Chest armor inserts. Size Small The Nucleon Flex Pro Ci is a high performance, super lightweight track and race chest protector featuring Nucleon Flex Pro Technology. Constructed from a high performance polymeric material blend for optimized flexibility and excellent weight saving properties. The chest protecto(s structure provides the perfect balance between force dispersion and energy absorption. Unique Nucleon Flex Pro microstructure is designed for advanced energy absorption. adaptivity, maximum breathability and air flow.