Flexion and extension

Opening scene

Think of your arm. Bend your elbow.

Did your arm get smaller? No. The angle between the two bones got smaller.

Now bend your knee. The same thing happened; the angle between the two bones closed.

Now the hard part: standing up, lift your arm forward and up. You didn't bend anywhere, your arm stays straight. But this carries the same movement's name too.

If this sounds confusing, you're in the right place. Because the first time you hear these two words, they sound like "bending" and "opening," and as long as you think that, half of it gets misunderstood.

This lesson is about putting those two words exactly where they belong.

What this lesson covers

First we'll define the two actions and you'll see why describing them as "bending and opening" falls short. Then we'll establish the neutral position; every action is measured from a starting point. After that, we'll talk about how the same name looks different at different joints, and why the ankle carries its own names. Then we'll see how to read these two actions in yoga poses. You'll work through this in real practice with sixteen examples. By the end of the lesson, you'll be able to say which joint is doing which action in any pose.

Starting point

In the previous three lessons you learned where your body moves: the three planes. Now you're learning what it does.

The sagittal plane, the forward-and-back plane, has two actions, and they're each other's opposite.

Now let's go back to the confusion from the opening. When you lift your arm forward and up, nothing bent, but this is a flexion. Why?

Because the angle being measured isn't an angle within the arm itself; it's the angle between the arm and the torso. When your arm hangs at your side, the angle between it and the torso is wide. As the arm lifts forward, that angle closes.

So here's the rule: you need to know which two parts you're looking between. At the elbow, you look between the upper arm and the forearm. At the shoulder, between the arm and the torso. At the hip, between the thigh and the torso.

This single question resolves the whole "is it bending or opening" confusion: which two parts?

Mechanism

Neutral position: where you measure from

To say an angle has closed, you need to know where it started from.

The elbow's neutral is fully straight. Bend from there and it's flexion; opening it further isn't possible for most people.

The shoulder is different. The arm is neutral when it hangs at your side; lift it forward and that's flexion, take it back and that's extension. So the shoulder can go either way.

The hip works the same way. Lifting the thigh forward is flexion, taking it back is extension.

The spine's neutral is its natural standing posture. Folding forward is flexion, opening back is extension.

That last sentence matters. Neutral isn't a point, it's a range. Looking for one single correct shape called "neutral spine" mistakes individual difference for a fault.

Same name, different look

Now let's get to the practical part.

Flexion looks different at every joint, and that's the hardest part of learning the term.

At the elbow, flexion brings the hand toward the shoulder.

At the shoulder, flexion takes the arm forward and up.

At the hip, flexion brings the knee toward the chest, or folds the torso toward the thigh. Notice, these are the same action; in one, the leg moves, in the other, the torso does. Which side of the joint moves doesn't change the action's name.

This last idea is useful. Standing in a forward fold, your hip is in flexion; lying on your back and drawing your knee to your chest, your hip is also in flexion. The pictures are completely different, the action is the same.

At the knee, flexion brings the heel toward the hip. And the knee has almost no extension at all; it reaches neutral when it straightens and doesn't go past that.

At the neck, flexion brings the chin toward the chest, and extension takes the head back.

Why the ankle has its own names

There's an exception here, and it's worth learning now, since you'll run into it.

Why separate names? Because at the ankle, which movement "closes the angle" works against intuition, and separate terms are used to prevent confusion.

Here's the practical payoff: in squats, lunges, and Downward-Facing Dog, what matters is dorsiflexion. A restriction at the ankle in this direction affects everything above it; the knee, hip, and spine all end up compensating for that restriction.

Two joints at once in the shoulder

There's one more place that creates confusion, and it's worth opening up now.

When you lift your arm, it isn't only the shoulder joint working. Your shoulder blade also glides and rotates over your ribs.

So what we call "shoulder flexion" is really the sum of two contributions: the arm's movement relative to the torso, and the shoulder blade's movement over the torso.

Why does this matter? Because the shoulder blade needs to rotate for the arm to keep going up. If the shoulder blade doesn't move, the arm can't continue past a certain height.

Remember the example from Y1.07: the instruction "Pull your shoulder back" had four different people doing four different things in the room. Now you can see why: that sentence never says which joint it's talking about.

Here's the practical payoff: if your arm isn't going up, the limit might not be the shoulder joint. If your shoulder blade's movement is restricted, your arm looks restricted too, and forcing the shoulder joint doesn't help.

Reading poses

Now let's put this together.

The way to read any pose is to ask joint by joint: is this joint in neutral, in flexion, or in extension?

In Child's Pose, the hip is in flexion, the knee is in flexion, the ankle is in plantar flexion, and the spine is in slight flexion.

In a Cobra-like backbend, the spine is in extension, the hip is close to extension, and the elbow is in flexion.

In Downward-Facing Dog, the hip is in flexion, the knee is close to neutral, the ankle is in dorsiflexion, and the shoulder is in flexion.

This reading habit gives you two things. First, you can see what a pose is working instead of guessing. Second, when a pose feels hard, you can narrow down which joint is the limit.

Reading the examples

Master layer

Now let's refine this a little.

I've described flexion and extension as two clean categories. In reality, even a joint's neutral can be debatable, and it shifts depending on the measurement method. That's why using direction and comparison is more reliable than fixating on degree numbers: the sentence "right hip flexion stops earlier than the left" is more useful than any degree figure.

Second nuance: a joint not reaching extension isn't always a restriction. It's normal for the knee not to go past neutral; cases where it does fall under hypermobility, and there the goal isn't increasing range, it's staying within the range you have. The same measurement points in two opposite directions in two different people.

Third, the difference between the name of an action and the work of a muscle. "Hip flexion" is the name of an action, not the name of a muscle group. Different muscles produce the same movement in different contexts; in some cases, what produces the movement isn't a muscle at all, it's gravity. What bends the hip in a standing forward fold is mainly gravity, and the muscles at the back don't produce the movement, they brake it. Knowing the name of an action isn't knowing who's doing it.

Fourth, and subtlest: these terms don't evaluate a movement, they only describe it. "Spinal flexion" is neither good nor bad. A common narrative treats a rounded back as automatically dangerous; but spinal flexion is a normal, necessary action of the body, done hundreds of times a day. The question to ask isn't "is there flexion," it's "is this load and this number of repetitions right for this person."

Common mistakes and misconceptions

Practice

Keep in mind

Flexion closes an angle, extension opens it; but without knowing which angle, both get confused, because what's being measured is the angle between two segments.

Every action is measured from neutral, and neutral isn't a point, it's a range that varies by person.

The same action looks very different at different joints; hip flexion is present both in a forward fold and in drawing the knee to the chest.

The ankle carries its own names, and a restriction in dorsiflexion affects everything above it in squats, lunges, and Downward-Facing Dog.

These terms describe, they don't evaluate; spinal flexion isn't a forbidden movement, it's an action that needs to be measured.

Bridge to the next lesson

You've named the two actions of the sagittal plane. What's left is the actions of the other two planes: moving away from and toward the midline, and rotating around your own axis. We'll open those up in the next lesson, and you'll run into a surprise there: because joints like the hip and shoulder can do these actions at the same time, a single movement's name is often more than one word.