oxygen in the alveolar
capillaries in the lungs and delivers it as needed in muscle
and brain tissue throughout the body
In the alveoli the partial pressure of oxygen is 104mmHg.
Under these conditions hemoglobin become saturated to
98% of its full capacity
As the blood circulates around the body, it passes through
tissues in which the partial pressure of oxygen is on
average about 40mmHg
The hemoglobin being in equilibrium with the tissue gases,
then gives up oxygen until its saturation is reduced to
about 75%
On the average then the blood returning to the heart is
only about 75% saturated