HFS may be short for Happy Feet Shoes,Hyper Flexible Soles,or, more likely, what you will exclaim when you try this 7.0 ounce (M9) true road runner, Holy ___ ___! Your feet will love the natural comfort of the wide toe box and zero-drop design. The tire-inspired tread keeps you sure-footed. The 37.5 sockliner and breathable engineered mesh keep you cool. Hidden instep and mid-foot tensioners give you the security you need to go for that new personal best.