How strong someone should be in order to fly without "power steering"?
You can't.  Most of these airplanes are flown with hydraulically powered control surfaces with no mechanical input possible from the cockpit flight controls.  The flight controls just operate servo control valves in the hydraulic actuators, like the bucket on a front end loader but a little fancier.  If it's FBW, the FBW system does the same thing at the actuators using torque motors operated by the FBW computers to drive the servo control valves.

There will be two, or three, hydraulic actuators driving each control surface.  The flight controls in the cockpit just control the extension/retraction of the actuators. Control feel in these airplane comes from spring devices in the control circuit to simulate "air loads".  There is no direct connection between the pilot and the dynamic forces acting on the control surface.  If you lose all of the hydraulic actuators, you lose the control surface.