Interval Training - Improve VO2max

The greatest benefit to interval training is to improve your VO2max. To get the most benefit from interval training is to accumulate a good bit of time working at 95-100% VO2max or 98-100% of maximum heart rate. It takes your body 2 minutes to reach the point where it's operating at maximum oxygen consumption, when running at proper I pace.

For example -
4 X 5 minute runs at I pace
2 minutes to reach max with each run
20 minutes of hard running you have accumulated 12 minutes at max even if you completely recover between each run

Since it takes 2 minutes to reach max, if you do intervals shorter than 2 minutes, you must reduce your recovery time

For example -
20 X 1 minute runs at I pace
30 second recovery between
20 minutes of hard running may accumulate to 10 minutes at max

Although the duration of workbouts is important (2 minutes to reach max) when it comes to interval training, intensity (training pace) is even more important. You can only attain VO2max when running at a particular intensity. Note that running at VO2max is running at your maximal HR. The least stressful intensity to produce the results you want is the ideal intensity. Keep your running at an intensity that still accomplishes a quality running session and provided you with maximal VO2max benefits.

Happy Running :)