Fluid Tensegrity is a term I've coined that refers to the tensegrity that occurs at the level of the skeletal articular joints.
I'll suggest here that this system is for the most part not consciously controllable. Its job is to maintain space within synovial (and non-synovial) joints so that the bones continuously float. This then allows the controllable body to be moved or stilled in a variety of configurations, some of which act as tensegrities ("phasic tensegrity potentials") and others which do not.
As an example of two possible extremes, fluid tensegrity allows complete relaxation and floppiness of bones relative to each other. It also allows complete rigidity so that bones are stabilized relative to each other. (Let's assume muscle tension short of the point at which bones are shattered).
It also allows a range of tension between these extremes. But in all cases it works to keep space in the joints. It provides the substrate for movement and stillness.
It also provides the possibility of phased tensegrity potentials. These overlapping structures are what could allow us to maintain tensegrity while moving or standing still. These are important because not only do they allow the body to be instantly responsive, they also make the body fully sensible.
Joint space in the knee joint can be used to predict knee problems. One website suggests that joint space narrowing can indicate that osteoarthritis is getting worse. (I used the search term "knee joint space narrowing"). The point that is important is that rather than bones resting on top of each other at synovial joints they actually have space between them.
The question is, how is this space maintained?
One of the main assumptions for fluid tensegrity are that ligaments are not passive structures. They are actually acted on and affected by muscle tension just as much as tendons are. (There are exceptions.)
Watch this Jaap Van de Wal presentation or read for more on the ligaments as active structures. In general he suggest a model where the belly of the muscle connects directly to ligaments as well as tendons. The conclusion is that ligament tension is affected by muscle activity just as much as tendon tension is.
One possible benefit of the ligaments as active (muscle affected) structures, is that they can affect joint capsule tension which in turn helps to maintain fluid pressure which in turn maintains joint space.
We could use a water balloon as an example of how this might work. Squeeze a waterballow in the middle and the ends bulge outwards away from teach other. Reduce the amount of squeeze and the ends move back towards each other.
It has been pointed out that the amount of fluid in the knee joint is about as much as you get licking your hand. The question then can be, if joint fluid isn't keeping the bones apart what is? If the bones are moving apart or being kept apart, something must be filling the space between the bones to keep that space present.
It seems the most likely answer is that fluid is used to maintain space between the bones and controlling tension of the joint capsule can help vary fluid pressure so that it resists bones being pressed towards each other.
Note that a water balloon is one example of a tensegrity. The skin acts as the tension element while the fluid inside is the compression resisting element. The fluid nature of the contents of the balloon allow the balloon as a whole to change shape and maintain its integrity. As the balloon changes shape the skin of the balloon redistributes stress within itself.
This could be one reason for maintaining space between the bones at the joints. It allows the bones to adjust relative to each other which in turn allows the joint capsule to freely redistribute stresses within itself.
We could look at abdomen as a joint that connects the pelvis and ribcage. The envelope of the joint is made up of the diaphragm, transverse abdominis and pelvic floor muscles.
The abdominal organs, contained within this envelope, could be viewed as a relatively incompressible fluid.
Applying tension to the transverse abdominis squeezes this abdominal fluid inwards and upwards. Applying tension to the respiratory diaphragm presses the abdominal organs downwards. Tension in the pelvic floor helps to push the abdominal organs upwards.
Because the transverse abdominis is made up of a series of horizontal fibers, it doesn't directly control the relationship between ribcage and pelvis. By that I mean that it doesn't directly twist or tilt the ribcage relative to the pelvis. The rectus abdominus, obliques and spinal erectors, however, can.
Assuming a starting point where there is equilibrium, with TA and Respiratory (and pelvic) diaphragm tension in balance, and the rectus abdominus and/or obliques helping to maintain stillness, an increase in respiratory diaphragm tension could cause the ribcage to move upwards. The abs not including the TA could release slightly to allow this movement. A decrease in diaphragm tension along with an increase in rectus abdominus tension could help pull the front of the ribcage down again.
In either case changes in tension of the joint capsule (the container of the abdominal organs) can be used to vary the relationship between ribcage and pelvis.
While there may not be enough fluid in, say, the knee joint for this degree of control, at the very least changes in ligament tension could be used to maintain space in the joint while changes in tendon tension moves one bone relative to the other.
Note that one special joint that does not need fluid tensegrity is that between the scapulae and the ribcage. In this case space is maintained by the layer of muscles between the two bones.
In the case of cranial sutures, which have to be able to adjust to allow growth, one team has modelled the skull plates and found that when pressure is applied to bones on either side of a sutured joint, suture tension increases helping to maintain space between the bones.
Tribology is the science of lubrication. Lubricants are used to reduce friction and wear between adjacent sliding surfaces. So that lubricants can do their job, there needs to be some mechanism for keeping the lubricant from being squeezed out from between the surfaces that it is meant to lubricate.
In hydrostatic lubrication separating pressure is created externally by a pump.
Assuming the synovial joints use hydrostatic lubrication, we would need some mechanism to maintain the separating pressure of the synovial fluid. This separating pressure would resist body weight or other forces which work to press bones together at the joints. Separating pressure could be created by varying joint capsule envelope tension.
For different positions (and loads) on the joint, tension would have to be varied to maintain separation pressure.
So even if you don't buy into the idea of tensegrity and instead choose to view synovial fluid purely as a lubricating fluid, their is still a requirement for separating pressure to keep the bones from resting on each other.
One of the benefits of understanding fluid tensegrity (and joint anatomy) is that it can provide a tool for improving movement and posture learning, and also a tool for fault finding problems.
Faulty muscle function may occur as a result of muscles "splinting" each other. But it also may be a result of the brain protecting joint capsules so that fluid tensegrity is maintained despite the problem.
One question that might arise is how does the brain know how much tension to apply. How much pressure is ideal?
First of all the actual amount of fluid within a joint can act as a limiting mechanism. Since there is about as much fluid as a lick of spit inside, say, the knee joint, that limits how far apart the bones of the knee can actually be pressed apart. But that doesn't answer the question of how the brain might be informed ast o the actual condition of the joint.
One of the points that Van De Waals make is that the dynament isn't just a functional action block. It is also a functional block of sensitivity. One part of this sensor package would be the actual work output of the muscle itself at any moment in time. The other part would be tension readings in the ligaments, tendons, and possibly in the investing fascia within the belly of the muscle itself.
Tension could be created within the joint capsule by the weight of the body pressing synovial fluid outwards against the joint capsule, wich depending on the amount of muscle activity could inform the brain that muscle activity is required to counteract the outward push of synovial fluid on the joint capsule.
Going the other way, joint capsule tension can increase separation pressure to the point where separation pressure resists further tensioning of the joint capsule. In either case the brain would have access to sensory information that tells it when the joint is within desired operating parameters.
One of the benefits of fluid tensegrity at the joints is that it gives us full freedom and the level of posture and movement. This is full freedom in the sense that we can choose postures and movements that have tensegrity, or not.
For why we might want to choose postures and movements with tensegrity read Creating tensegrity in yoga postures.
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