They had sport bras/bikinis?
The top part of the “bikini” worn by these girls consists of a breastband. Breastbands like this were already known in ancient Greece. The Greeks referred to the breastband as a mastodeton or apodesmos; the Romans called it a strophium. A breastband was often made of linen. Such articles of clothing were worn by women who engaged in sports or otherwise had an active lifestyle: the heroine Atalanta is sometimes depicted wearing a breastband.
And while men were generally expected to exercise naked in the Greek fashion; female public nudity was frowned upon. The ancient “bikini” allowed women to exercise in comfort without causing offence to ancient (male) sensibilities.