Electric motors can have quite a low efficiency in slow speed / high torque regime, where robots operate, because torque scales linearly with current, and power

Electric motors can have quite a low efficiency in slow speed / high torque regime, where robots operate, because torque scales linearly with current, and power loss (resistance -> heat) scales quadratically to current (P=UI, U=RI -> P=RI^2). In addition, when the motor is statically loaded, when it's just holding a position, it wastes a lot of power (depending on the torque) with zero efficiency, if it doesn't have mechanical brakes. I think only 1X is using tendon drives for all (?) joints in their robots, but most robots use them just for hands. 1X uses tendons just for moving the motor one link upwards the chain, not through multiple joints to the body (elbow motor is in shoulder etc.). Transmitting power from the body to remote joints with tendons is quite hard problem, and I'm not aware of anyone doing it this way. One problem is that there is no obvious way to decouple the motion of tendons when they go through a joint, e.g. if the elbow joint moves then it affects all tendons which go through the elbow. Also, it's rather complex and there is friction, stretching and vibration and so on, which might be hard to model / simulate. Hydraulics might work, but Boston Dynamics gave up on it, so I think it's probably not worth it. Hydraulics could in theory be very good because it needs just one motor for the pump and the fluid can distribute the power to many actuators. Moving the motors to the body is a good way to solve the mass / inertia problem, but no one has really figured out how to do it. Whatever 1X is doing might be the sweet spot.