Whole muscle 3- Length/tension relationship

Whole muscle 3- Length/tension relationship


– So I want to talk
about the relationship between the length
of an entire muscle organ and the amount of tension that
that muscle organ can generate. I want you to just do a little
thought experiment here. If you fully extended
your forearm at the elbow joint so your forearm is fully
straight, you’re almost in standard
anatomical position. And in this position I want you
to imagine that you have– like, keep your elbow fully extended,
like, not any kind of– there’s not flexion
at all in the elbow joint. I want it to be fully
extended straight out, almost hyper extended so
that your biceps brachii muscle, which is this massive
bad boy right here, is as long as it
can possibly be. You can’t make it longer
without, like, applying forces and pulling it
apart and stuff like that, which let’s not do that.
And I want you to imagine, and you might want
to pause and do this. Like, grab something heavy, and then slowly lift
the heavy thing. So do like a biceps curl, and so you’re slowly flexing
your forearm at the elbow joint, and then go all the way up. Carry that brick or whatever it
is that’s going to be the heavy thing
that you are lifting from a fully extended position
to a fully flexed position, and think about– see if you can figure out
at what point can you apply the biggest force. And at what point do you have,
like, is it the weakest? And do it.
Stop and go do it. And now you’re back, because you always
do exactly what I say. And I’m going to show you
what I’m talking about. When the muscle is short, you actually are generating
a small amount of force. And this is actually,
like, I totally feel like that
is an intuitive truth. If I am carrying something,
my brick, and I have to lift it, I’m actually going to
flex my forearm at the elbow joint before I lift it. I’m not going to try and lift it
from a fully extended position. And I don’t even
think about that. Like, it just feels like it
would hurt to lift the brick from a fully extended position. And so I’m just, like,
watching, like, my brain goes,
“Oh, I’m just gonna flex, even though I’m not
lifting it anywhere,” and now I’m going to lift it. Now I’m going to
finish the lift. But to go from that
fully extended position, it doesn’t feel comfortable. It doesn’t feel comfortable
because you really can’t generate very much tension when
you’re– I’m on the wrong end. When the length of your
muscle is really huge, you can’t generate
very much tension. But you get the sense when you
shorten that muscle just a tad, you can generate more and
more and more tension. This is supposed to
be a smooth curve. And there’s a point at which
you’re like, “Dude, go ahead. Throw somebody’s face
in front of that brick, and I’m gonna knock the holy
living daylights out of them.” That’s a little violent.
I wouldn’t actually do that. Well, okay, we’ll stop saying things that are
violent in nature. But there is a point
where you’re like, “Whoa. I can generate a lot of force.” It would be kind
of in the middle of the shortening
of your muscle. And then there’s a point
where you know what? You go back down. And when your muscle is super
short, I mean, even though you can’t
really move it very far, because you don’t
have much distance to move it, but it doesn’t feel very strong. Like, if I want to hit somebody
in the face with a brick, I’m going to start from here
and go like that. I’m not going to start
from fully extended, and I’m certainly not going
to start from up here. It just doesn’t
feel very strong. So my question for you is– my first statement is
this is the reality. This is the truth.
My question for you is why. Why is it like that?
Well, to answer that question,
let’s go back to our friend that you are growing
to know and love, the sarcomere. Keep in mind
that our myofibrils are nothing but a whole bunch
of sarcomeres in series. And so if our–
now imagine this for a second. If I could take these thin
filaments, the green ones, that’s what I was going for, and if I could
stretch them out so I’m going to fully extend the muscle
and stretch those out, so that maybe the amount
of overlap starts, like, right here. And I’m going to take it
all the way over here, and my Z line is going to
be all the way over here. Did you see what I just did? All
I did was just pull it apart. And so this space in the middle
is going to be really huge, and the overlap between the
thick and thin filaments is going to be really small. Why is that not going to
generate very much force? Is it totally intuitive?
If the overlap between my thin filament and my
thick filament is, like, two myosin heads,
how can I possibly generate very much force if I’m only
using two myosin heads? And it’s totally true.
If you contract a little bit, shorten the sarcomere
just a little bit, there’s an optimal zone where
we actually have– look at this. This is probably close to
my optimal zone where, dude, every single myosin
head has access to the thin filaments to
generate a contraction, and generate more force. There’s a point at
which the actins are completely
overlapped with the myosin, and you can’t get any shorter.
If you can’t get any shorter, you’re not going to be able
to generate very much tension. You’re not going to be able to generate or smash
a brick somewhere, that’s not very nice, with your muscles
fully contracted. And again, that makes
intuitive sense. Think about sprinters who start,
you know, if they’re in a race,
the 200 meters. They get down into
starting position. They don’t start the race
standing straight up, because they want to be
as explosive as possible. And in order to be explosive, they need to be
partially contracted. They can’t be all extended
in the case of standing straight up and starting
running from that position. All right.
That’s super cool. Let’s talk about flexibility. Okay.
Good idea.

24 Replies to “Whole muscle 3- Length/tension relationship”

  1. YOU ARE SERIOUSLY THE BEST! I love your down to earth "dude's" and a little bit of "violence" that makes learning this enjoyable! 😉 This is a very hard subject to grasp, but you definitely help make things make sense to us common folk! Thank you!

  2. So interesting!
    But I have one question:
    How come that the change in the actin-myosin overlap is only caused by the movement of the actin? It's clear for me that the actin is connected to the Z lines, and I thought that the myosin is connected to them as well… But now I see in your video that the myosin filament is not connected to the Z lines, so how does it stay static?

  3. Hate to disagree with your explanation but…the primary reason for the difference in strength over the range of movement for this forearm flexion movement is the lever effect which changes the line of pull of the muscle. When lifting against gravity (vertically downward force) the optimal opposing force (vertically upward force) is when the elbow is at 90*. You will note in the sarcomere length-tension relationship that within the body the range of length in the sarcomere is only over the short middle range of the theoretical total length of the sarcomere. Therefore the lever design for the biceps favors maximal contraction when the elbow is flexed and is the real explanation of the change in strength over the full range of motion.

  4. The intuitive part you are describing are proprioceptors. The muscle spindle detects the length of a muscle, while the Golgi tendon organ is responsible for monitoring the tension of a muscle.

  5. Literally the best way this could have been explained, thank you! Plus you have a good mic, pls make more videos

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