Most people associate the term "Aerobic" with cardiovascular exercise, such as brisk walking, running, cycling or using the elliptical. It is true that these activities are almost entirely aerobic. But perhaps you have heard that distance runners need not only engage in aerobic exercise, but also must increase their anaerobic capacity.
This might seem contradictory, since in their simplest terms Aerobic means "With oxygen," whereas Anaerobic means "Without Oxygen." You might think, "I am always breathing while running, so why would I need to run without oxygen?"
But the term anaerobic is not suggesting that you exercise without breathing; instead anaerobic capacity refers to the amount of extra work your body is able to do even when it is not receiving enough oxygen. In other words, even without oxygen your muscles are able to break down fats/sugars/etc, convert them into useful energy and perform.
As an example, at the end of an all-out 5k, your body is exhausted. You are breathing very hard and have reached your aerobic capacity. It seems as if you cannot go any faster, yet with 200 meters to go you dig deep and end the race with a deadly sprint. That kick at the end of the race is you engaging your anaerobic system! Through training your muscles, they have been taught to engage even when they don't quite have enough oxygen for fuel.
This knowledge about the difference between aerobic and anaerobic work begs two questions.1) How important is it to increase your anaerobic capacity?
2) How does one go about increasing his/her aerobic and anaerobic
The first question can vary greatly depending upon whether you are an 800 meter runner or a marathoner. For runners competing at events over 400 meters, aerobic fitness is incredibly important. Even distances as short as the half mile seem to require a significant aerobic capacity. But for distances from 800m to 5k, anaerobic capacity is also quite important. You need to have speed to compete well in those events. As distances climb, anaerobic capacity becomes less important, but even at the marathon distance having a bit of speed can provide the edge to push you to surpass your goals. So for shorter distances (800m to 5k) developing a high anaerobic capacity is integral to success and for longer distances (10k to marathon) anaerobic capacity, though not nearly as important as aerobic capacity, can help a runner achieve a higher level of success.
So since we know that developing both aerobic and anaerobic capacities are important for all runners who race over 400 meters, how does one go about improving those capacities? As distance runners, you are probably already aware of how to increase aerobic capacity. All those long, steady state runs you do are building your aerobic capacity. But how does one develop anaerobic capacity? Basically doing hard, fast speed work is the best way to develop muscles that can perform under conditions without oxygen. Repeats of 400 meters or less at near your top speed with full recovery will allow you to develop a proper anaerobic capacity. As an example, a 5 minute miler looking to improve his anaerobic capacity might do 8x200m at 33-34 seconds with 2 minutes recovery. The repeat would be very hard for that runner, taxing his muscles, and the recovery would be long enough that the workout does not tax his cardiovascular system.
So when developing a training plan, remember to incorporate both aerobic and anaerobic exercise to improve your overall fitness as a runner.