Mobility, if we look at the suffix, is going to be a word that describes a state or condition of the root word. In this case, mobile. To be mobile is to be capable of moving easily… therefore mobility is (the state of) being capable of easy movement. These are standard dictionary definitions. Note that there is a qualitative degree explicit in the definition (easily), and a physical degree (movement), but I believe the implication for the world of exercise, is that these properties must coexist along with capability – and that last word is where much of the confusion about mobility’s definition comes into play.
Capability (“capere = grasp/take + ability”) is best defined as the “practical ability” to do something – the you don’t just possess the ability, you have the tool-in-hand to do it. This is where it differs from ability (although they are virtually synonymous); ability is normally the skill or means to do something, but stops slightly short of capability. However, as long as we recognize the power to do something is inherent in our definition (be it from the use of the word capability or ability). The condition of being mobile (aka mobility,after we add the -ity suffix), must therefore have as part of its definition, everything intrinsic to that condition (as this is not a partial condition of being mobile, and no qualifiers have been added). The condition of being mobile, i.e. mobility, is that condition (or state of being if you prefer) under which movement is easy, and every quality and quantity necessary for same.
The preceeding definitions are standard English usage of these words and should hardly merit much debate. Whatever various bars we may set for this definition can be arbitrary or ad hoc, as long as they don’t alter the practical meaning of the definition itself.
Now when we define mobility as capable of moving easily, we see that much of the mobility we see in fitness does not fit the definition. So, under our own power, our ability to move physically, with relative ease, is what I say it means (or ought to mean) to be mobile. A mobile football player would be one that moves about the field with these descriptors applying, and face no sound argument otherwise. Conversely, if my arm is paralyzed, though able to be externally moved into all sorts of bizarre positions, we’d think it a bad use of the word “mobile” to say “I can not move my arm, but it is extremely mobile” (here we don’t want to conflate mobile with loose or malleable or able to be moved by an external force). This is because it’s not just enough to have the potential to move, one must also be capable of moving.
Explicit in the definition of mobile is the relative ease of that physical movement. For this reason stretching, and range of motion, although they often play a role in the overall mobility of a particular limb, are not essential qualitative or quantitative effects, despite their ability to either enhance or limit mobility. An extremely mobile quarterback is not overly dependent on their range of motion or flexibility, although they need a minimal level. We can certainly think of linemen who aren’t very flexible, but show the extreme mobility needed to chase a quarterback out of the pocket.
Range of motion, above a certain point, produces a diminishing return on one’s capability to move easily (and sometimes negatively affects it). The same can be said for flexibility. If I could willingly contort myself into all sorts of positions, but could not walk (or run) faster than a toddler, again, we would find it odd to call me “mobile”. A base (non-detrimental) level is necessary for these two qualities, but neither warrant specific inclusion in the definition of mobility, because we are not including things that need exist in a bubble not far above sickness/injury (or we would be listing all sorts of pathology that make us less mobile, like having a clubfoot or a painful Urinary Tract Infection). These things that can make us less mobile or even immobile, are covered under “capability” as their removal below a certain level would be sufficient to abrogate any of the three necessary qualities of mobility.
Our three necessary qualities are not sufficient, however – the lack of various conditions that may encroach on their respective status is also necessary. Therefore, saying that flexibility is part of mobility, is more appropriately listing an attribute that can affect one of three essential properties of mobility. Yes, if a thing itself is essential to another essential thing, to produce a state of being, then it too becomes essential – but in this case we define it as a dependent necessity -a thing that is only necessary because it affects another. Hence, we are not concerned with every possible thing that can affect one of our three essential qualities of mobility, we are only concerned that if we should come up with something with an effect, we can point to one of those three things and say “aha, having a sprained ankle limits ankle flexibility and range of motion, which makes us less mobile, because it limits the ease and power with which we can move, ” and therefore conclude that spraining an ankle will make us less mobile.
And clearly if the physical expression of my movement through space weren’t (relatively) easy, again, we’d find it odd to say I am “mobile” (as locomotion itself is difficult). Think about that in practice “I am very mobile and physical movement is not easy for me.” Again, this is nonsensical.
The best working definition of exercise based mobility is that which carries the three essential characteristics listed in the opening paragraph, but makes use of the standard parlance from which these words are derived, and perhaps eschews what they’ve grown to mean within the confines of the fitness industry (something to do with a bands or a foam roller or whatever).
Mobility is being capable of easy movement. There may be a near infinite list of issues that reduce mobility, and we may rightly say that by addressing any of them, we are “working on our mobility,” because to the extent that they limit our ability to move easily, working on them is working on our mobility. However, if flexibility is a limiting factor in my own mobility, though not yours, we could both be stretching and I can say I’m working on my mobility, but you can’t, even though we are performing the same stretch. If you require myofacial release to improve your range of motion, but I don’t, then the foam rolling we do together is you working on your mobility, and me practicing to roll a three foot long joint without using my hands.