Ladies and gents, please be civil and do not let this interesting thread deteriorate.
There is a number of people on this thread with knowledge of biochemistry; their opinions are welcome. If you are not one of these people (and reading a couple of hand picked studies about HIIT does not qualify), please obstain from expressing opinions and limit yourself to reading and asking respectful questions.
I will attempt to summarize so we can continue.
1. As Richard Feynman said, there is no total certainty in science; only degrees of uncertainty. In the field as mind numbingly complex as muscle cell biochemistry, the degree of uncertainty will remain high for a long time. So there is no last word.
2. There are two ways to progress athletic training knowledge: empirically and through the use of biological sciences. History shows that there have been a number of highly successful methodologies that were almost totally empirical—such as the Soviet weightlifting methodology. Usually a combination brings best results.
3. One has to take the results of brief studies on untrained subjects with a grain of salt. (And many HIIT studies are in that category.)
4. Oftentimes there is more than one way to get the job done due to the complexity of the human body and the multitude of possible stimuli and responses.
5. Sometimes methods delivering similar results can be diametrically opposite. E.g., one can train to run 800m by maximizing his glycolytic power and capacity—or by finding ways to delay maximal deployment of glycolysis.
In practical terms, when we are training to develop general endurance and WTHE with kettlebell quick lifts and stay healthy:
I. Glycolytic training pros and cons.
Pros:
* Promotes muscle building and fat loss through an endocrine response and muscle building through local effects of lactic acid.
* Can be used for peaking in some events.
Cons:
* Since glycolytic training is very stressful, it easily promotes basedowic overtraining if one is not careful in planning training and all aspects of one's life.
* For the same reason—intense production of stress hormones—one must be careful with glycolysis in the same manner one must be careful with the effects of excess stress on health.
* There is evidence that when acidosis is high, some mitochondria get destroyed (they literally fill up with water and blow up as they cannot keep up with buffering H+). Yes, there is a number of studies demonstrating that—in Russian. (And no, a few studies demonstrating mitochondrial biogenesis in newbies from HIIT do not cancel them out.)
* There is some evidence that the H+ and other electrically charged particles produced as a result of anaerobic glycolysis may damage other tissues—but the research is inconclusive. There is damage on one hand and a possible need for some oxidative stress to stimulate adaptation on the other (the hormetic effect).
* Glycolysis in the heart (sustained and repeated HR >190) does damage the heart. (Although there is no evidence that in healthy people high heart rates elevated for brief periods of time, e.g. a hard sprint or a set of snatches, are dangerous, you should trust your cardiologist's recommendations on high how and how long you can push yours.)
* Soreness and fatigue. Not a problem if you ride a desk; a life or death problem for some professions.
In other words, glycolysis is a strong medicine with possible side effects and one must take it in moderation.
Recommendations on how to do it:
http://www.strongfirst.com/long-rests/
II. Anti-glycolytic training.
Researched and experimented with in the USSR since the 1980s, AGT culminated with superb results on a number of Russian national teams in diverse sports in this century.
Original AGT work was aimed at minimizing glycolysis by maximizing the CP pathway and aerobic recovery. Later work was aimed at developing mitochondria in intermediate and fast fibers. The conditions for mitochondrial biogenesis: a fiber must spend a lot of total time in a
mild acidosis. Hence traditional interval and circuit training are modified to stop the sets at the onset of
mild local fatigue and to increase the rest periods to maintain the average session intensity at right below the AnT—hence the relevant HR discussions on this forum.
AGT develops exceptional endurance at a very low biological cost, with minimal fatigue and soreness.
AGT comes with WTHE—fat loss, power improvement, and some muscle hypertrophy. And the aerobic adaptations improves one's overall health (plenty of research on that).
Glycolytic and anti-glycolytic training can be periodized, but that is another conversation.