Convex vs Concave
There are a set of rules that exist within biomechanics that are meant to guide how we understand joint movement. The rules exist as a framework, a system of guidelines to facilitate joint mobility. Essentially, they describe how the surface of a bone, either convex or concave, moves with respect to a bone it is connected to. The interesting aspect of this framework is not necessarily the rigidity of the rules, but that the shape of a joint aids in understanding how it moves. Our entire body is structured this way. We develop curvature in one direction due to a lack of curvature in another. Convexity develops due to concavity. It’s like squeezing a semi-flat dodgeball. Squeeze it in one place and it flattens, while another place expands and rounds out. Those mechanics appear fundamental to motion.
We can think of this another way. Pressure is something most of us are probably familiar with. Apply pressure in one area, and another region expands to accommodate. The air in the tires of a car occupies space and puts pressure on the walls of the tire. If a leak develops, the aspect of the tire that flattens is the one on the ground. The aspect absorbing the most pressure. Air moves away from that pressure and fills the remainder of the tire. Your body moves the same way.
As an example: a suitcase carry facilitates pressure generation on the loaded side. When my foot strikes the ground, it begins to flatten as my body weight moves over it. Just like the tire did in the deflated example above. If we have a knee problem, and we’re looking to expand an area that looks to be compressed, we could bias the knee toward expansion via a suitcase carry. An aspect of the knee is now convex with respect to a concave portion of the foot, if only momentarily. The same rules apply, at apparently all scales.
Biomechanics has traditionally utilized rules of shape to understand and guide our interventions to promote joint mobility. If we repurpose that understanding and apply it more generally, we can see that those mechanics look to be more universal. Altering shape, managing pressure and reorienting our body is how we move.
Austin Ulrich, Physical Therapist
