I criticised a colleague the other day for using “shear force” to refer to the horizontal component of force measured by a force plate. He asked me “why?” Apart from me being a miserable old pedant who’s got nothing better to do than be annoying, the simple answer is that someone did the same to me a long time ago (it might have been Chris Kirtley, but then again it might not).
I don’t always trust Wikipedia but think it is quite good in distinguishing between shear forces which occur when forces are unaligned and cause a shear deformation (see figure above) and compressive forces when they are aligned and lead to pure compression or elongation. To distinguish between these you need to know how and where the balancing force is applied. The force plate only measures the ground reaction and I’d argue can’t therefore distinguish between shear and compressive forces. What it can do is resolve the overall force into components in different directions. I’d thus prefer to describe the components in terms of the direction in which they are acting rather than the assumed effect they are having on tissue.
If I was being really really pedantic I’d probably say that shear forces exist within a material rather than being applied to it. In most biomechanics it is actually the shear and compressive stresses and strains that are more important. These are caused by the external forces exerted on the material but are conceptually quite different being within the material. Generally speaking the vertical component of the ground reaction will give rise to compressive stresses. Given the complex arrangements of soft tissues in the foot and the irregular shape of the bones, however, it will also cause some some shear stress. Similarly although horizontal forces will result primarily in shear they will exert some compressive (and occasionally tensile) stresses as well.
Or am I being too pedantic? Anyone like to defend the use of shear force to describe what a force plate measures? It’s certainly very common usage.
Thought-provoking as usual, Richard makes a good point. The horizontal component measured by the force plate is dependent on friction, and I wonder if calling it a “horizontal friction force” would help to emphasize the fundamental importance of that friction to locomotion.