With the Boa® Fit System and a roomy toe box, this lightweight cross-country shoe is comfortable, efficient and ready to rip. Synthetic upper for supple feel over the foot + holes for ventilation. Efficient power transfer: With a 50 stiffness index, the injection-molded nylon composite sole is engineered to be moderately stiff and plenty light, with a good level of grip. Lightweight Boa® L6 Fit System for on-the-fly micro-adjustment, backed by the Boa Guarantee.