Re: Aerodynamic forces just get beyond human muscles
DC-9 in particular has control tabs (operated directly by the yoke and pedals) for all primary flight controls. The rudder is hydraulically powered, but will fall back to tabs automatically if lost. Also has a hydraulic pusher for the elevators that would be used when trying to recover from high AoA situations where airflow might be insufficient to allow the control tabs to move the elevators properly. It is a very 'analog' plane by today's standards.
To elaborate, control tabs are a smaller control surface that moves opposite the desired control deflection, and basically fly the control surface into position. As their effectiveness increases with speed (up to a point), controls remain reasonably light on a smaller aircraft.
For more details on this and secondary flight controls (flaps and spoilers are also hydraulic), see http://www.hilmerby.com/dc9/flap.html
- I had originally written in 'servo tab', but that is a related concept used to reduce control forces on otherwise directly linked surfaces... on DC-9, the control tabs are the only thing linked directly to the controls.