Elbow and knee (flexion and extension).

The idea

The elbow and the knee are both hinge joints, so each permits movement in one plane only: flexion and extension. Flexion decreases the angle between the two bones meeting at the joint, while extension increases that angle back towards the straight position. At the elbow, flexion carries the forearm towards the upper arm, as in the upward phase of a biceps curl, and extension returns it, as in the downward phase. At the knee, flexion carries the lower leg towards the back of the thigh, as in the recovery phase of a running stride, and extension straightens the leg, as in the drive phase of a jump. In multiple-choice questions you must match the joint named in the stem to the correct movement and to the correct phase of the sporting action described.

What earns the marks

• The elbow is a hinge joint that allows flexion and extension only.
• The knee is a hinge joint that allows flexion and extension only.
• Flexion decreases the angle between the bones at the joint.
• Extension increases the angle between the bones at the joint, straightening the limb.
• At the elbow, flexion brings the forearm towards the upper arm, for example in the upward phase of a biceps curl.
• At the knee, flexion brings the lower leg towards the back of the thigh, for example in the recovery phase of a running stride.
• Extension at the knee straightens the leg, for example in the drive phase of a jump.

Common mistakes and how to put them right

• Treating the elbow or knee as a ball-and-socket joint.

  Put it right: The misunderstanding is that these joints allow rotation and sideways movement, but the correction is that both are hinge joints permitting only flexion and extension.

• Reversing the definitions: the misunderstanding is that flexion straightens the limb, but the correction is that flexion decreases the joint angle and extension increases it.

• Confusing the movement with the muscle producing it.

  Put it right: The misunderstanding is that naming the biceps brachii or quadriceps answers a question about movement type, but the correction is that the question asks for the movement, so flexion or extension must be named.

• Applying the wrong phase of an action.

  Put it right: The misunderstanding is that the downward phase of a biceps curl is flexion, but the correction is that the downward phase is extension because the elbow angle increases.

Exam tips

• Read the stem carefully to identify which joint is named, then eliminate options that describe movements of the other joint.
• Check the direction of the movement described: if the angle at the joint is getting smaller, the answer is flexion; if it is getting larger, the answer is extension.
• Link each option to a familiar action, such as a biceps curl for the elbow or a squat for the knee, to test whether the movement named matches the action.