Some readers may have seen this idea in practice before in my discussion of runner's knee and IT Band Pain Syndrome. Both issues often impact the knee while really originating from the hip. In addressing the hip in specific ways, those injuries can often be improved. This understanding helps show that pain or issues at one joint usually come from a neighboring joint's lack of mobility or stability.
Joint and Primary Need
Lumbar Spine- Stability
Thoracic Spine- Mobility
Gleno-humeral (shoulder joint)- Mobility
Whenever a mobile joint becomes immobile, it forces the neighboring joint to take on that mobility. For example, if the ankle is immobile it forces the knee to become unstable as it tries to take on that mobility. Low back pain, to give another example, tends to come from a lack of hip mobility. Often these joints can be a chain of dysfunction- lose ankle mobility, get unstable knees with pain, lose hip mobility and get low back pain as it tries to be mobile rather than stable.
Each joint has a specific mobility or stability need. When those joints are not being used properly according to those needs, then the issues radiate outwards to the rest of the body. Often issues can be resolved by improving the mobility of the joints above or below the joint experiencing pain.
For runners, there is one joint in particular worth looking at even more than the rest-the hip. While needing mobility, the hip is also a bit more complex than the other joints- it can be both immobile and unstable. A weak hip can cause knee pain that many runners experience, and a tight hip can cause lower back pain.
Many of the bad habits runners get into can be explained easily through this model and by taking a look at the hip. For example, if the hips are too weak to either properly flex or extend (motions we do in running when driving the leg in front or behind you from the hip), then the lower back tries to make up for it.
Knees can often face pain due to a weakness in abduction (moving the outside leg sideways away from the body), which in this case is really the prevention of the reverse motion of the legs (sideways across and in front of the body rather than away).
If the psoas (the most important core muscle) and iliacus (one of the key hip flexor muscles) are weak or don't really activate properly then the lower back tries to be more mobile to make up for it. If the glutes aren't being used or activated properly, they will add to this issue by causing more compensatory motion of the lower back.
This issue creates a vicious cycle in many runners- as the spine moves more to compensate for low hip mobility, then the hips get more tight and less mobile, forcing the lower spine to be more mobile rather than fulfilling its natural function as a stabilizer. The hips keep getting weaker while getting tighter at the same time- requiring both greater mobility and greater strength. That's why hip strengthening sequences are so important for runners.
A final quick example is tight hamstrings or a tight lower back. While hitting the foam roller may temporarily help loosen them, stiffness could easily return because the need for stability nearby wasn't addressed.
In the joint-by-joint system, joints cannot be addressed in isolation because their issues are related to the issues of the joints nearby as well. For example, fixing the hip by itself won't be enough to prevent a return to stiffness and lack of mobility without additional stability created for the knees and lower back.
So the big takeaway here is to think in terms of training mobility or stability in a given joint area. When doing do, be sure to take a look at how the areas above and below are functioning as well. Are they properly stable or mobile as well? If not, those need to be addressed too.