The biceps and triceps are the principle muscles of the __________.

The biceps and triceps are the principal muscles of the arm because they are primarily responsible for the movement of the elbow joint. The biceps brachii, located at the front of the upper arm, is involved in flexing the elbow and rotating the forearm. In contrast, the triceps brachii, located at the back of the upper arm, is responsible for extending the elbow. This specific arrangement allows these muscles to work antagonistically; while the biceps contracts to bend the arm, the triceps must relax, and vice versa for straightening the arm.

In relation to the other options, the chest is primarily composed of the pectoral muscles, the neck contains muscles like the sternocleidomastoid and others not directly related to arm movement, and the back is dominated by large muscles such as the latissimus dorsi and trapezius, which are not involved in the flexion and extension of the elbow in the same manner as the biceps and triceps. Therefore, identifying the biceps and triceps as the main muscles of the arm aligns with their anatomical and functional roles.