I spent Saturday afternoon in a conversation with my son about Bending Moments, for his essay on Biomechanics for his university course work. 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.
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.
IT'S THE WRONG KIND OF PHYSICS.
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; 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.
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. 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. 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 through the history of biomechanics...or biomechanists that are measuring it. It is based on a false premise.
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, its not true, it isn't so, it never was. The End.