The Difference between mobility and flexibility

With a bit of understanding of the terms; flexibility, mobility and stability, we can go into the biomechanics of flexibility.

From a biomechanical perspective, flexibility of a joint is defined by the shape of the joint and how tight the joint capsule and ligaments are around the joint.  This is part of the reason why some people can touch their toes when they bend forward, and others cannot.  When you bend forward to touch your toes, the joints in your back are stretched.  This includes the joint capsules and the ligaments that surround the joint.  Knowing that when ligaments (and joint capsules) are stretched, they undergo permanent deformation and will never go back to the same length.  This does increase the flexibility of a joint, but the question is; would you want that permanent increase in flexibility?

Mobility is defined by the amount of muscular control you have over the entire range of a joint.  Talented athletes (gymnasts and dancers) have a tremendous amount of flexibility, but also have great mobility or control over the range that they do have.  Thus, many of them are able to avoid injury by controlling the amount of motion over a great range.

An injury will occur when we cannot control motion (such as through the spine) through its entire range.  Therefore, people who are too flexible will be more susceptible to injury near end ranges unless they are able to maintain control (mobility through range of motion/flexibility).  Not having the flexibility can also cause injury by not allowing one to move into a desired position (even if their mobility is good). This can lead to excess strain on joints and muscles.

The bottom line is that you want to be just flexible enough for your occupation or sport, and you want to have the control over that flexibility (mobility).