What type of muscles are responsible for straightening a joint, thereby increasing the angle between them?

Extensor muscles are specifically responsible for straightening a joint, which increases the angle between the bones involved in the joint. When an extensor contracts, it pulls on the bone, leading to the extension of the arm or leg at the joint. This action is crucial for various movements, such as standing up, kicking, and throwing.

In contrast, flexor muscles work to decrease the angle between the bones at a joint, enabling bending motions. Agonists are muscles that cause a desired movement, while antagonists perform the opposite action, assisting in regulation and control of movement. Therefore, the designation of "extensor" as the type of muscle that increases the angle at a joint is direct and clear in its functional role within the muscular system.