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Traditionally, mobility and flexibility have been used as synonyms for one another – with both words referring to the range of motion available at a joint or in a direction of movement.

To be a bit more technical about it, the distinction between these terms is sometimes taken a bit further, especially in clinical circles. In this context, mobility is sometimes used to refer to how far a joint can move, while flexibility refers to how far the soft tissues that cross a joint allow the joint to move.

These two uses for these terms have been the established standard definitions… until recently.

New definitions for old terms?

A growing trend in the yoga/movement world is to proclaim that mobility and flexibility don't mean what I described above, and that they in fact have new definitions.

Mobility is said to be one's active range of motion, while flexibility is one’s passive range of motion.

This proclaimed new difference between mobility and flexibility is generally followed up with a strong statement that "mobility" is the superior quality ✅, while "flexibility" is pointless at best and harmful at worst ❌.

Research doesn’t actually support this oversimplified, black-and-white claim, but it’s still prevalent. If you’re interested, Travis Pollen, PhD and I took a deep dive into the research on this topic here!

What are active and passive ranges of motion?

Quick biomechanics lesson:

• Active range of motion (aROM) is the range of motion of a joint that one’s neuromuscular system can control through active force production.

• Passive range of motion (pROM) is the range of motion that a joint can be moved through passively.

For a yoga example, consider standing big toe pose, a single-leg balance pose in which we hold one leg out in front of us, hooking the big toe with our fingers or a yoga strap:

When we use our arm to hold our leg in front of us at its end range of hip flexion, we're displaying our pROM because that leg is relatively relaxed (passive). Our arm is doing the work to hold the leg up.

However, if we release our clasp of the big toe and attempt to keep the leg high in the air, we’ll shift into our aROM because the leg muscles are now actively working to hold the leg up.

The main takeaway here is that in recent years, it’s become trendy to map the terms “mobility” and “flexibility” onto the concepts of aROM and pROM.

From a movement science-based perspective, I question this trend.

These new definitions are a recent invention of a branded certification system – they aren’t actually definitions that are used in the field of biomechanics.

Scientifically speaking, we already have words for these concepts: the original scientific terms active range of motion and passive range of motion themselves.

Therefore, co-opting alternate words (mobility and flexiblity) to represent these concepts is confusing and unnecessary.

In fact, nowhere in the scientific research on stretching (that I'm aware of, at least) do researchers use the terms “mobility” and “flexibility” to mean “aROM” and “pROM.”

If researchers are discussing active vs. passive ROM, they simply use the established and universally-understood terms “active ROM” and “passive ROM.”

And when researchers discuss how far a joint can move, they tend to use the terms “mobility” and “flexibility” interchangeably because these terms are basically synonyms.

Accessibility in our language

As yoga teachers, if we’re interested in an evidence-based approach to our yoga practice, I think it’s helpful to respect and use the existing scientific terms for the movement concepts we’re discussing.

I also think this is a helpful practice for clear and accessible communication purposes in general!

Although it may seem like “everyone knows” that mobility means aROM and flexibility means pROM within our own yoga/movement niche, if we step outside of our relatively narrow circle, we’ll certainly encounter people who aren’t familiar with these “new” definitions for these terms.

And if we’re talking with people who are actually familiar with biomechanics – let alone stretching research(!) – we’ll probably come across as confusing if we explain to them that mobility means aROM and flexibility means pROM – when this simply isn’t the case in the context of science!

Main takeaway?

For clear communication, we should treat mobility and flexibility as the well-established synonyms they are. Both can refer to “the range of motion available at a joint.”

And when discussing active and passive ROM, we should use the easily-understood terms “active ROM” and “passive ROM” that already exist for these concepts.

There! Pretty clear and simple, right? 😉

Want to clarify more common movement terms we hear in yoga?

Grab my free guide, “Yoga Language Clarified: The Top 7 Misused Movement Terms”!

I personally finding that clarifying the meanings behind the words I use makes me a more confident and intentional yoga teacher, and I have a feeling you will too! Grab my free guide here.

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