Why 40/20s Are Great for Aerobic Training—but Shouldn’t Be Your Only VO₂ Workout
Forty seconds hard. Twenty seconds easy. Repeat.
There is a reason 40/20s—and similar workouts such as 30/15s—have become so popular. They let you accumulate a substantial amount of work at a very high aerobic intensity without forcing you to hold that power continuously. Your heart rate climbs, oxygen consumption stays elevated, and the session can create a huge aerobic stimulus.
They work. That is not really up for debate.
But should 40/20s be the only VO₂max workout an athlete performs?
Probably not.
The reason is not that your body somehow “learns” to wait for the next recovery. There isn’t good evidence that short-interval training damages VO₂ kinetics or teaches the aerobic system to switch off every 20 seconds, even though I have thought and verbalized this before. Short intervals can improve VO₂max, VO₂ kinetics, five-minute power, and even longer-duration performance.
The better argument for including longer intervals is performance specificity. A cyclist who becomes excellent at producing high power with frequent micro-recoveries is not necessarily equally prepared to sustain severe-intensity power continuously when a race goes full gas—and stays there.
That is where three-, four-, and five-minute intervals remain important.
Why 40/20s Work So Well
At first glance, 40/20s can appear too fragmented to create a sustained aerobic stimulus. You are constantly raising and lowering the power, so it is easy to imagine oxygen consumption rising and falling just as quickly.
That is not what happens.
Power changes almost instantly when you push harder or back off. Oxygen consumption changes more slowly. During the 20-second recovery, VO₂ does not immediately return to baseline. Heart rate, cardiac output, breathing, and muscular oxygen use all remain elevated. If the recovery is short—and especially if the rider continues pedaling at moderate power—the aerobic system carries substantial momentum into the next repetition.
The power is intermittent, but the aerobic strain can remain remarkably continuous.
At the same time, those 20 seconds provide just enough relief to help the athlete produce more power during the next work interval. There can be some partial phosphocreatine replenishment, a brief reduction in muscular force, and a slowing of the ongoing metabolic disturbance. The rider also receives a small psychological reset before attacking the next repetition.
None of that amounts to complete recovery. Twenty seconds is far too short for that. But it may provide just enough relief for the rider to repeat a power output that would be unsustainable continuously.
That combination makes microintervals so effective: the aerobic system remains highly engaged while the legs receive small, repeated opportunities to survive.
Research from Rønnestad and colleagues has shown that short intervals can allow cyclists to spend more time above 90% of VO₂max than effort-matched five-minute intervals. If the goal is to accumulate time at a high percentage of VO₂max, short intervals may sometimes accomplish that better than traditional longer efforts. To make things even more confusing at times, even though athletes achieved more time at VO₂max, that did not always raise their actual VO₂max!!
Using all the tools is really sounding like the best idea.
Does Your Body Learn to Rely on the Recovery?
This is a common coaching explanation: if an athlete always performs 40/20s, the body supposedly becomes accustomed to the recovery and fails to develop the oxygen kinetics required for continuous efforts.
It sounds plausible, but the research does not support it particularly well.
VO₂ kinetics describes how quickly oxidative metabolism responds when exercise intensity increases. Faster kinetics means the aerobic system meets the rising energy demand sooner, reducing the contribution initially required from finite anaerobic sources.
Short on-off intervals do not appear to prevent this adaptation. Research comparing longer severe-intensity intervals with much shorter efforts found that both programs improved VO₂ on-kinetics similarly despite their very different power patterns. More recent work has also found that continuous moderate and heavy exercise, severe-intensity intervals, and sprint intervals can all improve the speed of the VO₂ response.
It would therefore be misleading to say athletes need five-minute intervals specifically to teach the aerobic system to turn on and remain on. Short intervals can develop the aerobic system very effectively.
The limitation is not necessarily the fitness they build. It is how specifically they prepare the rider to express that fitness.
Building Fitness Versus Expressing It
An athlete can improve VO₂max using 40/20s and still struggle during a continuous five-minute climb, bridge, or race-winning move. The aerobic adaptation occurred, but the athlete may not have developed the specific ability to express it without repeated reductions in power.
During 40/20s, every recovery changes the physiological problem slightly. Force falls, W′ depletion slows, and phosphocreatine begins to replenish. The rider can also adjust position, breathe, and mentally reset.
Now compare that with a race going full gas for five uninterrupted minutes.
There is no reduction in power or opportunity to divide the effort into small pieces. Above critical power, W′ continues to drain, local fatigue develops, and the effort becomes progressively more expensive.
That is why someone can look fantastic during microintervals yet unravel when asked to hold hard, steady power for five minutes. The rider may possess the aerobic capacity but lack the continuous exercise tolerance, pacing skill, or muscular durability needed to convert it into race performance.
What Longer Intervals Add
Longer VO₂ intervals are not inherently superior. They expose the athlete to demands that microintervals partially interrupt.
1. Prolonged local muscular fatigue
During a continuous severe-intensity effort, the athlete must keep driving the same musculature while fatigue steadily rises. This matters during sustained climbs, time trials, breakaways, and long pulls.
2. The VO₂ slow component
Above critical power, oxygen consumption can keep drifting upward even when power stays constant. A continuous interval forces the athlete to live inside that rising cost without backing off every 20 seconds.
3. Continuous W′ depletion
W′ is a useful model for the limited work available above critical power. During 40/20s, recovery can slow its depletion or permit some reconstitution. During a continuous effort above critical power, there is no interruption. That matters when the decisive move is sustained rather than a series of surges.
4. Pacing the effort
Microintervals let you attack because relief is close. A five-minute effort requires restraint: start too hard and the final minutes can become disastrous. Distributing effort over three to six minutes is a skill that must be practiced.
5. The sensation of having no escape
In 40/20s, relief is seconds away. In a continuous interval, the athlete must accept that discomfort will continue—and probably increase. That tolerance matters when a climb, bridge, or breakaway becomes brutally hard.
But Short Intervals Still Transfer
We should not take specificity too far. Short-interval studies have produced improvements across 30-second, five-minute, and 40-minute tests. Those aerobic adaptations absolutely transfer, but some riders still need direct exposure to continuous efforts before the fitness becomes visible in a decisive race situation.
How I Would Combine Them
Instead of asking whether 40/20s or five-minute intervals are better, ask what problem you need to solve.
If you struggle to accumulate quality work during longer intervals, microintervals may be the better tool. You can generate a large aerobic stimulus without early muscular distress ending the session.
If you crush 40/20s but repeatedly get dropped during sustained climbs or five-minute race efforts, more microintervals may not address the weakness. You probably needs continuous three-, four-, or five-minute work.
A block might emphasize 40/20s for several weeks, then move toward longer intervals as racing approaches. Options include three sets of ten to twelve 40/20s, five to eight three-minute efforts, four to six four-minute efforts, or four to six five-minute efforts.
These workouts should not use identical power targets. Short intervals permit higher work-interval power. Longer intervals require more control because repeated recoveries are not available to extend the effort.
The Bottom Line
Forty-twenty intervals are not a shortcut or an incomplete form of aerobic training. They are one of the most effective ways to accumulate time near VO₂max, and they can improve performance across a range of durations.
Your body also does not simply shut down and wait for the next 20-second recovery. The aerobic system remains highly active during those brief reductions in power, and short intervals can improve VO₂ kinetics perfectly well.
But fitness must eventually be expressed in the environment where it matters.
Sometimes racing gives you repeated surges with small opportunities to recover. Forty-twenties prepare you well for that. At other times, the race goes full gas for three, four, or five uninterrupted minutes. There is no reset, no reduction in muscular force, and no break in W′ depletion.
Longer intervals prepare you for that version of the problem.
Use 40/20s to build a massive aerobic stimulus. Use longer intervals to develop continuous exercise tolerance, pacing, muscular durability, and the ability to stay committed when there is no escape from the effort.
Use each workout for what it does best—and make sure you can express the fitness when the race goes full gas and stays there.