FuelCell midsole foam with approximately 3% bio-based content delivers a propulsive feel to help drive you forward. Bio-based content is made from renewable resources to help reduce our carbon footprint. Energy Arc pairs sport-specific carbon fiber plate geometry with strategic midsole voids, designed to increase stored energy to deliver a higher amount of total energy returned Angular midsole design reduces weight Outsole designed for superior traction 4 mm drop; due to variances created during the development and manufacturing processes, all references to 4 mm drop are approximate Preloved for 34 running miles. Just not a good fit for me.