"I have been thinking about this exchange over the weekend and want to make a couple of comments.

About energy systems. I have read this material (concerning various energy systems) and want to relay that most of this information is found in swimming coaching materials rather than in exercise physiology texts. I do not know the source of much of this information which, of course, always makes me nervous. Is this information fact, or is it supposition?

Certainly we know that there are three primary energy systems; stored, non-robic, and aerobic, available to provide energy to the working muscles. But eight different zones? I agree with Dr Hahn. Trying to identify eight zones, even under controlled situations doesn't seem possible. Individual variation, diurnal rhythms, and biological noise would seem to be issues that would confound things. Certainly, there exists a continuum in exercise intensity from rest to maximal exercise. Certainly there is a continuum from exercise that is sustainable for seconds to that which can be sustained for days. (One misconception that creates havoc is the idea that there is an anaerobic threshold beyond which there is a lack of oxygen in the muscle, blood etc. Big argument here that I can go into if you want but might confuse the issue.) This has been around for a long time and will probably continue for a long time.

Anyway, the Webster definition of continuum states: a continuous whole, quantity, or series whose parts can not be separated or separately discerned. The biochemistry of metabolism needs to be considered in this light. What we seem to have done is to interpret things that can be easily measured in an attempt to explain our model. Not knowing of course if the model works or not or what we measure is even important. Maybe this is an assigned importance rather than an actual importance. Odd to say, but how can you explain things otherwise? How can you build a model from parts you cannot see or measure?

So, can the eight 'zones' be measured and if so, using what criteria? If each zone were an equal percentage of the spectrum, we would have zones of about 12% or so in width. What is the precision, reliability, and repeatability of these measures? Can the measure account for the inherent variance? Needless to say, I am confused by all this and would like to know more. My current thought is this is just not practical.

In terms of the 'rest & recovery': again, a very interesting debate. Gerschler in the 1920's and 30's actually focused his training programs on the recovery interval, much more so than elements of the work interval. Moreover, the rest intervals were based upon recovery heart rate with rest comprising a much more significant element than it does today. His idea was that cardiac effects of training required a certain extent of rest and felt that for maximum effect, recovery must be allowed. Much was focused upon stroke volume and its interaction with heart rate. Today we know that differences in stroke volume are key and that increases in stroke volume drive increases in O2 delivery. So, maybe.