biomechanics of the major joints
Every joint in the body lives with the same conflict: it must be both mobile enough to let you move and stable enough not to give way. Different joints settle this trade-off differently, and that single design choice — favouring mobility or favouring stability — explains most of what each joint can do well and how it tends to break down.
Take the four big ones. The shoulder is built for mobility: a large ball sitting in a shallow socket gives it the widest range of any joint, but the cost is instability, so it relies heavily on surrounding muscles (the rotator cuff) and dislocates relatively easily. The hip makes the opposite bargain: a ball in a deep socket gives it stability to carry the body's weight for a lifetime, sacrificing some range — it rarely dislocates but bears huge loads, and its surfaces wear with the years. The knee is essentially a hinge that bends and straightens, but it also has to twist slightly and absorb large forces while standing on one leg; it leans on ligaments and cartilage pads (menisci) for stability, which is why it is so often injured by twisting loads. The spine is a stack of small joints and shock-absorbing discs that together must be flexible enough to bend and twist yet strong enough to protect the spinal cord and bear the upper body — a balancing act that makes the low back a frequent site of pain.
Knowing each joint's bargain shapes the whole approach to its rehabilitation. You protect the unstable shoulder by strengthening the muscles that hold it in; you respect the heavily loaded hip and knee by managing the forces crossing them (this is where moment arms and body weight matter); and you treat the spine as a system to be stabilised and moved carefully rather than a single part to be 'fixed'. The honest caveat is that a joint's mobility and stability are linked trade-offs — chase maximum range in an unstable joint and you may lose the very stability it needs, so the goal is the balance that suits the person's life, not the extreme of either.
After a shoulder dislocation, rehabilitation does not chase ever-more range; it strengthens the rotator-cuff muscles that hold the shallow ball in its socket. The joint's problem is too little stability for its great mobility, so the fix is stability, not more motion.
Each joint trades mobility against stability — the rehab plan follows that bargain.
Mobility and stability are linked trade-offs: chasing maximum range in an inherently unstable joint can cost the stability it depends on, so the goal is the balance the person's life needs, not the extreme of either.