So, I am going to challenge the orthodoxy here...it is what I do best 😉
Everyone talks about improving their mobility but what exactly do they mean? Do they mean they want to have more range-of-motion? Do they mean they want to move more freely? Do they mean they want less muscle tension? Or do they mean less pain?
OK, try this. Sit in the bottom of a squat for 5mins....if you are brave, do it for 10 mins. This is a great exercise for mobility isn't it? Or is it?
How many of you have done a sustained squat and get up feeling like you can't walk properly? Are you game enough to stand up after that and go for a 1RM? I know I wouldn't be!
You see mobility for me is how well you move with what you have. Flexibility is the amount of range-of-motion you have available to you.
A sustained squat improves your flexibility (and should be done with good form). But the sustained position compresses your joints and can make them "dry" - that is why you feel stiff afterwards...your soft tissues have been stretched (that's good) but your joints are not evenly lubricated...that's why you walk around shaking your legs and slowly do a few air squats to get the movement back into your legs (improving your mobility).
So static holds at end-of-range will improve your flexibility but decreases your mobility. Moving around a lot spreads the joint fluid around and improves your mobility.
For mobility, move more!