To provide you with a bit of context and go back to the actual definitions of each, we will find the following;
Joint flexibility: The distance and direction in which a bone joint can be extended or the joint range of motion dependent on the condition of the surrounding structures.
Joint mobility: The ability of a joint to be moved through its range across different planes.
In simpler terms, Joint Flexibility is the maximum available range of motion you have in that particular joint passively by an external force i.e. Momentum, force of gravity or even with someone else’s assistance. Joint mobility, on the other hand, refers to the available active and usable motion the joint can achieve independently.
Hip Abduction (moving your leg away from the anatomical midline), is one way to put this into practice. Stand straight in front of a mirror and kick your leg to the side using force (see Figure 1.A), and measure the range subjectively. Now stand straight again and lift your leg in the same direction, slowly controlling your movement, and compare the range of motion (see Figure 1.B). That difference in the range is the gap between your flexibility and mobility in that particular motion. You can implement the same concept on any joint and always compare right and left.
Now the question is, is this considered as weakness?
No! However, this is also dependent on your physical activity and whether you need this active motion or not. Because lack of mobility or “active range of motion” can increase the risk of developing a mechanical dysfunction which usually causes an injury on the long run.
This brings us to the next question; why should we work on our joint mobility?
In my opinion, working on mobility is as important as our daily workout, especially if we’re aiming for the same level of physical activity for the next 20 years. The more mobile an individual is, the more efficient, safe, and effective are his movement patterns. The human body has an active range of motion for a reason. To utilize it.
Now the last question is, how can we improve our mobility?
Well the concept is easy if we understood it. The gap between your joint flexibility and mobility is most probably due to SMCD (stability or motor control dysfunction) or in another words, your brain does not know how to activate the muscles at that range. One of the simplest ways to train the brain to activate the muscles is the following, take the joint beyond the mobility range and activate the muscles isometrically (muscle contraction without movement), hold it for 5-10 seconds for few repetitions.
In the following video I demonstrate the hip abduction mobility drill.