The basic definition of Flexion and Extension can be found in this blog, but there’s enough interesting nuance that I felt it deserved a full post so we can go through different common examples, so you can be confident which way to bend when someone tells you to flex!

 
As a quick recap:

 
Flexion: Closing a joint (decreasing the angle between two bones)
Extension: Opening a joint (increasing the angle between two bones)

 

As a really easy way to remember which one’s which, you can often just replace “flexing” with “bending”:

 

When you bend your elbow, you’re flexing it (bringing your Humerus closer to your Radius)

When you bend your knee, you’re flexing it (bringing your Tibia & Fibia closer to your Femur)

Then whenever you straighten them out again, that’s extension! Super simple!

 
Hinge joints like the elbow and knee can only (really) do flexion and extension. But what about more complicated joints that can move multiple ways, or larger areas of the body which involve lots of bones? How do we define which is flexion and which is extension?

 

A general rule of thumb is that flexion goes forwards (anterior  motion), decreasing the angle at the front of your body, and extension goes backwards (posterior motion), increasing the angle at the front of your body. For example:

  

Neck flexion would be to drop your head forwards, bringing your chin down towards the chest, decreasing the angle at the front of the body. Then extension is going back through neutral and drop your head behind you, increasing the angle at the front of the body!

 

When you lift your thigh towards your torso, or hinge your torso towards your thigh, you forward motion means you’re flexing your hip. Again, you’re decreasing the angle at the front of your body. You may hear this being described as “closing your hip”, then when you’re stood straight or extending your hips like in a Couch Stretch or Glute Bridge, you’re “opening your hip”.

 

But, it couldn’t just be that simple! The astute amongst you might have thought “Ok, but what about when I bend my neck sideways?!”. WELL smarty pants, I’m glad you’re here, because we also have Lateral Flexion & Extension. We know from this blog that anything lateral refers to your sides, and certain areas of the body can not only flex forwards, but sideways as well.

 

Your neck is part of your spine and can flex anteriorly  (forwards) and laterally (sideways), and so can the rest of your spine!

Bending to the right means your right side flexes, while the left side extends and vice versa. You can also do inbetween-ers: flexing diagonally! A combination of Anterior & Lateral flexion!

Alas, some joints have even more range of motion, let’s look at a common one:

 

Being a ball and socket joint, it has the largest range of motion of any joint and trying to define what’s flexion and what’s extension is seems almost silly.

Alas, it must be done. Once you know, it isn’t that strange… well, maybe a little bit:

Shoulder Flexion: Lifting your arm up in front of you & up over your head.
Shoulder Extension: Lifting your arm back, behind you.

This seems a bit counterintuitive, but remember that flexion & extension are defined anteriorly, so we look at what the front of the body is doing. When your arm is behind you, the angle at the front of your body is larger, then when your arm is overhead it’s smaller (or 0 if you can get your arms straight overhead).

 

In terms of the joint, in shoulder flexion we look at the upper arm bone (humerus) moving closer to the acromion of your scapula – decreasing the angle the same as other flexing movements.

 

So, when you lift you arms overhead and bend backwards remember that you’re extending your hips, extending your spine… but flexing your shoulders!

(By the way, if you’re wondering what it’s called when you lift your arms out to the side, that’d be abduction & adduction)

To finish, I thought I’d bamboozle you with two very silly examples of flexion and extension, which almost breakdown the neatness of our definitions so far: Wrists and Ankles.

 

To most people wrists both flex & extend.

 
If you remember from the Anatomical Position, palms facing forwards is the default position. This means that when you bring your palm towards your forearm, you’re bending forwards, or anteriorly, defining this direction as flexion. Similarly, when you move the back of your hand towards your forearm, you’re bending backwards and increasing the anterior angle, creating extension.

 

So far so good, right? However, any smarty-pants from before might have spotted that both these actions decrease the angle between the wrist & forearm (depending on which side of the forearm you look at)… so surely they’re both flexion – right?!

 
Yes… you’re right. 

 

Even though most normal people say that wrists flex and extend, technically they just flex:

 

Palmarflexion: Bringing your palm towards your forearm.
Dorsiflexion: Bringing the back of your hand towards your forearm.

So, depending on how fancy you want to be, feel free to use flexion/extension or palmarflexion/dorsiflexion when referring to the wrists!

  

Speaking of Dorsiflexion… isn’t that what ankles do?! The two sagittal movements of the ankle are usually called:

 

Dorsiflexion: The top of the foot moving towards your shin/shin moves towards the top of your foot

Plantarflexion: The sole of the foot moves back, and toes point towards the floor (/top of the foot moves away from your shin)

So, it seems our ankles, like our wrists, only flex and never extend!

 
After some research I found many different answers why this was the case, but ultimately it seems to come down to semantics and which angle we chose to define as increasing or decreasing. Dorsiflexion decreases the angle between the top of the foot and the shin, whereas plantar flexion decreases the angle between the heel and the calf. Take your pick! 

 

If anyone does have a good explanation why our ankles only flex, please let me know! Or maybe it truly is because extension “could be considered counter-intuitive” (as stated by Wikipedia).
 

Happy Flexing!