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  • Some Problems with Biotensegrity
    And how how to overcome them

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

    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.

    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.

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

    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.

    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.

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

    Tensegrity is the equivalent of “structure”. 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.

    When things start to go wrong with the body, it’s because of a failure, or the potential for failute of tensegrity at the joints. Joints act as tensegrities when joint capsules are tensioned 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.

    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.