Anaerobic metabolism

Anaerobic metabolism is energy production that does not require oxygen. It covers two systems: the phosphocreatine system, which refills ATP almost instantly for a few seconds of maximal effort, and glycolysis, which breaks glucose down quickly and can sustain very hard work for roughly a minute or two.

It matters because it is what covers the gap. Aerobic metabolism is efficient but slow to ramp up, so at the start of any effort, and during any effort harder than your aerobic system can supply, the anaerobic routes make up the difference.

The cost is capacity. These systems are fast and finite, and the metabolic conditions that accumulate alongside them, hydrogen ions and inorganic phosphate rather than lactate itself, are associated with the burning, heavy sensation that ends a hard interval.

Lactate has been miscast for decades. It is a fuel that muscle, heart and brain oxidise, it is produced continuously even at rest, and it does not cause next day soreness. It rises with hard exercise because production outpaces clearance, which makes it an excellent marker and a poor villain.

The two systems train differently. Short maximal efforts with long recoveries develop the phosphocreatine end; repeated hard intervals of thirty seconds to a couple of minutes develop the glycolytic end and the ability to tolerate and clear what it produces.

Aerobic fitness is what recovers you between those efforts, which is why endurance training makes repeated sprints easier even though the sprints themselves are anaerobic. The systems are collaborators rather than rivals.

In practice, use our heart rate zone calculator to keep hard intervals genuinely hard, and expect anaerobic work to be the part of training that needs the most recovery for the least volume.