.
So, let us say they all are contributing to
this particular action. So, I have let me
ah see if I can show you .
So, you have the biceps, you have the brachialis
and the brachioradialis; so, let me try to
draw that .
So, if I have 
just an approximate ; so if this is my forearm,
then I have 
the biceps; you will have to use your imagination
. Then I have 
the brachialis; acting something like that
and then I have the brachioradialis, which
inserts somewhere here ok. And then I have
the arm basically carrying the load ; so this
is the brachialis, biceps and the braccio
.
So, now when I draw the free body diagram;
I have the elbow, I have my ok let me use
the same F m 2 and then I have F m 3.
.
F m no no they are not, you can see here the
sample the muscle always pulls along that
like a 2 force number right when I cut the
weight and J Y; I also have the weight of
the arm and I have the weight of the load
ok; so, again muscles insert are known. And
I have to now look at what happens when I
write the equations for static equilibrium
ok . So, let us say these are the angles;
so this is theta 2; the line of action of
muscle and theta 3 ok, from the long axis
I know those angles at which they act.
So, assume the angles of pull are known; hence
of muscle attachments or the insertion points
and lengths of their momentums 
ok . So, now if I do J X, F m 1 does not contribute
anything to that plus F m 2 cos theta 2 . Now
I am pick a F m 3 cos theta 3 equal to 0;
m 1 plus fm 2 sine theta 2 plus F m 3; sine
theta 3 minus W a minus W L equal to 0. And
then sigma m about the elbow, if I take that
to eliminate J X and J Y all the components
axial components will not contribute to the
moment right .
So, it is only the perpendicular; the components
that are perpendicular there the Y components
that will contribute to the moments. So, let
us take I have let us say this is a 1, a 2
and a 3 and these are still b and c; the locations
of this is b and this is c . So, I have 
F m 1 3 into a 3 minus W a into b minus W
L because I am taking the clockwise moments
as negative; c equal to 0 .
So, now what are my unknowns in this?
.
Something's wrong? Yeah ok. So, if I look
at these equations .
Then my unknowns are I do not know the magnitudes
of any of the muscle forces hm, but how many
equations do I have?
.
1, 2, 3; so, I have more unknowns than equations
which makes it a statically indeterminate
system . So, there is additional information
I need in order to be able to solve the set
of equations . So, what are possible; ah what
other information could I use to solve this
system? I need some more information I need
2 more relations right something connecting.
So, what what are possibilities?
Ratios.
Ratios are forces. So, is there a way I can
find out how the forces are going to be distributed
among the muscles? One ah obvious way is to
based on muscle cross section area. So, I
can say that each muscle would produce a force
that is proportional to its?
.
Cross sectional area right . So, that is one
relationship I could use.
So, I can relate then that will give me a
ratio. So, I can say F m 2 by F m 1 equal
to a 2 by a 1 and I can call this k 2 1. Then
I could say F m 3 by F m 1 again or between
F 2 and is a 3 by a 1, I call that constant
k 3 1 . So, now all the muscle forces can
be expressed in terms of one muscle force;
basically F m 1 . And the other 2 muscle forces
are only are proportional to this F m 1 based
on these constants k 2 1 and k 3 1.
So, with this additional information; I could
solve for that. Other ways of knowing are
through what are known as EMG ah Electromyography;
basically measures the electrical activity
of muscles ok. it will not tell you how much
force is developed in each muscle; it just
gives you the signal when a muscle is active.
So, when you are doing this it will tell you
at least which muscles are actually active;
they put electrodes on the skin and look at
the whether the muscle is active at that instant
or not.
So, if you know for instance that a certain
muscle is not active; you can eliminate that
or if the activity of that it will tell you
when the activity starts and stops. So, it
will also give you an idea of; so for for
isometric forces there is some sort of a relationship
between the EMG signal and the force that
is developed in the muscle it is not true
for eccentric and concentric actions.
But for; so for a statics problem like this
looking at EMG and correlating that to the
ratios in which the forces are ah developed
in the muscles may be ok to do. So, that instead
of using the cross sectional areas; I could
look at EMG data; other more sophisticated
techniques include doing some kind of an optimization.
So, I say that ok I want these 3 forces to
act such that the overall muscle forces are
minimized.
