Alpinestars Bionic Back Protector is a slim, lightweight CE certified back protector with excellent ventilation performance. The ergonomic design of this protector makes it a necessity for any gravity fueled mountain bike. Designed to fit under your jersey. Ergonomic 5 plate construction follows the natural curve of the back with locking plates that lower the possibility of overextending the back. BNS compatible Back Protector. Well ventilated and designed to circulate air even if compressed tight against the body. Lycra piping surrounding waist straps and protector. Thin waist straps with shoulder belt closure system for a sure fit.