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- [Voiceover] When you
start using your muscles

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you can use them in different ways.

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I know it sounds kind of weird,

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like you know, your muscles
contract they move something,

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not always.

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In the case of an isotonic contraction,

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sure, the muscle is shortened in length

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that means it's moving a physical force.

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You're picking up a book,
you're grabbing a glass of water

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those are isotonic contractions.

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You're physically shortening the muscle

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and moving something.

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Now there are also a type of isotonic

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where the muscle elongates a little bit.

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The key thing is isotonic,
the muscle is moving,

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while isometric the muscle doesn't move.

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There's no movement,
you're not moving an object

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but in isometric
contractions, you're still

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putting force into it.

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Think this way, you put
500 pounds on a barbell.

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Try and pick it up,

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what's the odds you're
gonna pick up 500 pounds?

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Probably for the vast majority
of people in this world,

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not gonna happen.

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That would be an isometric contraction.

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You're trying, your muscles are flexing,

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you're trying to move them

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but the weight is not moving.

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The muscle's not shortening,

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you're not moving that weight.

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So you actually have
these two different types,

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isotonic, isometric.

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Now the amount of nerves impulse,

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the amount of nerve activation,

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the amount of signals basically,

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influences how much force.

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The more signals you have,
the more force you'll exert.

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The fewer signals you have,
the less force you'll exert.

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All depends on what you're trying to move.

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Now the way this is all broken down

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is called a motor unit.

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A motor unit is going to be a neuron,

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so motor neuron, that's
what carries the information

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from your brain and spinal
cord down to the muscle cell.

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But the motor neuron
can connect to several

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if not dozens of muscle cells.

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So that means every time
this one neuron fires,

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all the attached muscle
cells are going to contract.

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That's a motor unit.

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And this motor unit really is the smallest

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functional unit for muscle contraction.

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You have to stimulate motor units,

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you have to stimulate a group of cells

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to get them to move.

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And as they start to move,

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or even if they're holding steady

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they're generating some tensions,

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we're talking about muscle tension.

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Now, muscle tension is a mechanical force.

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It's what your muscle can
generate as it contracts.

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That means as the muscle's larger

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it could have more force, more tension.

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Muscle's smaller, has less tension.

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So this muscle tension,
how much tension or force

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is being generated is determined
on the motor unit size.

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How many muscle fibers are stimulated

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for a particular neuron.

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It's the number of active motor units.

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The more motor units you have,

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the more force you can generate.

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And the frequency of stimulation.

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If you can continuously, constantly

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stimulate a motor unit, it will
continuously and constantly

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create force and create tension.

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That's why you can vary the
different amounts of tension

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and force in your limbs, your body.

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You can pick up a one pound book,

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or you can pick up a 10 pound barbell.

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You differ the amount
of force you're using

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to move those objects.

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And when you do this, it's all or none.

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Once a muscle cell is stimulated

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it will contract fully.

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Once it is not stimulated
it will relax fully.

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There's no half contraction for a cell.

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Now obviously you can change
how much force you exert

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in a whole muscle, that's
because you're adding

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or subtracting motor units.

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You cannot change the strength

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of an individual muscle cell.

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So what happens is as you have
those muscle cells working,

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they're generating your muscle tone.

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Always your muscles have some tone.

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Even when you lie in bed, even when

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you're sitting down calmly,

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muscles are maintaining a
tone to maintain your posture.

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They maintain a tone
to keep you breathing.

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They maintain a tone to
make sure you're ready

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to react to any situation.

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So you always have some muscle tone.

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But if you need more force than
what that tone is providing,

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then you have motor unit recruitment.

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In the case of recruitment,

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it's gonna bring more motor units in,

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causing these motor units to
stimulate more muscle fibers.

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More muscle fibers means more force.

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So pretty much you're
recruiting more motor units,

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recruiting more fibers
to generate more force.

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Here's an example of a
neuromuscular junction,

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you can see the motor neuron
in the top figure is yellow,

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coming down and meeting
with a muscle cell.

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In the bottom picture, which
is actually a photograph

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of actual muscle tissue, you
can see the thick black line

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coming from the top right
and then kind of branching

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down into the red muscle cells.

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Those are the neurons coming down to meet

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with the muscle.

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That's gonna bring the
electrical impulse down

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and then transfer it into the muscle.

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The term muscle twitch is
referring to any complete cycle.

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It's a complete cycle of
contraction and then relaxation.

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Therefore any stimuli that can cause

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a contraction-relaxation
is generating a twitch.

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Most twitches are minute and small.

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To create overall movements,

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let's say taking your arm and flexing it,

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showing muscle, that's
multiple stimuli all in a row.

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All this electrical impulse,
all these muscle twitches

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can be observed using a myogram.

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A myogram is gonna be
an electrical reading

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showing these impulses.

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So here's an example,

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the first stimulus at time zero,

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what does that do?

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That is called the latent period.

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Nothing moves, that's when
the signal is being sent

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from the brain or spinal
cord down to the muscle.

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Once the muscle receives the signal,

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you go into the contraction,

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it's generating force,
generating tensions.

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Once you generate enough force,

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or you've had enough time
based on this stimuli,

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you go into relaxation.

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That's one single muscle twitch.

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But then if you notice,
if you move into this

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kind of more middle of the
graph, in that middle tone,

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you can see you have a stimulus,

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goes up and doesn't fully relax.

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You have another stimulus,
and another stimulus,

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and another and it keeps
causing it to climb

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higher and higher and higher.

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That means it's caused it
to have more and more force.

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That's called summation.

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But there's a limit.

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There's a limit to how much
you can go through summation.

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You can only reach your maximum tension,

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or maximum force.

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Your muscles can only move so much weight.

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Once you've reached that top,

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the muscle's not really
contracting or relaxing

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it's just staying in the
highly contracted phase.

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It's referred to as tetanus.

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I'm thinking tetanus as in tetanus shot,

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yes they're related.

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'Cause the tetanus shot

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is meant to obviously prevent tetanus.

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Tetanus is when a muscle
freezes or locks up,

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so the shot is to try
to prevent any muscles

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from freezing or locking up,

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that's why it's the tetanus shot.

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These muscle activities,
we talked about different

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types of movement, isotonic, isometric

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but there's also
different types of fibers.

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So, it's called slow twitch

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and that one's called fast twitch.

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In the case of slow twitch

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people that are working out
for long periods of time,

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jogging, biking, swimming,
long, long durations

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pretty much anything that needs endurance.

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So these slow twitch muscles

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are going to have a long
phase of contraction.

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They'll slowly contract over
a longer period of time,

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allowing each contraction to last longer.

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While the fast twitch, think of sprinting,

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weight lifting, tennis, very short quick

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durations of signal.

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Very short quick amounts of movements.

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They're meant for more
of the strength training,

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more of the quick reaction training.

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So it's all gonna be short
duration, fast twitch.

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Slow twitch, long duration.

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Alright, strength training

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now is this one of those things

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where you go out and start doing
a whole bunch of exercises?

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No, no, no this is
something where you have

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to kind of think about what you're doing,

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'cause very easily you could injure

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or tear one of your muscles.

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So when strength training one
way is resistance training,

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short very intense workouts.

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Because they're short and intense

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they're building more of
the fast twitch muscle.

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Aerobic training, well in aerobic training

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you want to make sure
you have plenty of oxygen

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getting into your muscles.

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And by doing that you're
training your muscles,

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you're training your heart
to become more efficient,

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to build endurance.

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It helps to increase blood flow,

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and usually once you hit
that target heart rate

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you want to hold it for 20 to 30 minutes.

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And ideally that should
be three times a week.

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So, aerobic training, great for the heart,

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great for the lungs, it really helps

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increase blood supply to
all your muscle cells.