There are a couple of things.
You’re right that the ability of your muscles to extend is an important factor, and can be trained pretty significantly in general (ignoring joint problems like rheumatism / arthritis or whatever) So yeah, most people would be able to get a *lot* more flexible with the right exercise and stretching, though there would certainly be those whose maximum flexibility is more / less than others.
There’s also how stretchy your ligaments are. This isn’t a good way to be flexible, though, as it comes with a consummate lack of stability that can lead to all sorts of “fun” problems. I have [EDS](https://en.wikipedia.org/wiki/Ehlers%E2%80%93Danlos_syndromes) and have had endless difficulty keeping certain of my joints from a variety of problems (chronic pain, alarming ease dislocating, poor capacity to cope with unaccustomed exercise…)
It turns out that keeping my level of exercise near how much my knees / hips / whatever can cope with at a given time is the way to go. The gap between “enough exercise to be beneficial” and “enough exercise to mess me up for a week” is razor thin, but can be managed with effort…
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