The best sounds and the longest lines are produced when we can draw the bow in a line, usually quite parallel to the bridge, so that the sounding point remains steady. However our bow arms work best in circular motions: our shoulders are ball-and-socket joints, and our elbows are hinged to allow the rotation of the forearm. If we are concentrating only on the straight line of the bow, we may inhibit the natural circular motions needed to keep the arm functioning well, and our sound may become stiff and thin.
The first bone of the arm is the shoulder blade. When we move our elbows, our shoulder blades also move. The movable shoulder blade (and therefore elbow) are an important part of the sound; engaging this area is what it means to “play from the back muscles.” When this area is accessible, our sounds become fuller, warmer, deeper, and louder; chords are also easier, because we have the full weight of the large back muscles. There will be a circular movement of the upper arm, turning inside the ball-and-socket joint. The elbow will also make small circular motions. To encourage this movement, use your left hand to turn the upper portion of your right arm. Notice how the turning results in your right elbow circling down and up. Can you feel the motion of your right elbow all the way back into your right shoulderblade?
The second important area is the forearm itself. Try holding your ulna bone and rotating your radius. You’ll be startled to see the range of motion possible without ever having to move your elbow. The small circular motions of the forearm give smoother bow changes and a more liquid, creamy quality to the sound. They also help in string crossings, particularly in fast passages. To develop this range of motion, place your right arm on a stand, with your elbow resting just beyond the left side of the stand. Now play Kreutzer #13 in the middle of the bow. Your upper arm will remain still while you cross strings using the forearm motion. This is one of the most useful capacities to develop in your bow arm; it will help you with fast barriolage passages, spiccato, and détaché.
As the forearm rolls, there is a resulting change in the hand balance which we call supination and pronation. The give of the hand into the bow facilitates these tiny adjustments. If we grip the bow too hard or are unable to bend the thumb, little or no forearm rotation is possible. There will be holes at the bow changes. We will not have enough variety of color or dynamic.