Takeaway

Giovanni Alfonso Borelli’s calculations of spinal biomechanics showed that spinal muscles alone were not sufficient to support the spine when carrying a heavy weight.

British statistician George E. P. Box is famous for saying “All models are wrong, but some are useful.” In the world of biomechanics, many of our models go all the way back to Giovanni Alfonso Borelli, who, for historical context, died on New Year’s Eve, 1679. 

Borelli was a polyglot, as interested in predicting the orbit of Jupiter’s moons as identifying the center of gravity of the human body. For the record, he was quite accurate with the former and not so much with the latter. Borelli’s experiments involved placing a supine body on a seesaw of sorts and moving it about until both the board and human achieved balance over the fulcrum. Based on this, he concluded the center of gravity of the body as being slightly below the pubis. While it is now a given that our center of gravity is at approximately 56 percent of our height when measured from the soles of our feet (slightly less than an inch [2.4 centimeters] south of the navel), that’s still a pretty good calculation for an inquisitive mind who died eight years before Newton published Principia Mathematica Naturalis, the tome that became the foundation of physics as we know it.

Despite that error, Borelli is considered the father of biomechanics and best known for his interpretation of the musculoskeletal system as a series of interconnected levers that magnify motion rather than force. That’s a useful model, but what happens to your wrist when you’re doing biceps curls with 5-pound weights? Or benching 200 pounds? To account for this, Borelli performed numerous anatomical studies and posited a system of pulleys along the muscles of the forearm to account for being able to hold a weighted object with an extended arm. 

A More Accurate Model 

The model of the elbow joint as a lever and the arm muscles as pulleys was useful for its time and is still used today as a default model in many texts and journals. But it’s also kind of wrong. If your arm is extended and you’re holding a weight, or a phone, or a baby, what exactly are those forearm muscles pulling against, given the horizontality of it all?

There are a few particulars this model fails to consider. One is the sheer number of muscles in the arm that cross both the wrist and elbow joints. Functionally, one has to think of them as working in relationship to each other. Then there’s the complexity of the wrist itself. Take a moment to flex your elbow. Now take your wrist through a range of motion. Slowly rotate, flex, extend, pronate, supinate, undulate, and dance through all the motions of this incredible bone structure. The coefficient of friction of the wrist, the dynamic ability for your carpal bones to slide relative to each other, is 0.05. By comparison, the coefficient of ice on ice is 0.04. 

In order to allow the wrist bones to hold up under load, the coefficient of friction would have to increase to 1.0. They would have to functionally fuse—think of sandpaper trying to glide on sandpaper. That’s not going to budge an inch, let alone a centimeter. The problem is that the forces required to achieve this stability would tear your arm muscles, crush your carpal bones, and exhaust the energy of the system. And biological systems operate in ways to conserve energy as much as possible. Why? Because, evolutionarily speaking, you never know when your next meal is coming. 

Clearly, something else must be at work here, and Borelli realized it. His calculations of spinal biomechanics, incredibly accurate even by today’s standards, showed that spinal muscles alone were not sufficient to support the spine when carrying a heavy weight. For his calculations, he imagined carrying a 120-pound trunk. From this, he surmised that the intervertebral disks must be of a viscoelastic nature, acting as both cushions and springs for the spine. From what we now understand about fascia, we know this is accurate. But there’s still something else at work. And that something is tensegrity.

Tensegrity in the Body

Coming from the worlds of art and architecture, not anatomy and physiology, tensegrity is a combination of the words “tensional” and “integrity” and refers to an object whose fundamental structure is composed of discontinuous compression members held in place by a contiguous tensional matrix. So, the traditional beams, columns, braces, and such that support and shape your treatment room or your house are still there, but they never actually touch. In the case of something like a geodesic dome, they are semi-flexibly held in place by a taut, interwoven wire (Image). This gives these structures remarkable resiliency. 

If a tree falls on your house, it’s going to do some structural damage. How much depends on what kind of roof you have and its condition. Whether you have brick, stone, wood, or aluminum siding exterior walls will also make a difference. If a tree falls on a geodesic dome, the dome will absorb the shock, distributing the strain throughout the structure. In doing so, it will “bounce back” against that stress and remain stable to the degree it was designed to do. Any increase in compressive tension will distribute that tension throughout the structure. Sound familiar?

It did to Donald Ingber, MD, PhD, when he was an undergraduate student taking a course in sculpture. Currently, Ingber heads the Wyss Institute for Biologically Inspired Engineering. But back in the mid-1970s, he was making small tensegrity models in his art class. He was fascinated that when he pressed down on them with his hand, compressing them, they flattened. When he removed his hand, they sprang back to their original shape. This reminded him of how living cells, when placed on a rigid surface, spread out and flatten. When placed on a flexible piece of rubber, the cells contract and become more spherical. This led him to wonder: What if cells are actually tensegrity structures?

Using a series of experiments, Ingber showed that the cross-linked filaments of the cytoskeleton (see Massage & Bodywork January/February 2023, page 78) behaved exactly like a tensegrity structure. Even though individual cells are surrounded and permeated with fluids and membranes, there is a network of molecular struts and cables that stabilizes the shape of the cell. 

Later experiments manipulated the tensile forces within and without the cells, and these manipulations changed the cells’ genetic expression. In summary, the cells that were stretched flat divided. You might say they overextended themselves. Cells that were not allowed to stretch retracted into themselves, rounded in shape, and died. Make of that example what you will.

Meanwhile, the cells that were neither overstretched nor understretched thrived. Instead, they differentiated and realized their cellular potentials. Liver cells secreted the appropriate liver enzymes, capillary cells formed hollow tubes, and so on—the Goldilocks principle at a microscopic level. Mechanical restructuring of the cells told the cells what to do, how to behave, and how to function.

Orthopedic surgeon Steven Levin later coined the term biotensegrity to differentiate biologic tensegrity from the artistic/architectural variety, but the principles remain the same. In essence, the 206 bones are struts pulling up against, and balancing in, gravity via the tensile nature of the myofascial web. It’s all connected indeed.

For all that interconnectivity, you still need to know how it’s all connected to best navigate and trace potential kinks in the system. Imagine a very elaborate garden hose or a badly wrinkled garment. The best-known series of tensegrity-oriented body maps are the Anatomy Trains, but many other linkages and slings are being proposed, explored, and studied. I look forward to these future inclusions. 

Still, Ingber put it best when he wrote: “From the molecules to the bones and muscles and tendons of the human body, tensegrity is clearly nature’s preferred building system. Only tensegrity can explain how every time you move your arm your skin stretches, your extracellular matrix extends, your cells distort, and the interconnected molecules that form the internal framework of the cell feel the pull—all without any breakage or discontinuity.”

Tensegrity. It’s not just a useful model; it may prove to be the tensional blueprint of all living matter. 

Resources 

Croskey, M. I. et al. “The Height of the Center of Gravity in Man.” American Journal of Physiology 61, no. 1 (1922): 171–85. 

Davidovits, P. Physics in Biology and Medicine. 5th ed. New York: Elsevier (2018): Chapter 1, Static Forces.

Gracovetsky, S. The Spinal Engine. October 2008 ed. Self-published (2008): 174. 

Ingber, D. E. “The Architecture of Life.” Scientific American 278, no. 1 (1998): 48–57.

Lowell de Solórzano, S. Everything Moves: How Biotensegrity Informs Human Movement. Philadelphia: Handspring Publishing (2020).

Pope, M. H. “Giovanni Alfonso Borelli—The Father of Biomechanics.” Spine 30, no. 20 (2005): 2,350–55.