What are the two primary actions exhibited by muscles that act on the knee joint?

The two primary actions exhibited by muscles that act on the knee joint are flexion and extension. Flexion refers to the bending movement that decreases the angle between two body parts, specifically decreasing the angle between the thigh and the lower leg at the knee joint. Extension, on the other hand, is the straightening movement that increases the angle between body parts, allowing the leg to return to its straightened position.

These movements are crucial for activities such as walking, running, and jumping, where the knee joint performs both flexion and extension to facilitate mobility. The quadriceps muscle group primarily controls knee extension, while the hamstring group is responsible for knee flexion. This functional relationship between these actions at the knee joint is critical for normal locomotion and stability during physical activities.

While other actions like rotation and stabilization are important for knee function, they are secondary to the fundamental movements of flexion and extension that are most commonly associated with the mechanics of the knee.