Interval Training for Cardiovascular Fitness. Part I: What is it? Why do it?

I have decided to time my mile as part of my fitness testing.  In the past I have done a six minute test.  The six minute test is simply seeing how far you can run in six minutes.  Either process is not pleasant.  So why do it?

The speed at which you can maintain a pace for six to eight minutes is representative of your velocity at VO2 max.  VO2 max is the maximum volume of oxygen your body can use while running.  In general, the greater the VO2 max, the better your aerobic system is at delivering oxygen to the active muscles.   The time limit that people can maintain a pace at VO2 max is about 6-8 minutes.   Running out of steam before six minutes indicates that you are running at above the velocity of using VO2max and that you are using a higher percentage of the anaerobic metabolism.

Below the velocity at VO2max, more than 90% of the energy required for the work you are doing is generated by the aerobic system.  As the intensity of work increases above the velocity at VO2 max, the percentage of aerobic (versus anaerobic) metabolism decreases.  For this reason, it can be helpful to know your velocity at VO2 max.

It is thought that the greatest increases in aerobic capacity will occur by working out at intensities near the velocity of VO2max.  However, this pace can only be maintained for six minutes before fatigue.  The fatigue is thought to be caused by the buildup of lactic acid.  To avoid lactic acid buildup, we would have to work out at intensities in which only the aerobic system is used.  This would correspond to about 80% of the velocity of VO2 max.  At this intensity, about 99% of the metabolic energy comes from the aerobic system.  This is a pace that can be maintained indefinitely.  But at no point will you actually be using all of your aerobic capacity (you will not be using the maximum amount of oxygen that your body is capable of using) .  So working out at this pace does not produce gains in the aerobic system as rapidly as working out at higher intensities.

So the problem is, how do we work out at intensities in which we are using as much oxygen as possible without tiring out from lactic acid buildup?

The answer is interval training.  The whole point of interval training is that you can extend the amount of time you spend using your VO2 max with significantly less lactic acid buildup in the muscles.

There is still a lot of debate what type of interval training produces the best results.  Intervals as short as 15 seconds of work and 15 seconds recovery have been studied.  Recovery between working intervals has been studied at rest, walking, or at 50% velocity of VO2max.  The variables that can be adjusted in interval training can seem endless.  Deciding what kind of interval training you want to do can get very confusing.

In part II we'll look at some factors to consider when incorporating interval training into your conditioning program.

Related Posts

• Interval Training for Cardiovascular Fitness. Part II: Planning your interval program
• Interval Training for Cardiovascular Fitness. Part III: Three interval protocols I’ve used successfully