The last couple of key workouts I’ve been doing 4 sets of 4x 15-seconds all-out and 90 seconds off, with 5 minutes rest between. Its actually harder than it sounds. Here is yesterday’s workout:
Got dizzy in 3rd set and had to get off the bike for 60 seconds during a 90 second recovery. Tasted some bile in my mouth at the end. Some of that is because of smoky air from fires burning in California, but it is definitely a hard workout.