Some ideas behind the Joint by Joint

Many people would wonder - "what determines if a joint gets placed in the mobile or stable category". For some, certain joints intuitively fit into one or the other category. For example the lumbar spine is inherently a region not built for large ranges of motion and functionally acts as a reactive stabilizer. Therefore it gets placed in the "stability" category. With this mind I have good reason to believe one's ability in separating joints into categories has everything to do with their knowledge of anatomy, both functional and non-functional. Yet some may still struggle to figure out how/why you placed the "lesser known" joints in their respective categories - specifically the smaller joints of the foot and hand or the proximal distal articulations of long lever bones.So in order to avoid the cop out answer "just go study anatomy and you'll know whether it gets placed in the mobile or stable category". Could you afford the less informed readers of some guiding principles and standards, be it anatomical or physiological, to which they should hold joints up too when deciding its categorical placement?I do think there is quite a bit of truth to looking at joint anatomy to begin to understand joints' optimal function.  The shapes of the synovial joints don't necessarily dictate mobility and stability in terms of the joint by joint, but they do indicate bony approximation that lend to stability.  If you think of the knee's condyles, the fist-like femoral condyles can literally bump into the tibial plateau in frontal plane movement.  The bony contour lends to stability in the frontal plane.  The Advanced Joint by Joint notes the Frontal Knee as "stable."Similarly, the ball and socket or condyloid joints allow for free and smooth movement.  Most likely, these are going to be part of mobile joint systems.While the above is worthy of mention, particularly because you mentioned it first in the question, the essence of the Joint by Joint Theory lies in the principles of stiffness.  Stiffness is defined as change in tension over change in length.  We see examples in stiffness with 4 sizes of Jumpstretch or Superbands.  All the bands are the same length, but based on their thicknesses, they each have different presentation of stiffness.  It takes much more force to pull a Blue band to the same length as a Miniband.  The Blue is more stiff than the Mini.Stiffness is managed through joints through it's planes of motion.  To some level all joints can exhibit multi-planar motion, so all sides of the joint can contribute to the stiffness of a joint in a certain direction.  Contributions can come from bony shape that we already mentioned, ligaments' restraint, muscle hypertrophy, muscle force generation, joint surface integrity, and neuromuscular control.So back to classifying joints or joint systems as mobile or stable, it is all a construct of relative stiffness.  Mobility and stability can only be descriptors in relation to 2 joints interactions in a given movement.If you want to squat high to 50 degrees, the hip is mobile when you get to 50 degrees.  We may entertain this as a potential hip mobility problem, but only because the reference is the Deep Squat ATG.  To see what qualifies as mobility and stability is simply a relationship of the stiffness of the neighboring joints.The mid-foot is stable as it must control it's own large range of motion to allow it's neighbors at the ankle (talo-crural) and big toe to be more mobile.  Again reference what you believe to be technical proficiency, and the descriptors find themselves.In (a) blog post, you briefly state:"The first thing I look for in evaluating an exercise choice is if it meets Coach Boyle’s Joint by Joint Theory when it is executed with technical proficiency."How specifically does an activity, when done to technical proficiency, "meet" the Joint-by-Joint theory? What about that exercise inherently makes it Joint-by-Joint acceptable? What are the guidelines and boxes we need to check off before giving our stamp of approval?So as above, stiffness can be created by desirable or undesirable means within a joint.  Mobility and stability are just relative terms in what we want to see joints do in relation to each other, and it must be to a referenced movement.The exercises we choose are those referenced movements.  And when we see more movement (mobility) vs. less movement (stability) in the exercises we choose, that is how measure them against the Joint by Joint.  Do we see and/or feel range created in a joint vs. range controlled in a joint?  And does that look like what we think technical proficiency should look like.Now there are bad movements done with their own technical proficiency, and these movements demonstrate movement creation at joint systems that equate to injury when moved too much or too often.So it's first understanding stiffness.  Then it's about following injury patterns at joint systems that move too much.  Those are stable joints.  The neighboring joint is mobile.  That is the measuring stick.