If ever, I'd try a 3D image of the hand, from which all movements can be determined. Two cheap cameras may be sufficient for making 3D images, plus image processing software for tracking the position of (visible or virtual) markers on the hand. This task requires much computing power, can not be accomplished with a small Arduino. One or more Arduinos; depending on the number of muscles; can be used to make the robotic hand perform the observed moves.