Peroneus longus and Peroneus brevis plantarflex evert the ankle. They can get overworked with a lot of side to side motion, such as with Zumba dancing.
to release a cramp in the peroneus muscles hold the foot in an everted position, then have your client try to invert their foot against resistance. They will be using Tibialis anterior and Tibialis posterior, and this will inhibit the peroneus muscles.