Bending Moments vs Moments Bending

Touch and seeing are much more than sister senses in the human experience.

Top pic shows the whole arm as if it were a system of open levers; which it only  looks like  from the outside, in silhouette. It cannot be a working model; the body is a closed system...

Top pic shows the whole arm as if it were a system of open levers; which it only looks like from the outside, in silhouette. It cannot be a working model; the body is a closed system...

Man-made structures are constrained by the properties of the materials used…while biology is constrained only by the laws of physics with different patterns and shapes naturally developing because they are the most energy-efficient configurations. The inevitable generation of bending moments and shear stresses within a class 2,3,4, etc system could simply not be sustained as part of normal biological function (Levin, 2002)…

Graham Scarr, Biotensegrity, The Structural Basis of Life, Handspring, 2014.

I spent Saturday afternoon in a conversation with my son about Bending Moments, for his essay on Biomechanics for his university course work (a Master’s Degree in Sports & Exercise Science). Bless him, he said "I'm boxing above my weight here mum, because I really want to understand it and not many people can help me, because they don't either. I'm pretty sure it's this:" He gave me a good explanation of ground reaction forces through a joint and what's in the literature that he was working with and trying to learn and apply enough to write about it.

Here's the thing: the moments we spend bending, be it in a yoga class or reaching over to make a bed, run a bath, pick up a child or a bag, stroke a dog, do up our shoes (and so on) do not - in truly madly deeply scientific terms - involve ANY "bending moments" as such. 

Regardless of all the science based upon this topic (sorry to every ivory tower built upon this premise) there can be no bending moments (or shear forces, or levers, or upright inverted pendulums) in healthy human beings or animals or trees. There's a very simple reason for this.


Human beings, like any other animal on the planet, abide by the laws of SOFT MATTER, not hard matter. That makes us non-linear biologic forms. We don't bend at the knee, ankle, wrist, spine, hip, neck or fingers as an open chain mechanism, such as a lever (is by definition); which has bending moments. Every movement is at least a 4-bar closed kinematic chain. (See pic of the lower three images). It just is that way. A bit like gravity really - you can say what you like; it relentlessly is the way it is. Of course, in a semi-academic book, one has to write about this with references. However in my own blog, I can say it how it is: just wrong!

Don't be deceived by the softly spoken Happy Yogini voice in which I'm saying this. As a daughter and ex-wife of two very smart engineers, I have been brought up to ask how things work and seek to make sense of them, much as my son is doing. There's a couple of decades of homework with some of the world's leading Orthopaedic Surgeons, anatomists, embryologists, scientists and biomechanical specialists in the mix here. They have spent their working lives with living beings, children, elite athletes and every kind of human shape in all ages and stages of fitness; in the field of health and human performance. No matter whom it is applied to; THIS IS TRUE, of ANY non-linear, biologic form. We do not have bending moments in our joints. We are rounded, multi-dimensional interconnected whole beings calibrating motion through our joints, with no pins. It doesn't stop there and it doesn't get explained away in a 2D diagram, however many experiments there are through the history of biomechanics...or bio mechanists that are measuring it. It is based on a false premise. The wrong paradigm, the wrong physics. You can’t measure a non-linear form accurately using linear measuring tools. Duh-uh!

Linear, non-biologic hard matter forms can have bending moments. That doesn't mean we do, just because from the outside your knee or your elbow (in silhouette) looks like the corner of a lever. It is not a lever - not even because it is a commonly held belief/example/in-the-books basis of most university curriculums and classical biomechanics. It's a 2D explanation for 3D (or more) motion. It's out of date, it’s not true, it isn't so, it never was. The End.