Creating an artificial limb such as an arm is a very difficult task, even with all of our modern technologies. Furthermore, a natural arm is controlled through the nervous system's motor pathways with tremendous precision, and signals from the brain to the muscles are transmitted within fractions of a second. Natural selection has had a long time to develop this brilliant system - one which is not simple to simulate. The most basic mechanical arms are controlled with cables. There is, however, much effort to better utilize modern technology in order to create prosthetic limbs which have more effective, and more natural finctions.
Myoelectric prostheses are electric limbs which are controlled by functioning muscles in the body.
Small electrodes are placed on the skin over functional muscles. These electrodes measure the electrical activity of the musclus they are over, and through practiced sequences of muscle contractons, one can control the movements of the robotic limb. The new feet and ankles come in all shapes and sizes. The frames, made of carbon fiber graphite, bend and flex. It will still be quite a while before we will see prosthetic limbs which can completely make up for the absence of a natural limb.