Female gymnasts do not wear protective devices under their leotards,
and for that matter male gymnasts don't either.
Gymnastics is not a contact sport where participants must be wary of
body strikes from an opponent. As in any athletic activity, there is
the possibility of sustaining a blow by falling or hitting oneself,
but gymnasts are trained to avoid such trauma by contorting their
bodies in the proper way. This training is deemed sufficient, for
there is no opposing player who attempts to strike one's body in the
course of a sporting event. Body blows are not a normal or expected
part of the activity, as they are in other sports.
Furthermore, groin cups, shin pads, and other kinds of firm shielding
device or body armor would constrain the gymnast's motions during her
performance. What gymnasts do find helpful to wear are supportive
undergarments that will not slip or tear during intense physical
exertion and extreme bodily contortion. Rather than skimpy underwear,
then, gymnasts are advised to wear athletic female undergarments that
can be purchased at any sporting-goods store. The same is true for
dancers, who carry out the same kinds of motions. The more mature the
gymnast's or dancer's body, the more support she will need. A younger
girl's needs for supportive undergarments are more limited.
Examples of underwear recommended for gymnasts and dancers are shown
below, on a site devoted to the subject.
All About Dance: Undergarments
Such items are not restricted to gymnasts and dancers, but are readily
available, as I've mentioned, under the general rubric of athletic
undergarments or supportive underwear.
See below for examples of such items.
SportMart: Women's Athletic Wear
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gymnastics wear undergarments