-: Okay, when we look at the knee joint,
and it's the body's ability to flex the knee
through a full range of motion here.
We got a variety of muscles that connect up at the hip
and cross the knee,
and then we've got a couple of muscles
that cross from the ankle above the knee,
namely the gastroc, which is drawn here,
the lateral head and the medial head.
And they basically interdigitate with the hamstrings
in order to help out with that motion.
Depending on where in the range that we are at,
the gastroc is going to take primarily,
either a primary mover role,
or it's going to take more of a stabilizer role,
as the hamstrings kinda take over the rest of that.
The gastroc, as a knee flexor,
is gonna primarily take care
and initiate the first about 15 degrees
before they come and hand off to the hamstrings.
And these hamstrings here,
we've got the all these different colors here.
The most medial one is gonna be the semitendinosus,
which starts at the ischial tuberosity, most medial,
comes down and actually attaches in the front
to the anterior tibia.
And right next to it,
we're gonna have one called the semimembranosus,
which has the thickest belly of them all
and kinda wraps around to the side here,
which is attaching to the medial tibia.
And then also, kind of more lateral,
slightly more lateral there as well.
The biceps femoris attaches laterally
on that same ischial tuberosity here.
If we had a hip, I'd show that but,
as that comes down, and it attaches directly to the fibula,
which is this bone, and this is the opposite leg,
but we can basically demonstrate that
by showing you that the bicep femoris
attaches right into the fibular head
and a little bit into the tibia, slightly more in.
And there's one more piece here of the hamstrings,
which is,
if we can turn the leg a little bit, what we'll see is,
there's something called the short head,
and the short head of the biceps femoris
is this little guy right here.
And it also attaches to the head of the fibula,
and it's only going to be utilized
as a knee flexor completely,
whereas, these other hamstrings have an additional function
of being hip extensors as well,
which we'll talk about more in detail later.
Other muscles that are involved in knee flexion,
kind of as a secondary function,
we are going to have the gracilis,
which is a primary adductor, but it will,
once this knee crosses this axis, will become a helper,
as far as knee flexion is concerned.
And also, a little guy that we have yet to mention
is called the popliteus,
and the popliteus attaches
from the medial side of the tibia, all the way,
and it crosses and attaches
at the lateral condyle of the femur.
And what this guy does,
because it still has a moment to flex the knee,
we will consider it a knee flexor,
but it's also considered the key that unlocks the knee.
Because even though we think that
this joint is primarily a hinge joint,
there's just a small amount of rotation that's allowed,
and before the knee,
basically, before the knee completely
or initiates its bend from fully locked out,
it needs a slight bit of rotation first,
in order to allow it to come out
of that fully locked out position.
'kay, the last muscle that's important to mention on here,
and Chelsea, can we flip you over,
is gonna be the sartorius,
which has a main function of a-bducting
and externally rotating the whole leg,
but because it attaches
right on the medial side of this tibia,
which is also known as the Pizancerine Complex,
it's going to have an ability to create knee flexion.
And then on the other side, it attaches right at the ASIS,
right in the front of the hip.
So these are your primary knee flexors.
-: So, when we look at the knee flexion,
in terms of application,
that relationship between the hamstrings and the gastroc
gives us a lot of considerations,
in terms of doing like leg curl based exercises.
So when we consider the fact that
the gastroc is gonna be the main prime mover
in that first 15 degrees,
and then it's gonna shift over to the hamstring,
that can change the intent, or the loading,
or even the range of motion that we would choose to use
when training knee flexion,
depending on if we're trying to bias the hamstrings,
or the gastroc, or just work knee flexion as a whole.
So if we're working just a quality of knee flexion,
we're just gonna work on the dynamic that we want,
but if we're focusing on one muscle or another,
in terms of gastroc versus hamstring,
we're gonna try and make sure that
that muscle takes over the majority of the load.
So that would be, if I'm focusing on hamstrings,
not launching out of the bottom
or maybe choosing a resistance
that's actually going to peak
in this top portion of the range.
Maybe restricting the range of motion
to everything outside of the 15 degrees
for maybe at least some of the reps,
but not all of the reps, lots of different strategies.
But what we see is the main thing that people tend to use
is they tend to do either plantar flexion or dorsiflexion.
So in this dorsiflexion position,
we have lengthened the gastroc around the ankle.
So when we get to this fully stretched position,
it's more lengthened, and as we come up,
it doesn't lose length as fast.
Now, this does not necessarily change much
the mechanics at the knee, but it does,
in an exercise like the leg curl
where the foot is not on anything,
give that gastroc a better opportunity
to create tension across the knee,
because that tibialis and all those plantar flexors,
or all those, sorry, dorsiflexors on the other side
are giving opposing tension for the gastroc to make
that can apply to both knee flexion and knee stability.
If we were to plantar flex,
then we're gonna put the gastroc
in both a mechanical disadvantage,
from a like shortening perspective,
and for the fact that we're not getting
that tension on the other side for it to be able to use
to create tension on this side.
So all of those dorsiflexors on this side,
if we're plantar flexing,
they're not giving us tension to work against
to act around the knee joint.
Now, that affects the output of the gastroc,
but what it doesn't do
is necessarily make it more hamstrings,
when we consider the fact
that what the net effect is gonna be
is actually less load.
So mechanically,
nothing's really changing at the hamstring
in terms of its mechanical advantage.
What it's having to do
is maybe make up for the lack of recruitment
or lack of involvement of the gastroc
at this more lengthened position,
so it is going to work harder,
but not necessarily harder
than if you were to equal out the tension
with the appropriate load
and still leave the gastroc as part of the equation.
So one thing to consider
is whether or not we are better off
trying to get more hamstring by taking out the gastroc,
which is a very potent knee stabilizer,
or whether or not we should leave that gastroc in
and use things like tempo, intent, range of motion,
and resistance profiles
to make the hamstring have to work harder.
And that's the direction I lean towards in my programing,
is trying to figure out ways to make the hamstring do more
by adding more work for the hamstring,
finding ways to get more tension in it,
not necessarily taking away the ability
for something else that's stabilizing
or acting on that joint to create tension.
And this works on the other side as well,
when we talk about a leg extension.
So the gastroc,
as we become more and more flexed at the knee,
becomes more of a stabilizer.
So if we actually look at the knee joint,
so when we get to this flexed knee position,
the gastroc is really gonna be
pulling the two bones together this way.
So it's pulling these two bones together,
so it's pulling the femur down
into the tibia and the fibula.
When we're in this position,
it has that ability to create that prime mover,
so when we're down in this position,
the hamstrings are actually pretty good
because they come around remember, attach the front here,
and they can flex that knee really good.
They have a good mechanical lever
at the knee for that flexion.
So from the bottom of a leg extension,
if my hamstrings have to do too much stabilization,
they're actually going to resist
and create more of a co-contraction around the knee,
so I'm gonna get less output out of my quads.
And my quads are gonna have to work artificially harder
to overcome that extra resistance,
because what I would really like in the situation
is for that gastroc to be doing the majority
of the stabilization work.
And if you imagine, the gastroc actually pulling up,
if we think of the gastroc actually pulling up,
it's almost, kind of, not directly, but in a way,
almost could actually assist that knee extension to a degree
before it actually starts
to get its levers for deflection again.
So in this position, the gastroc,
because of that compressive force,
works more synergistically with the knee extension
and is less opposing in relationship with the hamstring.
It's not technically a knee extensor,
but it is synergistic with the quads
in the extension in that lengthened position.
So when we look at something like a leg extension,
where our foot is not on the floor,
it's more important even then
that we're contracting these anterior muscles on the shin,
these dorsiflexors, to be able to get some stability,
some tension in that gastroc
to stabilize out of that lengthened position.
And that's also one of the reasons that I think,
from a loading perspective,
a leg extension is gonna be an exercise
that is best suited to load in the short position
or use an intent, where we go slow at the bottom,
and then we either accelerate,
or we find a way to create more tension
or more time at the top,
whether that's through pauses, quarter reps,
manual resistance, accommodating resistance,
using a smart cam or torque arm,
to bias that short position
that we don't get in any other exercise.
And take advantage of the fact that,
or at least account for the fact
that the foot's not on the ground, so therefore,
I'm missing an integral stabilizer
or at least disadvantaging it,
compared to when I'm getting in say,
squat mechanics, or leg press, or hack squat,
or split squats, lunges,
anything where the foot has contact with the ground.