Centration is a term I first saw used in Todd Hargrove's book A Guide to Better Movement.
In Hargrove's book a centrated joint is one that is as close to centred as possible given the constraints of the pose or action. It implies maximum surface contact between the two bones of that joint.
If you buy into the idea of tensegrity, the ends of two bones don't actually come into direct contact. In fact one surgeon, a strong proponent of tensegrity states that in knee operations he observed the bones of the knee pulling away from each other.
One explanation for this is that the joint capsule (the envelope of tissue that contains the synovial fluid of a joint) actually pressurizes the fluid in such a way that it forces the bones apart. It could be like squashing a balloon in the middle so that the ends of the balloon move further away from each other.
And so the idea of maximal surface contact could be misinformed.
The Idea of Ligaments?
According to J.C. (Jaap) Van der Wal MD PhD the idea of ligaments is an anatomical fallacy. Ligaments are actually created when anatomists dissect a cadaver.
Instead what Van der Wal suggests as a basic architectural element of movement and proprioception is a dynament which is two bands of connective tissue, each attaching to a different bone, and muscle fiber that connects these two bands. There are no actual discrete ligaments in the body. Instead the connective tissue network has within it elements that perform the required functions. As a result, when muscles are activated, they don't just add tension to their tendons, they also add tension to the ligaments.
We are used to thinking about ligaments as coming into play only at the extremes of joint positioning to prevent the joint from going being it's design limits. If ligament tension is affected by muscle activity then the implications are that ligaments don't just function at the end range of a joint, but ideally throughout the whole range of movement of a joint.
As a result of this, muscle activity not only causes movement or stability it also changes pressure in the joint capsule fluid via changes in joint capsule tension.
Van der Wal also goes on to suggest that this functional element acts as both a control mechanism and a proprioceptive mechanism. It is thus not only an architectural unit of control but also a basic unit for sensing the body.
My own experience and understanding of the body is that tension is one of the primary quantities that we can consciously learn to feel. With the right amount of tension, we can not only feel our body but it also becomes more responsive.
Learning to Feel when a Joint is Centrated
So how does this relate to the idea of centration?
With training we can learn to feel our joints, or actually what we may actually learn to feel is the tension in the connective tissue around the joint or that crosses the joint. These networks of tension with their voids filled by bones and joints, can give us a three dimensional sense of our body.
How do we recognize an absence of centration (or "balanced tension")? One way is to notice an absence of tension or "feel". Another, perhaps easier to detect is an excess of tension, either connective tissue tension or actual muscle tension.
How do we then recognize when we have achieved centration? By the feeling of balanced tension all around a joint. The feeling is subtle but with relatively simple training perceivable.
Moving Slowly and Smoothly
One of the best exercises for learning to feel this tension is tai ji, particularly if you focus on isolated movements to begin with. The main attributes of tai ji that make it excellent for learning to feel tension are slow and smooth movements done well within the bodies range of motion.
We don't need a tai ji master or even teacher for this, we can focus on smooth isolated movements of the body, while focusing on simple elements of the body so that we can learn to feel it.
Centrating the Shoulder Blades
A very simple set of exercises is learning to feel and control the shoulder blades. We can thus learn to centrate the shoulder blade with respect to the ribcage by balancing tension in all directions against the weight of the arm itself.
Centrating the Spine
Another way to practice centration is to practice bending the spine backwards and forwards, in isolated groupings. We could focus on movements of the pelvis and lumbar spine, movements of the thoracic spine and ribs, and movements of the cervical spine and head with respect to the ribcage.
By practice slow and smooth movements of these elements, through their possible ranges of motion (twisting, bending, sliding) we can learn to get a feel for these elements and the tension that controls them. We can then begin looking for the position of balanced tension or centredness.
Centrating the Hips
We can also practice this with the hips, particularly in one legged balancing positions. What position of the pelvis gives the supporting hip both room to move and balanced tension in all directions bearing in mind that the pelvis can turn (left or right), tilt (left or right) and roll (front to back.)