Women's ULTRA CompressRx Recovery Sock Size: 2, 3, 4 Color: Black Fabric: ZoneRx - Muscle-specific zones of compression stabilize the calf muscles reducing muscle damage during exercise; CRx - Graduated compression from the ankle to the top of the calf that helps remove lactic acid and improve blood circulation back to the heart. SynchroRx provides moisture management and temperature regulation; Achilles tendon support; Padded foot soles prevent foot irritation