Matthew LimThere were two episodes of Sailor Moon S with the titles of "Usagi in Tears! Glass Shoes for her Birthday" and "The Pure Heart Stolen! Usagi's Biggest Crisis" which were both changed to Birthday Blues Part 1 and 2 respectively.
The episodes featured a villain named Persian Cut that was transformed from a glass slipper that was given to Usagi as a birthday present. In the Sailor Moon S arc the villains would steal the 'pure hearts' of people, this power being activated a different way by each individual villain. Persian Cut's power was activated by her flexing her bicep.
The flexing in the original Japanese series didn't happen until "The Purest Heart Stolen! Usagi's Biggest Crisis" but when the series was translated into English "Birtday Blues (Part 1)" cut down a lot of content from "Usagi in Tears! Glass Shoes for her Birthday" so that it would get to Persian Cut a lot faster, so she happens to flex in both episodes.