The rubber bands then close the hook and provide the pinch force necessary to hold an object. This below elbow prosthesis opens and closes with an electric motor. The signal to open or close is made by the presence or absence of small electrical charges in the muscles of the arm as it contracts. Small contacts that fit intimately against the skin detect the minute electrical pulses that race down a muscle as it contracts. When one is detected, circuitry tells the motor to open or close.
More complex circuitry can even tell the hand how fast to open or to rotate at the wrist.
These systems can use a hook or a hand and utilize an interface design that requires no harnessing. Although myoelectric arms are more cosmetic and technologically advanced, they are expensive, heavy, and require a certain amount of maintenance. For obvious reasons the patient must avoid moisture when wearing the prosthesis. The inner workings of the myoelectric arm reveal the circuit board, electrical connections, and electric motor. Patients must use rechargeable batteries and need to replace them frequently.