According to Wikipedia:

Anaerobic exercise is exercise intense enough to trigger anaerobic metabolism. It is used by athletes in non-endurance sports to promote strength, speed and power and by body builders to build muscle mass. Muscles energy systems trained using anaerobic exercise develop differently compared to aerobic exercise, leading to greater performance in short duration, high intensity activities, which last from mere seconds up to about 2 minutes. Any activity after about two minutes will have a large aerobic metabolic component.

A study from the National Institute of Fitness and Sports in Japan compared the effects of 6 weeks of traditional aerobics training to high intensity interval training.

The aerobic group exercised at 70% of their maximal aerobic uptake for 60 minutes 5 days a week during the study. At the end of 6 weeks the endurance group did not increase their anaerobic capacity and only increased their aerobic capacity slightly (from 53 to 58 ml/kg/min).

The interval training group also exercised 5 days a week during the 6 week trial. Their workouts consisted of 7-8 sets of 20 second sprints, followed by a 10 second recovery. After the 6 weeks, this group increased their aerobic capacity slightly MORE than the aerobics group (7ml/kg/min increase for the sprint group compared to 6ml/kg/min for the aerobics group). The interval training crew also increased their anaerobic capacity by 28%!

Let’s compare total exercise time for the two groups over the six week study:

Aerobics:
6 weeks x 5days/week x 60 minutes = 1800 minutes

Intervals:
6 weeks x 5 days/week x 4 minutes = 120 minutes

So… in less than one tenth the time, the interval group beat the aerobic group in both aerobic and anaerobic improvement!

Hooray science!

If you’re interested in checking out this study, here’s the citation:

Med Sci Sports Exerc. 1996 Oct;28(10):1327-30.
Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max.
Tabata I, Nishimura K, Kouzaki M, Hirai Y, Ogita F, Miyachi M, Yamamoto K.

And here’s a link to the abstract: Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max