Flexibility is our ability to put ourselves passively into an extended range of motion. Imagine standing and using your arm to lift one leg up into flexion as high as possible. Mobility is your ability to control your ranges of motion, so now imagine letting go of your leg and trying to hold it in that same position with control. If you cannot hold it there without outside assistance then you have little to no strength in that range. While most people understand that passive range is mandatory before adding weight, it’s lesser known that problems and injuries can arise just as easily when an athlete has the passive mobility to get into a position but has no active control or strength in that same range.
 
In any given range of motion our tissues only have so much capacity to support a load effectively. Once we achieve a new range of motion we have to build up the tissues responsible for that movement in that new range. If we don’t, and we find ourselves in that position under a load that our tissues have not been trained to handle, we’ll be at a high potential for injury.
Let’s take twisting an ankle as an example. I can invert my foot passively to 70 degrees or so and I can bare all of my weight while balancing on the outer edges of my feet. If I’m hiking and I roll my ankle either within those 70 degrees and/or at a load that doesn’t exceed what I’ve trained my tissues to withstand in that range, I’ll be able to shake it off and keep going. If on the other hand I roll beyond 70 degrees of inversion OR I exceed the load bearing capacity of my tissues, if I fell for instance, I’m at risk of an injury. The process remains the same even if you only have 30 degrees of passive inversion. As long as you can produce force at that angle and you don’t go beyond those limits you’ll be fine.
Could I train my ankle to invert further than 70 degrees and be able to withstand more load at that range? Yes, and it would allow me more room for error, but at the moment that’s not a priority of my training. If I was a trail runner that would be different. When considering our mobility training we need to look at the types of movements we wish to do and assess first, whether or not we can passively get our individual joints into those positions, and then build strength in those ranges so they become useable. To continue developing more and more passive range without the equivalent strength work leaves us more susceptible to injuries and mishaps.