Named for the atomic mass of Oxygen, this VO2max workout is designed to help you make better use of that oh-so-precious resource. The workout features two main sets. The first is a block of intervals above threshold, pushing your body to work and recover repeatedly. The second is a sustained sub-threshold effort that asks you to work hard on tired legs. You get to breathe easy - and grab some extra O2 - during the long warm up and cool down.