When I was first taught what a joint axis was I recall it being described to me as a door hinge. A fixed point about which motion will occur (more often than not 90 degrees’ perpendicular to the hinge/axis). This imagery tends to stick and there is no doubt it is a reasonable analogy, particularly for someone who may be completely new to human movement and biomechanics.
However, this is just an analogy (or ‘model’). A simplified version of reality. Completely acceptable to use; we use analogies/models all the time when it comes to the human foot. Why? When the reality is so complex that it becomes a hindrance in furthering understanding or applying clinically, then a model may be appropriate to use. This said, it is sensible to remind ourselves regularly that we are using models, and to acknowledge the possible short comings of them.
So what are the short comings of using a door hinge to visualise a joint axis? Well here is an important point to start with: Door hinges exist. Joint axes don’t. Take a look at the door nearest to you – you can see the hinge right there in front of your eyes. Ever seen a joint axis? No. You could spend all day every day for the rest of your career in a Podiatric Surgery unit and you still won’t see one. Because they don’t exist. They are an artificial construct; an imaginary line.
Why is this important? Well a door hinge is real, and sits there fixed in time and space. As such it dictates the motion of the door. The door rotates about this fixed hinge. We hopefully all agree that joint axes aren’t actually real. Therefore, they can’t sit fixed anywhere in time and space. So how can they dictate motion??
When biomechanists model human movement they can calculate joint axes. They do this by looking at the way two segments move relative to each other, and can then calculate where the axis is based on this. So, if there is no movement then there is no axis. As such, it may be reasonable to consider that a JOINT AXIS DOES NOT DETERMINE THE MOTION AT A JOINT; THE MOTION DETERMINES THE AXIS.
Therefore, joint axes will vary in their spatial and temporal locations through different periods of the gait cycle, vary when doing different activities and, of course, often vary significantly from person to person. This is sometimes depicted as a ‘bundle of axes’ for a given joint. Motion at certain joints is much less restrained than at others due to things such as bony morphology and ligamentous orientation (e.g. the midtarsal joint Vs subtalar joint for example), so its fair to conclude that some axes will have ‘looser bundles’ than others. More on this in another post.
For now, a summary: joint axes don’t exist and the door hinge analogy is a very simplified version of the truth. Using models is cool, but never be tricked into thinking they are reality. For further reading on this Nester & Findlow’s work on the Midtarsal joint (2001) isn’t a terrible place to start.
