Muscle wire works by heating usually by the current flowing through it. The heating current required is measured in amps or milliamps, the current that 5 volts pushes through the wire depends on its resistance which is affected by its thickness, what its made from and its length. The arduino could control it directly if it uses less than 40 mA or via a transistor or relay if it uses more than 40mA. Where does the potentiometer enter into the equation if you're driving it from the arduino ?