Electric motors can't generate the torque to drive something like a knee joint, without being very large and heavy. Motor torque is limited by the current you

Electric motors can't generate the torque to drive something like a knee joint, without being very large and heavy. Motor torque is limited by the current you can run through copper wire, times the magnetic field you can generate with permanent magnets. The best motors are only about 2 Nm / kg (continuous torque / motor weight). A knee joint requires torque around (weight of robot/2 * length of tibia) in order to squat, say 100 nM for a human-sized robot. Direct-drive motors would weigh 50 kg each! So in practice, you use a 20:1 gearbox and 2.5 kg motors. Superconductors could totally change that some day.