[Scott Johnston]

Todd:

Thanks for the thoughtful questions and comments.  I’m glad you are paying close attention and are provoked to think more deeply about these things.

You may recall this quote from Per Astrand, one of the fathers of modern exercise science:

“It is an important but unsolved question which type of training is most effective: to maintain a level representing 90% of the maximal oxygen uptake for 40 min, or to tax 100% of the oxygen uptake capacity for 16 min’’

You are asking this same question essentially, and no one knows the answer definitively.

The study I cited is: Adaptations to aerobic interval training: interactive effects of exercise intensity and total work duration. S Seiler, K Jøranson, B V Olesen, K J Hetlelid Scand. J. Med. Sci. Sports 23:74-83, 2013 I recommend you read it since you are curious about it.  The summary I was making when I said that the 4x8min group improved their endurance was that they showed significantly greater gains than the other groups in the following qualities:  maxVO2, power at maxVO2, power at 4mMol/l blood lactate, and time to exhaustion at 80% of max power.  This test was done with cyclists, hence the power data.

All of those qualities combined, but especially the last one, indicate increased endurance.

For anyone but an elite-level athlete, a maximum effort of 8 minutes is going to give an intensity of just over the AnT.  Local muscular fatigue will end up being the limitation to sustaining this output. The same holds that for most people, the 16-minute reps are going to be just below AnT or Zone 3 intensity.

This study is not saying that 4x8min is the holy grail of interval workouts and that is all you need to do. It is only showing that these did produce the biggest gains in endurance qualities.

The thing you need to understand is that just as the aerobic base serves as foundation for all the higher intensities but increasing the size of the vacuum cleaner in the ST fibers, each increasingly faster twitch group of fibers serves as the vacuum cleaner for the next higher level of FT fibers just above it.  The reason intervals of different lengths are important is that the longer, lower-intensity ones serve as aerobic support for the shorter, higher-intensity ones.

I hope this helps.

It sounds like I might need to write an article or do a podcast about this topic.  Thanks for prompting me.

Scott