The short answer
A rate and a quantity, not two engines
People tend to picture aerobic and anaerobic as two systems taking turns, with a switch between them. That is not how the measurements behave. VO2max describes how fast oxygen can be used. Anaerobic capacity describes how much work you can do on top of that before you run out. One is measured per minute, the other is a total. Keeping that distinction straight explains most of what follows.
The curve that joins them
For hard exercise, the time you last gets shorter as the pace gets higher, and it does so in a specific hyperbolic shape that is remarkably consistent across different species and different forms of exercise1. That curve is fully described by just two numbers.
The first is critical power, the asymptote of the curve. It is the highest rate of aerobic energy production you can sustain without continuously drawing on your reserves1. The second is W prime, written W', the curvature of that hyperbola. It is a fixed amount of work, measured in kilojoules, available to you above critical power, made up in part of anaerobic energy sources1.
Critical power sits below VO2max. That gap is the whole story: between your highest sustainable effort and your aerobic ceiling lies a region you can enter, but only on a budget.
Why running out is predictable
Below critical power, your body can reach a steady state and hold it. Above it, it cannot. A major review describes critical power as a genuine fatigue threshold for exactly this reason: it separates the intensities where physiological responses stabilize from those where they do not2.
Once you are above critical power, you spend W' at a rate set by how far above it you are. When W' is gone, VO2max is reached and you stop2. The further above critical power you go, the faster the tank drains, which is why going out slightly too hard costs you disproportionately later.
How fast the tank refills
W' is not spent permanently within a session. Drop below critical power and it recharges, but more slowly than most interval sessions assume. When researchers exhausted subjects and then gave them different recovery durations, W' came back to 37% after 2 minutes, 65% after 6 minutes and 86% after 15 minutes, with a half-time of roughly four minutes3. Critical power itself was unchanged by the prior effort.
This is why short rests between hard repetitions make each one harder than the last: you are starting the next rep with a partly empty tank. It is also why that is sometimes exactly the point of the session.
Training them is partly separate
The clearest demonstration is one we have covered before. In the original Tabata study, the group doing 20 seconds hard and 10 seconds rest raised both VO2max and anaerobic capacity, the latter by 28%. The comparison group cycling at a moderate pace for an hour, five days a week, raised VO2max but saw no change in anaerobic capacity at all4.
Same outcome on one measure, completely different outcome on the other. That dissociation is the practical argument for treating these as two qualities rather than one fitness number. More detail on the protocol is in Tabata intervals for VO2max.
Roughly what trains what
Long intervals at a hard but controlled effort, of the 4 x 4 minute kind, push the aerobic ceiling and critical power. Short, very hard, repeated efforts with incomplete recovery lean on W' and are the ones that build tolerance for spending it. Easy aerobic volume supports both without directly maximizing either.
Measuring them honestly
Anaerobic capacity has an established laboratory measure, the maximal accumulated oxygen deficit, which levels off for efforts lasting about two minutes or more. In the study that established the method, the maximum accumulated oxygen deficit varied between subjects from 52 to 90 ml/kg5, a wide spread that is largely independent of how high someone's VO2max is.
Critical power and W' are estimated from several maximal efforts of different durations, ideally on separate days. Watches and bike computers that display these numbers are modeling them from your training data, which is useful for watching a trend but is not the same as a measured value. If you want to know what your own curve looks like, the most honest low-tech version is to record genuine best efforts over a few different durations and see how they change over months.
Try this workout
Tabata for anaerobic capacity
Who it is for: Anyone already training regularly who wants more to spend above their sustainable pace.
- 1Warm up. 10 minutes of easy cycling, building gradually.
- 2Main set. On a bike: 8 rounds of 20 seconds as hard as you can, 10 seconds rest. This is the protocol that raised both VO2max and anaerobic capacity, the latter by 28% 4.
- 3Then go easy. Finish with easy riding, not another hard effort. Your reserve above critical power was only about two thirds back after 6 minutes 3.
- 4Each week. Once or twice, alongside a long-interval session such as 4 x 4 minutes for the aerobic side.
Put it into practice
Sources
All research- 1Critical power: implications for determination of VO2max and exercise toleranceJones et al., 2010. Medicine & Science in Sports & Exercise 42(10):1876-1890.
- 2Critical power: an important fatigue threshold in exercise physiologyPoole et al., 2016. Medicine & Science in Sports & Exercise 48(11):2320-2334.
- 3Effect of recovery duration from prior exhaustive exercise on the parameters of the power-duration relationshipFerguson et al., 2010. Journal of Applied Physiology 108(4):866-874.
- 4Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2maxTabata et al., 1996. Medicine & Science in Sports & Exercise 28(10):1327-1330.
- 5Anaerobic capacity determined by maximal accumulated O2 deficitMedbø et al., 1988. Journal of Applied Physiology 64(1):50-60.
General fitness information, not medical advice. If you have an injury or a health condition, check with a professional before starting a new kind of training.