Which muscle has its proximal attachment at the anterior surface of the medial distal humerus?

The brachialis muscle has its proximal attachment located at the anterior surface of the distal humerus, specifically on the medial aspect. This placement allows the brachialis to play a significant role in elbow flexion, functioning as a primary flexor of the forearm. The medial position of its origin enables it to effectively generate force during flexion regardless of forearm position, distinguishing it from other muscles that may have more varied attachment points.

In contrast, the biceps brachii primarily attaches at the scapula and has a different action due to its positioning and attachment, often involving supination in addition to flexion. The coracobrachialis originates at the coracoid process of the scapula, which does not match the specified location on the humerus. Lastly, the triceps brachii is associated with extension of the elbow and primarily attaches to the posterior aspect of the humerus and scapula, making it unrelated to the anterior medial distal humerus attachment point. Thus, the brachialis is the only muscle that accurately corresponds to the description provided in the question.