30-30s were popularised by Prof. Stephen Seiler's research on time spent at VO2max. The protocol accumulates more time above 90% of VO2max than traditional 4- or 5-minute intervals because the 30-second recoveries don't allow HR and oxygen uptake to fall. Typical session: 2-3 sets of 13 × (30s on / 30s off), with 5-8 minutes recovery between sets. Fast to complete (45 min total), brutally effective for VO2max development, and well-tolerated by riders who struggle with longer VO2max intervals. Cadence and power matter less than hitting the required intensity.