The "Convex-Concave Law" in Joint Movement: The Ingenious Code of Human Movement

Experts from Hunan Rehabilitation Hospital said that when we raise our hands, legs or turn our necks, seemingly simple movements actually follow a precise rule called the "Concave-Convex Rule." This rule reveals the wonderful relationship between the shape of the joint surface and the direction of movement.

Human joints are composed of convex surfaces (such as the humeral head of the shoulder joint) and concave surfaces (such as the glenoid cavity of the scapula).

When the joint moves, the convex surface slides on the concave surface, and the sliding direction is opposite to the overall movement of the bones; conversely, if the concave surface moves actively (such as the radius of the wrist rotating around the carpal bone), the sliding direction is consistent with the movement of the bones. This kind of reverse or same-direction sliding, like the meshing of gears, ensures that the joint is both flexible and stable.

Take raising your arms as an example:

When the humeral head (convex surface) rolls upward, it will slide downward synchronously to prevent the shoulder joint from getting stuck. When the knee joint is flexed and extended, the tibial plateau (concave surface) slides forward to cooperate with the rolling of the femur (convex surface) to achieve a smooth squatting movement. If this law is violated, joint wear or limited movement may occur.

Therefore, when we treat frozen shoulder, we often slide the humeral head downward to expand the range of motion of the joint. When fitness enthusiasts squat, if they feel discomfort in the knee, it is often because the tibia fails to slide in a coordinated manner, resulting in a sudden increase in cartilage pressure.

Understanding the law of convexity and concavity not only allows us to protect our joints more scientifically, but also reveals the movement wisdom that the human body has honed over billions of years of evolution - every movement implies a perfect balance of geometry and mechanics.

Hunan Medical Chat Special Author: Luo Zhenjie, Hunan Rehabilitation Hospital

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(Edited by YT)