The Eye of the Beholder - S2-E6
Revealing mistake: In the episode, all the people are shown from the neck down to hide their faces (which at the end we see this is on another planet as they are alien looking). But as the first nurse seen in the episode walks to the window and turns around, even in shadow you can see that she is normal looking, that is, no prosthetic or make-up applied for the alien effect.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.