The Eye of the Beholder - S2-E6
Other mistake: For someone whose entire head has been bandaged for several weeks, Janet Tyler's curly hair is remarkably unflattened when the bandages are finally removed, and her thick mascara isn't smeared in the least. (00:00:15)
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.