MariposasMobility
Mobility is the ability to actively move a joint through its full intended range of motion, under control, using your own muscular strength. That last part is the piece most people miss: passive range of motion, where someone else pushes your leg into a stretch, doesn't count as mobility. You have to own the position yourself. Poor mobility shows up as compensations, like a squat where the heels rise, the lower back rounds, or the knees cave, because the hips or ankles can't get where they need to go. Flexibility is often confused with mobility, but flexibility just describes how far a muscle can lengthen passively. Mobility is flexibility put to work, combined with the stability and motor control to make that range of motion useful. In a training context, limited mobility in one joint frequently dumps load and stress onto the joints above or below it, which is a common root cause of overuse injuries.
Example
A lifter working on overhead pressing might have plenty of shoulder flexibility when lying flat, but the moment they try to press a barbell overhead from a standing position, their lower back arches aggressively to compensate for limited thoracic spine and shoulder mobility. Dedicated mobility work, such as controlled articular rotations for the shoulder and thoracic extension drills, gradually builds the active range needed to press in a stable, stacked position. Over several weeks, that arch reduces and pressing strength often improves alongside the improved joint control.