Zwift Slowtwitch Indoors: Sub-Threshold To VO2 Over-Under Building upon the previous Sub-Threshold XX session, this time we are decreasing the time between surges and increasing the number of surges. By challenging your ability to process lactate at near threshold power, you will increase your time to exhaustion and increase endurance. All this is achieved with a 30min steady state effort that dips briefly into the realms of VO2. bike