Musculature (Continued)

The superficial muscles of the arm and leg are well known, at least by outward appearance. Their mechanical function, however, is not clear because their attachments remain hidden. The latissimus dorsi, for instance, is a very large muscle whose origin is found in the posterior thoracic cage. Its action is made by the bone it actually moves, the humerus in the arm. It is sometimes classified as a back muscle, yet it actually contributes very little, if anything, to the movement of the back. It moves the upper arm, just as the most prominent chest muscle, the pectoralis major. The iliopsoas, unlike the latissimus dorsi or pectoralis major, is a muscle that is not even visible on the surface, but has a very strong mechanical effect on the upper leg, thanks to its insertion at the neck of the femur. If you were to see this muscle within the body, you would see that its origin is not very far from that of the latissimus dorsi, but because it is differently inserted, it is a deep leg muscle instead of a superficial arm muscle.