Have you ever noticed that some people can flex and extend their thumb into crazy positions, while others have very limited thumb motion? What in fact is a “normal” amount of thumb motion?
This is a tough question to answer given that the metacarpal-phalangeal joint (MCP joint) of the thumb has a somewhat unique property to it. This is arguably the only joint in the body that has a large amount of bending and extending variability from person to person. Clinically, I have seen anywhere from twenty (20) degrees of flexion to as much as one hundred degrees (100) or more. Hyper-flexible people will also often exhibit a huge amount of hyperextension (30 degrees or more of “backward bending”) in the joint.
Anatomically speaking, the wide variability in thumb motion can be explained by the shape of the metacarpal head. Some metacarpal heads tend to be more rounded, which results in more range of motion. Conversely, a flatter metacarpal head results in less range of motion.
From a practical standpoint, people who have large amounts of movement in this joint of the thumb tend to have problems with stability and are a bit more likely to damage the joint with sudden, jerky motions. Hyper-flexible individuals should also be careful when placing the full weight of the body through the hands (e.g. with push-ups and yoga positions).