Deep Muscles of the Arm and Hand (Posterior View)

[Continued from above] . . . and the forearm is bent or lengthened by muscles of the upper arm, which is lastly rotated at the shoulder joint by muscles attached to the shoulder, scapula, chest, and clavicle. At each joint from the shoulder down to those between the phalanges (finger bones), the motor muscles create finer movements than the relatively gross motor functions provided by the muscles directly above them. Prime movers, such as the deltoid muscles in the shoulders, are powerful initiators of force; whereas the fine muscles within the fingers can manipulate tiny objects through small movements. Yet those finger muscles are still capable of exerting great force by themselves or conducting it from the large muscles above.

Interaction with the many joints of the upper extremities lends this muscular system its flexibility, from the many bones of the fingers and wrists to the complex movements possible in the elbow and shoulder joints. Frequently, this is accomplished through antagonistic muscle pairings—flexor and extensor muscle pairings such as the biceps with the triceps muscles—providing flexion and extension, aided by assistant muscle systems offering additional articulation of the limb; for instance in rotation, pronation, supination, and abduction of the hand.