Final answer:
Flexion of the elbow joint decreases the angle between the forearm and upper arm, while extension increases this angle, straightening the elbow. The elbow's stability is supported by collateral ligaments and the annular ligament.
Step-by-step explanation:
Flexion and extension at the elbow joint refer to movements that affect the angle between the forearm and the upper arm. Flexion is the bending movement that decreases the angle between the forearm and the upper arm, such as when you bring your hand closer to your shoulder.
On the other hand, extension is when you increase the angle between the bones of the forearm and the upper arm, effectively straightening the elbow. The muscles involved in flexion include the biceps brachii and brachialis, which contract to produce the movement. In contrast, muscles like the triceps brachii are responsible for extension, lengthening the elbow.
The elbow's structure, supported by the ulnar collateral ligament on the medial side and the radial collateral ligament on the lateral side, along with the annular ligament that encircles the head of the radius, ensures stability and proper joint functioning during these movements.