FitSpaceFitSpaceFitSpace
Short answer: Mobility is a joint's ability to move through its full range under muscular control — not just a muscle that's been stretched. Regular stretching lengthens a muscle but doesn't always improve control through a joint's range, which is why a "flexible" person can still move stiffly and get injured. Ten minutes of mobility work before training plus two to three short sessions a week does more for joint health than one long stretching session once a week.