Which muscle assists in elbow flexion when the arm is partially flexed and semi-pronated?

The brachioradialis muscle plays a significant role in elbow flexion, particularly when the arm is in a semi-pronated position, which is approximately halfway between palm up and palm down. This positioning allows the brachioradialis to effectively contribute to the flexion of the elbow joint, especially during activities that involve lifting or pulling motions when the forearm is not fully supinated or pronated.

While the biceps also contribute to elbow flexion, their primary function is more pronounced when the forearm is fully supinated. The triceps, on the other hand, are primarily responsible for elbow extension, making them not relevant to this particular action of flexion. The gluteus medius, being a muscle of the hip, doesn't play any role in movements of the elbow. Therefore, in the context of elbow flexion with the arm in a semi-pronated position, the brachioradialis is uniquely positioned to assist effectively in this motion.