Final answer:
During a prone push-up with elbow extension, the correct motion is the concave ulna moving on the convex humerus, which describes the elbow joint's action in elbow extension. The correct answer is options B.
Step-by-step explanation:
The question refers to the action of the elbow joint during a prone push-up, specifically the elbow extension phase. In this movement, we are examining the interaction between the humerus (the upper arm bone) and the ulna (one of the forearm bones). The correct answer is B. The concave ulna is moving on the convex humerus. Extension at the elbow involves increasing the angle between the forearm and the upper arm.
The ulna, which has a concave surface where it joins with the humerus, rotates around the convex surface of the humerus as the elbow extends. This matches the general principle that convex surfaces move in relation to stationary concave surfaces in diarthrotic (freely movable) joints of the body, known as the convex-concave rule.