Aerobic Capacity

Aerobic Capacity: what is it?

Coaches use "aerobic capacity" in two ways, and conflating them causes most of the confusion around it. Narrowly, it is a synonym for VO2max - a ceiling measured in ml/kg/min. Broadly, it is the whole aerobic engine: the ceiling, the fraction of it you can sustain, and how much oxygen it costs you to move at a given speed. The broad version is the one that predicts performance, because two athletes with identical VO2max values can be minutes apart over 10 km.

The three components

Endurance performance rests on three things that are trained differently. VO2max is the ceiling. The fractional utilisation - what percentage of that ceiling you can hold for the duration of your event - is largely a threshold quality. Economy is how much oxygen it costs you to hold a given speed or power, and it is driven by technique, elasticity and, in running, by strength and stiffness.

The practical consequence is that a plateau in one component does not mean a plateau in performance. A trained athlete whose VO2max has stopped moving can still improve substantially by raising their threshold and their economy, which is exactly what happens across most experienced endurance careers.

The adaptations behind it

Central adaptations increase oxygen delivery: the heart's left ventricle fills more and ejects more per beat, plasma and total blood volume rise, and cardiac output goes up. Peripheral adaptations increase oxygen use: capillary density around the muscle fibre rises, mitochondrial volume and enzyme activity increase, and the muscle gets better at oxidising fat, sparing glycogen.

The two respond to different stimuli, and that is the whole basis of polarised training. High-intensity work is the strongest lever on the central side; large volumes of easy work are the strongest lever on the peripheral side. A programme that is all intervals and a programme that is all easy volume both stall, for different reasons.

Building it in practice

The distribution that works for almost everyone is a large majority of easy work with a small deliberate slice of genuinely hard work, and relatively little in the middle. For a general-fitness client training three or four times a week, that means two or three easy sessions of thirty to sixty minutes and one hard interval session - not four moderate sessions.

Progress volume before intensity, and progress it slowly enough that the client can absorb it. Aerobic adaptations take weeks to months to appear, which is a hard message for a client used to seeing strength move week to week. Set the expectation early: this is a three-month project, and the marker is that their easy pace speeds up at the same heart rate.

• Track pace or power at a fixed easy heart rate. When that improves, aerobic capacity is improving.
• Do not judge a block by the interval session. Judge it by what the easy days look like.
• Strength work supports economy, particularly in running. It is not competing with the cardio.
• Retest with the same field test every 8 to 12 weeks, under identical conditions.

Why it matters beyond endurance sport

Aerobic capacity is one of the strongest markers of long-term health, and it also determines how well a client recovers between hard efforts - between sets in the gym, between intervals, and between training days. A lifter with a poor aerobic base is worse at their own sport, because their between-set recovery is slower.

For a general-fitness client, the honest sell is not the VO2max number. It is that a bigger aerobic engine means everything else in their week costs less: stairs, kids, a long day, and the last set of every exercise they do.