[rojeee]

Depends what you want to do. Run a fast 5k? Improve aerobic system? Something else? Short fast bursts of running will improve your anearobic capacity. Slightly longer intervals run a bit slower may target your lactate threshold and lactate clearance rates. Slower running for longer distances will target your aerobic system.

As people, and the featured article, notes, short intense intervals may burn the most calories but I would argue for more well rounded fitness you need to do a bit of everything.

Due to HIIT, there's loads of people around the world now who have great anearobic metabolism but non-existant aerobic metabolism. The effects are increased autonomic stress from all the anearobic work and an inability to exercise for long durations without quite a lot of stress on your body.

[scp3125]

That... doesn't sound like HIIT. HIIT shouldn't average out to anaerobic heart rate ranges, otherwise you really lose the benefit of keeping your heart engaged in aerobic ranges during the rests. And to be honest, HIIT used for recreation shouldn't even peak in the elevated ranges above 167 bpm, if you're talking about doing it safely over years for health. Higher than that might be effective in training to increase aerobic capacity or lower your overall heart rate, but may damage your heart over time.

The effect of taking those controls on HIIT should result in reducing the autonomic stress response of /all/ exercise on heart rate, not increasing it. The demonstrated effect of HIIT is being able to efficiently scale aerobic capacity without increasing the time required and prolonged physical stress on the body. That's why Olympic athletes have been using interval training for a century, and from the 90s to this day, have used HIIT techniques like Tabata to increase their overall ability to exercise for a long duration, without having to actually commit to exercising for repeated long durations.

I'm not sure where you got that piece of information about loads of people with great anaerobic "metabolism" vs aerobic "metabolism". It would truly be very difficult, but not impossible to consistently raise your anaerobic capacity, (which I'm guessing is the implied relation to processing lactic acid efficiently as a fuel source for exercise?) but not raise your aerobic capacity also. Most of the ways I have seen that involve some kind of ketosis, or something that would otherwise deprive the body of the baseline glucose to drive aerobic capacity up in tandem with anaerobic capacity (because there's just not available excess glucose to store as glycogen in muscles beyond basal mechanical muscle operation and may lead to atrophy from burning muscle as a fuel source a.k.a. awful orange piss). And generally you would have a hard time building up extra muscle in that state as well, so it would be generally not advisable/possibly extremely dangerous to do so without proper diet and nutrition.

disclaimer - I am not an exercise physiologist or certified in any way. I'm an idiot on the internet, so please feel free to correct me if I'm wrong or don't have it quite right

[rojeee]

My point is that if the capability of your aerobic system is very poor - and it is for the vast majority of people, including many HIIT advocates - then whatever exercise you do, you'll very quickly be approaching anearobic territory and find it very difficult to stay at a metabolic steady state for long durations of exercise. When I mean anaerobic, I don't mean "zone 5". I just mean that there will be a much higher percentage of anaerobic glycolysis happening than perhaps should be for a particular level of exertion.

For these people, their aerobic system is so bad that they need to walk to stay in "zone 2". It stands to reason that if they are doing a HIIT session then they are very quickly in the territory of zones 3 and 4, so we are looking at lactate threshold or critical power levels of exertion. So you are looking at more than 60-70% of power coming from anaerobic metabolism. That puts a lot of stress on the body.

For these people to develop their aerobic systems, they need to do a lot of slow jogging or fast walking. Eventually if they do that enough then they'll be much healthier, find HIIT sessions easier and recover much faster. People that spend too much time doing intense exercise will eventually end up with some degree of autonominc dysfunction. I've seen it plenty of times with HIIT people that want to start doing running. They train multiple times a week. Look stressed and knackered. Think they are aerobically fit but can't run a 5km in less than 20 minutes and can't run a 10km at all.

I do HIIT all the time, e.g. 20x400m on the track with 3min walking recovery. But as you noted, one doesn't do these sessions to increase ability to exercise for a long duration, you do long runs e.g. 25km at aerobic threshold for that. I don't do these sessions to primarily target adaptions to my aerobic system either. Instead, I do HIIT sessions like 20x400m with varying levels of recovery to target different apations. E.g. with long recovery I can target anearobic capacity. With shorter recoveries and a lower amount of reps, I can improve lactate buffering and usage.

Finally, it's reasonably well known in athletics circles that if you improve your lcatate threshold, i.e. increase the intensity that you are able to stay sustainably aerobic, then you end up decreasing your anaerobic capcity. The reason why is that the lactate threshold training reduces your ability to produce lactate.

Source: I've been running a long time and have coached runners of various levels.