Radio-ulnar Articulation

1. Movement around the radio-ulnar articulation is a rotary motion of the forearm.
2. Points not in the axis of rotation will describe arcs of circles and not straight lines.
   
3. The wrist joint permits rotation in two fundamental planes.
   1. The one generated by flexion and extension at the wrist.
   2. The other by abduction and adduction.
4. In both cases the movements of distal points are arcs of circles.
5. With flexed fingers the length of the radius is the length of the hand from wrist to hand knuckle.
6. With extended fingers the length of the radius is from the wrist to the fingertip.
7. The finger-joints generate motion in arcs of circles, the radii of which are the lengths of the various phalanges.
8. As simple hinge-joints they cannot generate rectilinear motion.
9. From the observations we may formulate three principle which have practical bearings upon problem of piano technique.
   1. All movement generated by motion at a single joint is curvilinear.
   2. Any motion of a part of the arm in a straight line results from simultaneous movement at more than one joint.
   3. Simultaneous motion in two or more joints can generate both rectilinear and curvilinear movement.
10. Given a straight line movement, we know that it is caused by the participation of several joints.
11. Given a curvilinear movement, we cannot determine from this condition whether it has been generated by motion at single joint or at several joints.
12. This must be determined by a study of the
    1) skeletal parts involved in the movement, 2) their spatial displacements and, 3) the degree of curvature in the movement.
13. Motion at a joint demands muscular coordination.
14. Movements in straight lines involve the coordination of motion at several joints.
15. Movements in straight lines are physiologically more complex than movements in the arc of a circle caused by one-joint.
16. A straight descent of the finger-tip in a finger stroke is mechanically more complex than the curved stroke resulting from a fully extended flat finger.
17. A straight descent of the finger involves motion at each of the three finger joints.
18. The curved stroke of the finger involves motion at the hand knuckle.
19. Any movement in a straight line means that several joints are participating in generating this movement.