Webs of Tension, Isolated Compression
The main ingredients in a tensegrity structure are space and tension.
Tension elements in a tensegrity form an interconnected network that distributes forces within itself. Any changes affect the entire network. Space-creating elements (or Compression-Resisting Elements) float within the tension network. It is because the space creating elements can move relative to each other that the tension elements can freely and easily distribute stress amongst themselves.
Tensegrity via Gravity
The solar system (any solar system) is an example of a tensegrity system. Gravity provides tension while the movement of the planets themselves resists that tension.
Tensegrity via Inflation
A balloon is a tensegrity. Push in on the balloon at any point and it responds. Tension is distributed throughout its tension network. Meanwhile the space creating elements redistribute because they are free to move relative to each other.
The rubber of the balloon's skin acts as the tension element. It is entirely connected. Meanwhile the molecules of air inside the balloon are the space-creating/compression-resisting elements. Free to move relative to each other the molecules of air push outwards against each other and against the skin of the balloon.
Note that within a balloon pressure is the same everywhere. However, tension within the skin of the balloon, it's rubber envelope varies according the diameter of the segment in question (hence the "neck" of a round balloon being "softer" than the main body of the balloon).
Tensegrity via Tension
A bicycle wheel can also be viewed as a tensegrity. The old type made out of steel are easier to understand in this regard.
The spokes are the tension elements. The hub and rim are the compression-resisting elements.
In a balloon, as air is blown, the skin resists. This creates pressure within the balloon. The skin resists, creating tension.
In a bicycle wheel, spokes are tightened against the resistance of the rim and hub to add tension to the spokes and "pressure" to the rim.
In a "loose" bicycle wheel, the uppermost spokes have the most tension when supporting the weight of the bike and rider. While rolling, as the spokes approach the uppermost position they experience an increase in tension. As they approach the lowermost position their tension decreases.
With spokes tightened, there is less disparity in tension between the spokes. They can better share the load no matter which spokes are uppermost, though the tension in each spoke won't necessarily be the same (as anyone who has ever tried to true a wheel will know).
This spoke/rim/hub system lets the wheel bear the stress of the bike and riders weight with minimal weight and maximal resiliency by redistributing stress as the wheel rolls.
As for absorbing the bumps in the road, the air filled tires, like a balloon, are also tensegrities. They handle the bumps, helping to provide a smooth ride.
Experiencing Changes In Tension
When a bicycle wheel rim gets bent, we can use a spoke key to adjust tension in the spokes near the bent part of the rim. Turning the spoke one way or the other we can increase or decrease tension. These changes in tension either pull the rim in one direction or allow it to move in the other. Thus we can vary spoke tension to pull a wheel back into true.
Proprioception wise, when trying to true a bicycle wheel an interesting thing happens. The spoke key becames more difficult to turn as tension increases. It becames easier to turn as tension is decreased.
How do we know if a spoke has a lot of tension? It takes more muscular effort to tighten it further.
And so part of truing a wheel is adjusting spoke tension to be as even as possible while at the same time pulling the wheel into true.
Anatomy, Space and Tension
Our body is neither a bicycle wheel, nor a balloon, nor a solar system, though it can share similiar properties.
Elements like the skull, the pelvis, the spine, have all been modelled successfully as tensegrities. One group worked on modelling the suture joints of the skull. With their model they couldn't push the individual bones of the skull together. The harder they pushed, the more sutures resisted the bones being pushed together. The sutures along with the shape of the joint kept the individual compression-resisting elements apart.
Even the elbows, knees, and other joints of the body can be modelled as part of a tensegrity structure. In these cases, rather than bones resting on top of each other, the fluid filled joint capsule creates space between the bones while maintaining their relationship. The joint capsules give connected bones "float" the ability to adjust relative to each other even while maintaining their overall relationship.
The liquid inside the joint capsule can be thought of as a space creating element. The combination of joint capsule envelope and the fluid inside ties the ends of the bones together but also keeps them apart so that they have the freedom to find the best position relative to each other.
(Dr. Stephen Levin1 relates first hand experiences of knee surgeries and seeing the bones being pushed apart.)
Muscle tissue has been found to connect not only to tendons, but also to the joint capsule2 so that muscle tension also affects the joint capsule as well as the relationship between the bones it works on. Increased joint capsule tension would increase the pressure of the fluid within the joint which then resists the ends of bones being pushed together.
(It may be that) because joints are held together and maintained by space and tension, they can actually grow while remaining intact. And in the course of changing shape and moving, then the bones have enough freedom of movement at the joints to adjust relative to each other meaning that stresses can be shared by the connective tissue elements that cross those joints.
This could be thought of as "designed" tensegrity. It maintains the ability of our joints to move and change shape. But what about the postures and movements themselves? Can these be considered tensegrity structures also, or is tensegrity a goal that we can work towards in movement and stillness. If so, why bother?
Connective Tissue and Muscle, Generating Tension and Distributing It
Muscles are the engines of shape changing and shape maintaining. Connective tissue is the medium that distributes the forces created by muscles. Together they form an adjustable tensioning network that can maintain the relationship between bones or change them.
Because muscles themselves are all connected by this connective tissue network, the tension that one muscle creates can affect the network as a whole and in so doing affect other muscles within the network.
Because muscles affect each other via this network, muscles can help each other, substitute for each other, hinder each other. Because muscles are all affected by this network, the network also provides an intermuscular communication medium. Information is delivered via tension... or it may be more accurate to say that the tension itself is the information.
Smart Tensegrities
(Morphic Tensegrity Potentials)
To get an idea of how muscles communicate via tension we can go back to the bicycle wheel. We are going to make some changes, some modifications. We're going to turn it into a smart tensegrity.
We'll replace the spokes with muscle tissue embedded in connective tissue. And we'll orient and connect the muscles to the rim and the hub just as "normal" spokes are oriented and connected.
To add tension to the muscle contracts. To relax tension the muscle relaxes. The connective tissue that connects each muscle to the hub and rim is important not only to transmit muscle tension but as a sensing device. Connective tissue acts as a stretch transducer telling us, as long as it is under some tension, how much stretch or tension it is currently experiencing.
As a starting point we contract all muscles so that all spokes have some tension and hold the hub centered relative to the rib. We can control the relationship of the rim and hub by varying activation of the various groups of muscle-spokes.
- Adding tension to either backward or forward angled muscle-spokes, we can cause the rim to rotate relative to the hub.
- Adding tension to the spokes that attach to one side of the hub we can cause the hub to shift laterally relative to the rim.
- Tightening spokes near the "top" of the rim we can pull the hub closer to that part of the rim.
Note that for any of these actions, not only do we shift the rim relative to the hub, but because the opposing spokes already have tension in them, then any change in relationship between hub and rim is going to change tension in those other spokes spokes.
- Contracting the muscle of one spoke is going to cause opposing spokes to experience lengthening or stretch.
- Relaxing muscle tension in one spoke is going to cause a relaxation of tension in opposing spokes.
These changes in muscle activity and tension gives us a means both of controlling the relationship between hub and rim and, equally important, of sensing that relationship.
How does the sensing occur?
Sensing Muscle Output and Tension
In order to sense the relationship between hub and rim, one of the qualities that we can look at is muscle output. Another is stretch.
Sensing muscle output directly is equivalent to turning a spoke key and noticing how much effort it takes. Sensing stretch can also give an indication of effort, but it also gives us a clue as to the distance between the hum and rim along the length of the muscle-spoke in question.
The difference between muscle output and stretch is easier to feel than it is to explain.
Try slowly straightening one elbow.Then try to straighten it further. Pause, and then release. You should also notice that your triceps activates when you try to straighten your elbow further. If it doesn't, then don't do this exercise!
Repeat a few times to get a "feel" for these sensations.
The sensation at the front of the elbow when you open it could be described as an "opening" sensation. Or it could be described as a stretching sensation. This sensation is markedly different than the sensation of your triceps activating. That could be described as muscle activation sensation.
Muscle activation sensation only occurs when a muscle is active. It gets louder the more a muscle shortens while active. It's signal becomes attenuated as the muscle is lengthened while active.
Stretch sensation generally occurs when a muscle is lengthened to some degree. It's signal becomes louder the more the muscle is stretched. It tends to fall of the more the stretch is reduced.
By deliberately varying spoke muscle activity and tension and noticing the resultant change in relationship between hub and rim we can calibrate muscle activity and spoke tension to the actual relationship between hub and rim.
After calibration, if we want a particular relationship between hub and rim we know exactly which muscles to activate and by how much, and we can recognize when we have attained that relationship both by muscular output and changes in stretch.
We can also change muscle action and tension with the goal of varying wheel stiffness.
By increasing tension in all muscles we can make the relationship between hub and rim more rigid (stiffer) and thus more resistant to change. By reducing the tension we can make the relationship less rigid (softer).
The benefits of the maximum tension position include greater resistance to change.
The benefits of the minimum tension position include less effort to maintain plus it can be easier to make rapid changes in position.
Because we can vary muscle tension directly and in real time, the best position would be the minimum tension position, so long as we have the facility to anticipate when we need to ramp up the tension.
Measuring Tension and Changes in Relationships
In a zero external stress environment (the equivalent of learning to drive in a car park without having to worry about other moving cars) we can measure and calibrate muscle action and tension against changes in relationship between hub and rim.
Then we begin to add external perturbations. Again we play with tension versus relationship. How do we maintain a relationship under certain conditions, how do we move between two relationships under certain conditions?
Experiencing muscle activity and connective tissue tension and the accompanying change (or lack of change) in the relationship between wheel and hub, we can learn to change hub-rim relationships or maintain them no matter what is happening.
Note that so long as we maintain space between hub and rim, and as long as we maintain some tension in all the spokes and as long as we can handle changes in environmental conditions while maintaining space and tension then we have tensegrity. Tensegrity allows us to sense the current relationship and changes in relationship. And it allows us to handle changes in the environment while maintaining sensitivity and responsiveness to future possible changes.
Balancing Muscle Action and Tension to Find Tensegrity
Our body is slightly more complex than the Smart Tensegrity bicycle wheel. However the same basic principles can apply to operating our body. We can aim to give any pose or movement tensegrity so that we not only can feel our body but so that we can sense changes and respond to changes.
One way in which we can work towards tensegrity is to create space or bigness in our yoga poses. Working to create space or length we add tension to our connective tissue.
Create Space
At a minimum we can create space in the spine by first learning to move ribs away from pelvis and skull away from ribcage.
Space can be created at the shoulders by moving the shoulder blades in the direction that the arms are reaching. (Creating just enough space can also lead to shoulder stability.)
Space can be created at the hips by reaching the thigh bones out of the hip socket.
Creating space adds tension to connective tissue and we can learn to feel this tension. And this tension is created by the action of opposing muscles.
Add Tension
Further tension can be added by creating compression along the long axis of the arm and leg bones.
Draw the elbow towards the shoulder joint or vice versa. Draw the knee towards the hip joint. In addition try drawing the wrist towards the elbow and the ankle towards the knee.
Try to create space first, then add the tension on top of it.
Vary Tension
Once you get a basic feel for adding tension try to vary it. Slowly go to maximum, then see what the minimum tension is that you can create while still being able to actually feel that tension.
I'll suggest that unless you are lifting weights, look for the minimum tension still maintaining feelability.
The control of tension can then then be further refined by learning to activate or relax muscles at will. The better we get at feeling tension and controlling it, the better we can choose the optimum amount of tension.
Optimum tension is somewhere within the range of the minimum tension we can feel and the maximum tension where we can still respond in the minimum amount of time necessary.
Maintaining our body in a state of tensegrity we can both feel the elements and control how they relate. Via the tension of tensegrity we can feel any changes and respond in such a way that we maintain space and tension.
Related
References
1. Stephen Levin: Biotensegrity.com
2. Jaap Van Der Wal: Architecture of Connective Tissue
Published: 2014 04 11
Updated: 2020 10 24
