To better understand sign language, it may be useful to have some background information about how the hand operates and how complex its movements can be. This requires an examination of the internal structures of the hand.
Figure 2.1: The joints in their hand together with their
associated degrees of freedom.
Figure 2.2: The tendons that are present in the hand. Again,
note the high complexity of the muscles and tendons within the hand,
leading to a wide range of possible motions.
In figure 2.1 we can see that there are a total of 23 degrees of freedom available solely in the hand above the wrist, to which we usually add another six degrees of freedom, which cover its movement in 3 dimensions. A hand can move not only up-down (y-axis), left-right (x-axis) and forward-backward (z-axis), but also rotate about these axes. For examples, palms can face up or down (rotation about z-axis -- known as roll), about the ball of the wrist -- for example when indicating stop to being lame-wristed (rotation about the x-axis -- known as pitch) and from side-to-side, as when ``dealing cards'' (rotation about the y-axis -- known as yaw). A number of signs differ with respect to some of these factors only. For example, weight and doubt differ only in the yaw of the palms -- weight has fingers pointing outwards and doubt has the blade of the hands parallel to the chest.
We can see the great number of muscles involved in motion of the hand in figure 2.2. Many of the muscles are actually in the forearm, and are connected via tendons to the hand itself, with the tendons transmitting the force to the hands. Thus any device trying to measure the effect of these muscles is going to be limited or expensive or both.
Two other important measures of finger movement are flexion (the ``bend'' of a finger along its axis) and abduction (the degree of bend between fingers). These are illustrated diagrammatically in figure 2.3. The thumb, in particular has a wide variety of possible movements -- in addition to the flexion/extension and adduction/abduction of the other four fingers, it has much more complex movements with both radial and palmer abduction, and anteposition/retroposition.
Figure 2.3: The wide number of possible motions that the hand
can make.