High-intensity interval training (HIIT) can boost your VO2 Max specifically for cycling. A few sessions include 40/40 intervals, or 40 seconds of work followed by 40 seconds rest; 2 minutes at 115-150% of FTP, 4 minutes at 115-125% of FTP and more complex ones like 30-15 intervals (30 seconds of work followed by 15 seconds of rest). Such sessions must be performed 2 times in a week with sufficient recovery, and are ideal 6-8 weeks before a major competition.