So now let’s go through
how force is produced.
Force is produced in two different
ways in skeletal muscle.
The first way force is
produced is how you would
think about it, as you go
through power strokes.
As you contract a muscle,
you’ll develop tension.
Interestingly though, tension is developed
be dependent upon the length of the muscle.
So you get this kind of formation where
you have a peak and two valleys.
So at a certain length,
your muscle will be the
strongest or able to
develop the most tension.
And then, as you stretch it too far,
it doesn’t develop as much tension,
or if it is too contracted, it
doesn’t develop as much tension.
So there’s an optimal length
for tension development.
So this is the active process.
When we put back in now
a passive tension,
and that is it’s less of an
ability to be stretched.
If you stretch it to
a greater degree, it
develops more tension
just like a rubber band.
If you pull on a rubber band,
it starts to resist you
as you get to a higher
and higher length.
That is the passive component
of muscle tension.
To determine the total muscle tension,
you have to add these two together.
And here, you have a
peak in force at a given
length associated with
active force production
and then you have a
second peak associated
with some active and
some passive components.
The other item that we need to consider
is what is driving this process.
It’s all based upon a nerve.
We discussed alpha
motor neurons earlier,
now I’m going to group them together
as an entity, as a motor unit.
So that motor unit encompasses all the
different fibers that it innervates,
and one alpha motor neuron is going
to innervate a number of fibers.
The motor unit then stimulates the muscle
by releasing acetylcholine binding to
an acetylcholine receptor that causes
an action potential on the sarcolemma.
But what happens if you stimulate this
particular muscle by electrical stimulation
instead of how the nerve normally
would stimulate with acetylcholine?
Here, each of the individual lines in red
are stimulations by electrical stimulation.
You can see if you pulse a couple
of stimuli that are fairly
wide in terms of their duration,
you get individual twitches.
If you start to train them faster, you
will get an individual twitch, will not be
able to fully relax before it gets another
twitch, before it gets another twitch.
So you can start to summate
twitches if you train
them or to give them in a
shorter duration of time.
You can also cause
something called tetanus
if you stimulate the muscle
at a higher frequency.
There is both fused and unfused tetanus
dependent upon the cycle frequencies.
The difference between the
two visually, you can
see here, is that unfused
a jagged kind of appearance
every time you get a
pulse and a fused tetanus
is more of a flat line.
But also notice that with fused tetanus,
you do get more contraction than you do
or forced production than you
do with unfused tetanus.
So why is this important
for us clinically?
Well, this process starts to get at the
point of you need to develop tension,
and how you develop tension
is via calcium release.
So there are ways to release more calcium
in certain times than there are in others
and your body will utilize
this component of
individual twitches and
having multiple stimulations
to a muscle to make greater and greater amounts
of force all mediated through calcium.