Algorithms for the segmental motor innervation of the extremities

The pattern of the segmental motor innervation to the extremities is reflected in the kinematics of their joint linkages. More proximally located joints have more complex movements and are controlled by more spinal cord levels than more distal joints with simpler movements. More proximally located joints are also controlled by more cranially located groups of spinal cord levels while more distal ones are innervated by more caudally located groups. Algorithms are presented that summarize these principles in the form of two simple, concise charts and a stick figure. The cord levels given by the algorithms for the innervation of any particular muscle are in good general agreement with recognized authorities. In addition, these algorithms are easily learned and readily applied to clinical problems by placing the patient in the position assumed when playing the piano or operating an automobile. In this posture, the most cranial two spinal cord levels of a group controlling a joint are responsible for "upward" motion of that joint and the most caudal two or three control "downward" motion thereof.