What are intervals?

"The Finishing Kick" in the October Running Times has an interesting point of view on how to run intervals (can't link to it because it doesn't appear to be on the RT web site). In the article, Roy Benson cites the example of Woldemar Gerschler, the "father of interval training":

In the late 1930s, Dr. Gerschler used his refinements to develop Rudolf Harbig into the world record holder at 800 meters with a time of 1:46.6. As an expert on cardiac output and blood volume pumped during exercise, Gerschler's contribution involved using pulse counts to determine the length of the recovery period. According ot Dr. Gerschler's calculations, a runner was ready to run the next segment of the workout when his pulse had dropped down during the recovery interval to 120 bpm.

Leaving the obvious imprecision of the 120 bpm calculation as being "recovered" (what about older athletes with lower maximum heart rates, or those who have naturally occurring lower max heart rates? 120 bpms might not be low enough to indicate adequate recovery), I think Benson's large point is valid: Recovery should probably be measured by cardiovascular data, not by the very imprecise measurement of time or distance jogged during a recovery interval. After all, who reading this blog right now doesn't set out to do 12 X 400 meters with a 200 meter recovery jog between, or 2 X 20 minutes on the bike with a 10 minute recovery spin? A tired athlete might not sufficiently recover in that time period.

But on the other hand, some interval workouts are designed to not allow total recovery as a way of training athletes to duplicate race-day conditions. An 800-meter runner, Harbig's race was mostly an anaerobic battle of fast-twitch muscles and lactate tolerance. For Harbig, to duplicate race conditions, 400 meter intervals should have been done at nearly top speed to stimulate the fast-twitch muscle fibers and test his ability to continue running with a high lactate load, for which a long recovery period is necessary. But for a 5,000-meter specialist, or a marathoner, the distance requires the ability to function under moderate lactate load and at a steady heart rate. In this instance, I believe a shorter recovery period is called for to duplicate the length of the race.

An example: Last summer, my nephew, at the time a high school cross country runner, was visiting, so we went out to the track to do some intervals. I was training for sprint duathlons (5K/30K/5K), so I was training as if I were a 5K/10K runner. He was training for the 5K distance. I did my intervals as a 400 meter run at race pace with a 200 meter recovery (about a minute or a minute and 10 seconds). After two or three, he was feeling very whipped, because his coach had never given his runners that short of a recovery period. In essence, he was having them go all out and then giving them two or three minutes to recover. That's great training for an 800-meter runner or a miler, but not ideal for the 5K(in my opinion) because it doesn't train the body to go near continuously at race pace.

This Running Times article probably best explains how to approach interval workouts as a runner. Most importantly, it notes

The two types of speed training most critical to distance running success are: VO2 max training and technique training.

VO2 max training is designed to improve your maximal aerobic capacity. Your VO2 max is determined by the maximal ability of your heart to pump oxygen-rich blood to your muscles, and of your muscles to extract and utilize that oxygen to produce energy aerobically. By improving your maximal aerobic capacity, this type of training will help to improve your sustained speed, which is most important for distance running success.

The most effective training to improve your VO2 max consists of running intervals of two to six minutes duration (typically 600 to 1600 meters) at your 3K race pace. Your recovery jogs between intervals should take about 50 to 90% of the time it takes to run each repetition. (emphasis added) The stimulus to improve your VO2 max is provided by the amount of time that you accumulate in the optimal intensity range during a workout.

Given that over 90% of your energy in races of 5K or longer is produced aerobically (and the proportion gets higher the longer the race), it is this sustained speed that you need to improve your racing performances. Running your intervals faster than 3K race pace will reduce the stimulus to improve your VO2 max by building up high levels of lactate in your muscles and shortening the duration of your workout.

Yes, heart rate should be a factor in determining your recovery period, but achieving total recovery can be counterproductive, depending on what your goals are.