The title of this book naturally catches a Rolfer’s attention; and it is worth some attention. The book is written for those who are neither engineers nor physicists; it’s a layman’s introduction to aspects of both. There is a lot in it about tensional and compressive structures, for those of us who are wondering about Tensegrity, and in general there is plenty for us who wonder why we don’t fall down. Here are some excerpts:

…the molecular structure of animal tissue does not often resemble that of rubber of artificial plastics. Most of these natural materials [i.e., animal tissues] are highly complex, and in many cases they are of a composite nature, with at least two components; that is to say, they have a continuous phase or matrix which is reinforced by means of strong fibres or filaments of another substance. In a good many animals this continuous phase or matrix contains a material called ?elastiri? , which has a very low modulus and a stress-strain curve something like [Fig. 1]. In other words elastin is only about one stage removed, elastically, from a surface tension material. The elastin is, however, reinforced by an arrangement of bent and zig-zagged fibres of collagen, a protein… which has a high modulus and a nearly Hookean behaviour. Because the reinforcing fibres are so much convoluted, when the material is in its resting or low-strain condition they contribute very little to its resistance to extension, and the initial elastic behavior is pretty well that of the elastin. However, as the composite tissue stretches the collagen fibres begin to come taut; thus in the extended state the modulus of the material is that of the collagen…

“The role of the collagen fibres is not merely to stiffen the tissue at high strains; they also seem to contribute very much to its toughness. When living tissue is cut, either accidentally or surgically, in the first stage of the healing process the collagen fibres are re-absorbed and disappear, temporarily, for a considerable distance around the wound. It is only after the gap has been filled and bridged by elastin that the collagen fibres are re-formed and the full strength of the tissue is restored. This process may take up to three or four weeks, and in the meantime the flesh around the wound has an almost negligibly low work of fracture. It is for this reason that, if a surgical wound has to be reopened within two or three weeks of the original operation, it may be difficult to get the new stitches to hold.

“Collagen exists in various forms, but it may consist of twisted strings or ropes of protein molecules, and its resistance to extension is basically due to the need to stretch the bonds between the atoms in the molecules: that is to say, it is a Hookean material much like nylon or steel.”(from pp. 164-167)

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DEFINITIONS OF TERMS

Modulus
Or “Young’s Modulus”. A number, equal to the Stress divided by the Strain. “It expresses how floppy a material is.

Stress
“The stress in any direction at a given point in a material is simply the force or load which happens to be acting in that direction at the point, divided by the area on which the force acts.”

Strain
“Just as stress tells us how hard – that is, with how much force – the atoms at any point in a solid are being pulled apart, so strain tells us how far they are being pulled apart – that is, by what proportion the bonds between the atoms are stretched.” [Italics added]. Strain is equal to the extension of a material under load divided by its original length.

Strength
“Strength is not the same thing as stiffness. To quote from The New Science of Strong Materials: ‘A biscuit is stiff but weak, steel is stiff and strong, nylon is flexible and strong, raspberry jelly is flexible and weak. The two properties together describe a solid about as well as you can reasonably expect two figures to do.’

“The strength of a material is the stress… required to break a piece of the material itself.”

Tensile strengths of biological and other materials, in pounds per square inch:
Muscle Tissue (fresh but dead)- 15
Stomach Wall- 62
Cartilage- 430
Fresh Skin- 1,500
Fresh Tendon- 12,000
Fresh Bone- 16,000
Human Hair- 28,000
Traditional Cast Iron- 10,000-20,000
Commercial Mild Steel- 60,000

Surface Tension Material
In a surface tension material, such as water:

1. “The tension force does not depend upon the strain or extension but is constant however far the surface is stretched.

2. “Unlike a solid, the surface of a liquid can be extended, virtually indefinitely and to as large a strain as one cares to call for, without breaking.

3. “The tension force does not depend upon the cross-sectional area but only upon the width of the surface. The surface tension is just the same in a deep or ‘thick’ liquid as it is in a shallow or ‘thin’ one.”

Hookean Behavior,
Hookean Material
“When we come to plot the stress-strain diagram for metals and for a number of other common solids we are are very apt to find that, at least for moderate stresses, the graph is a straight line. When this is so we speak of the material as ‘obeying Hooke’s law’ or sometimes of a’Hookean material’.”

Work of Fracture
Or “Fracture Energy”. The energy required to tear a material apart so that two new surfaces result; “…the strain energy which could be stored in one kilogram of tendon would ‘pay’ for the production of 2,500 square metres (over half an acre) of broken glass surface…”