Most movements of the arms and legs involve two joined muscles, one of which stretches while the other contracts. These are known as agonist-antagonist pairs, which the biceps and triceps are a good example of – when a person bends their elbow, the biceps muscle contracts as the triceps muscle stretches. That stretching motion relays sensory feedback information to the brain, helping it to keep track of the arm's position.
In most conventional limb amputations, the joined ends of the paired muscles are cut off, keeping them from properly communicating with one another. As a result, the muscle signals that are sent to the brain are confusing. This means that many amputees end up having to visually watch what their prosthesis is doing, in order to guide its movement.