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  • Some Problems with Biotensegrity
    And how how to overcome them

    Biotensegrity is the study of tensegrity in living creatures, particularly ourselves. And the focus on this article is biotensegrity at the level of posture and movement.

    A Balance of Tension and Compression

    Tensegrity structures distribute local stresses globally through a tension network. This tension network tends to pull inwards.

    To prevent a tensegrity system from collapsing inwards, space creating elements are required. These resist the inwards pull of the tension network.

    A Tensegrity is a Tension and Pressure Distribution System

    One quality of tensegrity structures is that local stresses are distributed throughout the system.

    In a system with solid space creating elements, the stresses are distributed in such a way that the shape of the structure is maintained though the distance between solid elements may change.

    In a system with pressurized space creating elements (such as an air or water filled balloon) the space creating elements freely redistribute so that local stresses are globally redistributed.

    A Tensegrity System is Responsive

    An important quality of tensegrities are that they are responsive. They adjust to any changes in tension or pressure so that those changes are distributed instantaneously throughout the structure.

    The Problem with Applying Biotensegrity to Posture and Movement

    Applying the idea of tensegrity to movement and posture (biotensegrity) there are some problems.

    Total Relaxation

    The body can relax totally.

    In this case you could be lying on the ground. In this state, the body as a whole is not a tensegrity, since local forces cannot be distributed globally. There's too much slack in the system, akin to slack guitar strings or an unstrung bow.

    In this state the only parts of the body (at the level of movement) that have tensegrity qualities are the joints which, in the case of synovial joints, are fluid filled capsules holding bones together. (Vertebral disc "joints" could be viewed similiarly.)

    The fluid within these capsules, pressurized by the joint capsules, keep the bones from impinging but also keep them tied together.

    In a resting state (where they aren't bearing the weight of the body) tension is sufficient to pressurize the liquid just enough to keep bones from butting together.

    In this state changes in tension or pressure are not redistrubuted throughout the system musculoskeletal system of the body.

    Standing

    Stand a body up. You can be in a pose or doing an action that doesn't have tensegrity.

    The legs could be strong and stable, but insensitive. Other parts of the body may be relaxed enough or tense enough that in either case any stresses to the system are not distributed globally.

    However, at the joints, enough tension is maintained in the joint capsules so that bones are kept from butting into each other because of fluid pressure within the joint. This can be despite bearing the weight of the body or dealing with excessive muscle tension.

    In the case of the arms hanging down, tension in the capsule, plus suction (preventing a vacuum from occuring) helps to prevent joints from being pulling apart.

    So in this case, while there is tensegrity in the joints (likewise when fully relaxed) there isn't tensegrity in the posture as a whole.

    Recognizing Postural or Dynamic Tensegrity

    Now, do a pose or action that does have the qualities of tensegrity. The feeling could be described as a balance between expansion and contraction. There can be a feeling of readiness without excessive tension. You have sensitivity or awareness and responsiveness, the ability to respond instantaneously to change as it is perceived.

    Now you have tensegrity, but it's something that most of us have to work towards. This same state can be utilized while moving.

    And while it can be a worthwhile goal to work towards, I'd suggest that it is only a halfway point.

    Tensegrity is a Responsive Structure

    Tensegrity is the equivalent of "structure", a responsive one. But depending on what we are doing, the body doesn't need structure all the time. Parts can have "structure" and other parts not. And what allows these possibilities is that tensegrity is (ideally) maintained at the joints.

    Joints Need Tensegrity

    When things start to go wrong with the body in terms of movement or posture, it's because of a failure, or the potential for failure of tensegrity at the joints.

    Joints act as tensegrities when joint capsules are tensioned by movement of muscle (or both) so that fluid pressure is enough to keep bones from bumping into each other. The trick is maintaining the required tension as the joint changes shape.

    If you want to understand biotensegrity and how it applies to controlling the body, you need a model of a joint, particularly a model to help understand how synovial joints maintain their state of tensegrity despite whatever the body is doing.

    Finding Suitable Tensegrity Models for Joints

    The bicycle wheel as a model of the hip is a potential starting point.

    Most tensegrity structures that use solid compression resisting elements are a poor model for the body because they have a single basic shape. Exceptions are models where tension in individual tension elements can be varied to vary the relationship between elements.

    A better model is a balloon or tire.