Which term describes an increase in the angle between two bones?

The term that describes an increase in the angle between two bones is extension. Extension occurs when a body part moves away from the other, resulting in a straightening motion. For example, when you straighten your elbow or knee, the angle between the bones in the joint increases. This action contrasts with flexion, which involves decreasing the angle and bringing the two bones closer together, such as bending the elbow.

Understanding the distinctions between these terms is essential for comprehending human movement mechanics, especially in fitness and rehabilitation contexts. While terms like lateral flexion and hyperextension are related to movement around specific axes and at particular joints, they do not specifically define the basic action of increasing the angle between two bones. Thus, the correct term for this movement is extension.