“It's so vague though, and so many different interpretations, that who knows!”
Because it’s not a “real thing” as such, lol
I spent a fair bit of time talking about it with an ortho specialist because my last dog had elbow dysplasia.
Basically you want to build up the muscle supporting joints nice and gradually - but some things need limiting. But that’s too vague, so people start going, well how long? and 5 minutes per month is fairly easy to remember and does build it up gradually, so someone came up with that...then my assumption is that people have a time so they get all rigid about it... and you end up with a hard and fast rule that doesn’t really make sense.
But really it should be - fetch is terrible for joints, stairs and jumping on and off furniture are also really bad, keep an eye out for them overdoing it if they do things like play with other dogs and don’t march them for miles on lead...but their normal pace and just moving freely is fine.
Little and often is better for building muscle than long walks.