Anaerobic literally means “without oxygen”.  Anaerobic running is when you exceed the speed and/or distance for which you have trained. The muscles are pushed beyond their capacity and need more oxygen than the body can supply. For a limited period of time, muscles continue to function by utilising chemical processes that free oxygen from within the muscle itself. The amount of oxygen available this way is quite limited, large amounts of waste build up (lactic acid) and the muscles get tight and sore. After the exercise is over, this oxygen must be “paid back” to the muscle.

Anaerobic exercise is used by athletes in non-endurance sports to build power and by body builders to build muscle mass. Muscles trained under anaerobic conditions develop differently, leading to greater performance in short duration, high intensity activities, which last up to about 2 minutes.

One of the main purposes of speedwork is to give you anaerobic experience in measured doses, if you follow it with sufficient rest, you’ll train your body to deal with oxygen debt.

See more definitions in the Definitive Guide to Running Terms.