Which joint is classified as uni-axial?

The elbow joint is classified as uni-axial because it allows movement primarily around a single axis, akin to a hinge. This means that the elbow can flex and extend but does not allow for significant movement in other planes, such as rotation or lateral movement. Uni-axial joints are structured in such a way that they permit motion along one specific axis only, which is observable in the way that the forearm moves relative to the upper arm during activities like lifting.

In contrast, the shoulder joint is a multi-axial joint that allows a wide range of motions, including rotation, abduction, adduction, and elevation, making it much more versatile. The hip joint also falls into this category, as it allows for movement in multiple directions. The wrist joint, while permitting a variety of movements, is primarily considered a bi-axial joint, allowing movement along two axes—flexion/extension and radial/ulnar deviation—resulting in a greater degree of freedom than a uni-axial joint.