VO2max is the ceiling — the most oxygen your body can take in, transport and use per minute. It is not the number you race at. It is the number every other number lives underneath, and lifting it makes threshold cheaper, easy pace faster, and the whole range beneath it more comfortable.
You raise it by spending time near it. Not at it briefly, not above it — near it, repeatedly, for long enough that the system has to adapt. That is what these intervals are for, and everything about their shape follows from that one requirement.
Why the Rep Is Four Minutes Here and Three on Foot
This is the difference that matters most, and it is worth understanding rather than memorising.
Oxygen uptake does not jump. When you start a hard effort, it takes ninety seconds to two minutes to climb to its maximum. Everything before that is the ramp, not the stimulus. So the useful part of an interval is whatever is left after the ramp, and the ramp has to be paid for every rep.
On a run you can be at full effort in the first stride. Your legs move at target pace immediately, the whole body is loaded, and oxygen uptake climbs quickly. A three-minute running rep buys you a good ninety seconds at the ceiling.
On a bike the ramp is slower. You are seated, you are recruiting less muscle mass, and there is no gravity forcing the pace — you have to build into the power rather than simply arriving at it. That pushes time-to-ceiling toward the far end of the window, and a three-minute rep on a bike would give you perhaps sixty seconds of the thing you came for, with the same fatigue cost. Four minutes is what makes the rep worth doing. Push it much past five and you drop below the ceiling from fatigue, which is a different session.
Same principle in both places: the rep is as long as it needs to be to accumulate real time at the target, and no longer. The number differs because the ramp differs.
Why Recovery Is Three Minutes
Long enough to make the next rep possible, short enough that you do not start from cold.
You are not trying to fully recover. If your heart rate falls all the way back to easy, the next rep pays the full ramp again from the bottom, and you spend the session climbing instead of sitting at the top. Three minutes brings breathing back under control and heart rate down substantially, while leaving the aerobic system elevated so rep two arrives at the ceiling faster than rep one did.
Three, rather than the two a running session uses, because the rep is longer and the recovery scales with it — and because a bike recovery is genuinely free. You can stop pedalling entirely. A running recovery is still running.
Spin easily through the recovery, don't stop. Light pressure clears the legs faster than freewheeling does.
Finding the Effort
Heart rate: 89 to 96% of maximum. Below a running VO2 session's 92 to 100%, because every bike band sits lower — seated position, less muscle recruited, lower heart rate at the same effort. If you chase 95% of your running maximum on a bike you will be riding above what you can repeat, and rep four will fall apart.
Heart rate will not be there for the first ninety seconds. That is the ramp described above, and it is expected. Pace the rep by effort and breathing, and read heart rate in the second half as confirmation.
Cadence: 95 to 105 rpm — higher than any other session in the catalog, and deliberately. A fast cadence at a given power shifts the load off the legs and onto the cardiovascular system. That is precisely the system you are training here. Grinding the same effort at 75 rpm turns a VO2 session into a muscular one and puts the stimulus in the wrong place.
Feel: hard. Three or four words at a time. The last minute of each rep should be genuinely uncomfortable, and you should believe you could do one more at the end of the last one.
Executing It
The first rep should feel too easy. That is right. Almost every failed VO2 session fails because rep one was ridden as hard as the athlete could go, which makes reps four through six impossible and turns a six-rep session into a three-rep session with a long, pointless tail.
Ride every rep at the effort you could sustain for all of them. If the last rep is the same as the first, you have done it correctly. If the last rep is much slower, you did not do a VO2 session — you did one hard effort and then some tired riding.
Stop early rather than late. If a rep drops noticeably in power or heart rate cannot reach the band any more, the session is finished. Nothing useful happens after that point and the recovery cost keeps accruing.
Indoors is better for this one. Four minutes of uninterrupted hard riding is hard to find on open roads, and a junction in the middle of rep three ends it.
Where It Sits in the Week
This is a genuinely hard session. It is easy on the legs and expensive everywhere else — heart, lungs, hormones, nervous system. Being off your feet changes the muscular cost, not the systemic one.
So it is spaced like any other quality session: not next to a threshold session on either delivery, not the day before a long effort, and not at all in a week already carrying two hard days. In taper and race weeks it does not appear.
Practical Guidelines
- Structure: 15 minute warm-up · 4 to 6 × 4 minutes hard, 3 minutes easy spin between · 10 minute cool-down.
- Total: 50 to 64 minutes.
- Intensity: 89 to 96% of maximum heart rate. RPE 8.
- Cadence: 95 to 105 rpm during the reps.
- Pacing: every rep the same. If the first one was your best, the session was wrong.
- Frequency: once a week at most, in build and peak only.