“Flexibility” and “mobility” get used interchangeably all the time — including, at points, by major health organizations. Before asking what research says about either one, it's worth being honest about a more basic problem: the field itself hasn't fully agreed on what the words mean.
The Terminology Problem Is Real
A 2026 commentary published in Sports Medicine makes this point directly, examining how inconsistently “range of motion,” “flexibility,” and related terms are defined across the research literature. The authors point out that even the American College of Sports Medicine has used two different definitions of flexibility in its own materials at different points — at one point describing it as simply the range of motion available at a joint, and elsewhere describing it as the ability to move a joint through a full, pain-free range. Those aren't quite the same idea, and the inconsistency isn't unique to one organization.
Clinical resources aimed at the general public often don't sharply separate the terms either — it's common to see “flexibility” described as involving both range of motion and mobility in the same sentence, without a clean dividing line between them.
A Working Definition (With the Caveat Above in Mind)
With that inconsistency acknowledged, here's a reasonable everyday distinction: range of motion describes how far a joint can move, full stop. Flexibility is generally used to describe the passive extensibility of the muscles and tissue around a joint — how far it can move when something else, like a practitioner or your own bodyweight, is doing the moving. Mobility is more often used to describe how far you can actively move and control a joint under your own power.
You'll frequently see mobility described, especially in fitness and training contexts, as roughly “flexibility plus strength” — the idea being that having a muscle able to lengthen doesn't help much if you can't also control and produce force through that range. That's a useful mental model, and it's not unreasonable. But it's worth being clear that it's a popular simplification rather than a single, standardized scientific definition agreed upon by every professional or research organization.
Where the Simple Story Gets More Complicated
Research also complicates the idea that stretching is the only, or even the primary, path to better range of motion. A systematic review and meta-analysis comparing strength training to stretching found strength training was similarly effective at improving range of motion — suggesting flexibility gains aren't uniquely tied to stretching the way that's sometimes assumed. Separately, the 2024 Sports Medicine paper cited above notes there isn't strong evidence tying short-term range-of-motion increases specifically and exclusively to stretching, as opposed to other forms of movement and loading.
“Mobility equals flexibility plus strength” is a useful, widely used simplification — not a settled scientific formula. Research suggests movement and control generally are more tangled together than a clean equation implies.
Why This Distinction Is Worth Knowing
Being precise about these terms matters because it's easy to see “stretching improves flexibility” (well supported, see Does Assisted Stretching Improve Flexibility?) and assume it automatically means “stretching improves how well you move and function” — a broader claim that current research doesn't cleanly establish. The honest position is that flexibility is one input into how you move; strength, motor control, and other factors are inputs too, and the research on how they all combine is still developing.