Ok, I see a contradicition here. it is probably a misunderstanding on my part, but I will ask anyway. You say that tendons can't be stretched and that there isn't any muscles in the hand itself. Well, then how do you explain someone hand's? I know that they can be stretched and limbered up. So (in my mind) either the tendons can be stretched or there are muscles in the hand. Please explain.
boliver
1. I never said there aren't any muscles in the hand. Of course there are! There are no muscles in the fingers, though.
2. With "stretching", one usually associates flexibility around a given joint, or better, increased range of motion around a joint. There are several ways to increase this range of motion. The range of motion is primarily determined by the ability of muscles to relax and contract, the elasiticity of the muscle tissue (once injured, muscle tissue loses a lot of flexibility), elasticity of the tendons across the joint, elasticity of the ligaments, elasticity of the skin, and finally, the joint structure itself, the type of joint and surrounding bones.
Generally, muscles can stretch a lot, and this is and should be the primary means of achieving greater flexibility. Ligaments can be stretched a little, and tendons should not be stretched at all. They provide stability of the movements around a joint. Some people do indeed stretch their tendons, e.g. contortionists, gymnasts, and Circus athletes. However, stretching tendons is very difficult, because it has to be done in addition and beyond stretching muscles. Tendons are designed to connect muscles to bones without stretching. If you stretch a bone-tendon-muscle system, muscles stretch first, then the tendon (with the muscle at the limit at which point it may rupture). Importantly, when a tendon is stretched, usually through trauma, the surrounding tissue gets permanently damaged or the tendon simply snaps (which is what usually happens; look at soccer and football players.) Connective tissue and skin can also be worked on to increase flexibility if they are the limiting factors.
3. Back to the hand. I assume you mean increasing the hand span. This is achieved through increasing the range of motion around the joints in the wrist where the fingers attach, not by increasing the range of motion at the knuckles. These lateral movements of the fingers are carried out by muscles in the forearm. You need to stretch those. The additional lateral movements from the knuckles are assisted by muscles in the palm of the hand. You can work on these as well, but working on the connective tissue and the skin between the fingers might give faster results.
Topic: steeley tendons on the inner forearm