Theoretically, extended periods of high intensity aerobic training could basically turn your muscles into cardio fuel. If you are in a deficit and do this kind of thing, you run out of easy energy to burn and your body can start breaking down muscle tissue for fuel. To maximize muscle retention, if you do an HIIT session, you don't get to this point, but, you use enough large muscle groups and fast twitch muscles that your body releases alot of fat burning hormones. These hormones can boost your metabolism and burn more fat for a full 36 hours after your workout. So basically, longer high intensity sessions can burn more calories while you're working out, but you run the risk of losing muscle tissue while dieting, but HIIT doesn't pose that risk, and the hormones that are released are what is doing most of the calorie burning. You'll get similar effects from doing squats. However, I still have no idea how you could possibly compare the two on a calorie burning basis. How in the world could anybody know how many calories were burned due to a certain exercise over a 36 hour period?

I've brought up similar questions before though, mainly because I also do alot of high intensity stuff for extended periods of time. I race mountain bikes and I normally stay at 92% of my max heart rate for 2 hours, often going up to 97% for awhile on hill climbs, and every now and then getting down to 88% on a long downhill. My question was that if this muscle loss was as big of a deal as is theorized, seems like I should be getting smaller, but instead I'm just getting stronger and bigger and more defined.

Personally, I don't really purposely use extended high intensity aerobic training for fat loss purposes anymore, only for training. If I've got a race or big ride coming up, I eat the hell out of everything I can find. I think I actually gain weight on race weekends.

Ok, so this is where I get frustrated. I agree because I think that I'm looking better and better but then I'm heavy at the moment so I think anything makes me lose fat haha. But you're a good example Corndogggy because you're already fit and you do extended HI exercise and yet you're getting stronger. I think that makes more sense to me than the burning muscle scare that I'm obsessed with.

My entirely unqualified opinion is that the muscle loss is relatively insignificant compared to the calories burned during extended high intensity exercise. If your goal is to build mass, I might think HIIT is the way to go, but if you are trying to lose fat, I'm guessing that HIIT is less beneficial. Either way, it's probably not a measurable difference.

There is a little bit of an apples and oranges effect here, so to speak. High Intensity Interval Training, and High Intensity Continuous Training are not directly comparable by the term "high intensity". If you take the same person applied to both workouts, basically you would find that their HIIT sessions were still of considerably higher intensity than their HICT sessions. Both are called high intensity, but it is a relative term. HIIT should still always be far more intense than any continuous training workout. There is some leeway in this rule in only the most elite of athletes, for example, sub 4 minute 1600m (1 mile) runners. They have trained to a point where they are running an extended time period pretty close to their maximum effort. However, this extended time period is still only 4 minutes, not 20 or 30. Basically, it is impossible for a person to run at the same intensity continuously as they should be running for HIIT. This inherent difference is what makes HIIT beneficial in different ways. Some research has shown that HIIT is the most effective (perhaps only?) way to increase VO2max (or however you type that out) substantially. This supposedly being a significant measure of fitness levels. also, part of the reason for doing HIIT instead of longer continuous training is that there is a time frame for completion of a workout, so to speak. most people tend to do weights and cardio in the same session, but trying to do an extended session of cardio after 45-60 mins of weight lifting means you are putting your body at risk of being overstressed and producing counter-productive hormones. most suggestions call for workouts to be completed in under 75 mins partially for this reason. of course, you cant do HIIT for every cardio session for reasons quite similar.

also, total calories burned from exercising shouldnt be such a stressed factor. long term continuous training may burn more in one workout than HIIT, but you can only safely maintain just so much of a caloric deficit, and the majority of that deficit should come from a healthy diet. exercising extra to make up for poor diet is most likely not going to lead to long term results and overall fitness.

all-in-all, each type of cardio has its place, and picking one over the others will most likely lead to either detrimental (too much HIIT for instance) or sub-optimal (nothing but slow and steady) results. A person must take care not to over-stress the body, while still working it well enough to make improvements. though over stressing may yeild results initially, if kept up things will reverse and get negative fairly quickly. If under-working, stalls in performance are more likely.

I would really suggest taking a look at what Steve has set up cardio wise for TomO. It is a very good example of balancing things out, and I believe Tom has made some amazing improvements in a relatively short amount of time.

Hopefully this helps to answer the original question

Both of these comments are made in such a manner that you're still talking about typical low intensity cardio. That's not what is being asked. The question is in regards to balls-to-the-wall efforts for extended periods of time, which is a far cry from what most people do.

Personally, I believe that the extended efforts burn more fat, it's just a matter of whether you believe the deal about muscle loss, or at least how significant this issue is.

I mean, most real HIIT sessions are going to last maybe 12 minutes. Most of that time is spent walking. If you take off sprinting for 10-15 seconds after resting for a minute, what, are you going to hope that you hit 97% max heart rate, and hope that you can do this 10 times? You're going to be tired enough that you can't max out your heart rate as much after the first few times anyway, you can't get your heart rate up to the same level every time. There's just no way that this is burning more fat than going balls-out in a mountain bike race for 2 hours. If I didn't do both, I wouldn't make such a comment.

Most of what I've read when it comes to moderate extended bouts of cardio seem to put the use of protein / amino acids for energy purposes at about 3% - 5% ( max is somewhere around 10% ) of the calories expended during the actual cardio session. They suggest very litte protein is used up during brief and intense cadio sessions ( like HIIT ). But as the cardio becomes longer and longer, and depending on the intensity and things like the status of glycogen stores, the use of protein for fuel goes up - but seldom beyond 10%. But again, for most gym rats who do a 30 minute to 60 minute run, I'd guess protein is somewhere around 5% of total energy expended.

My biggest question on this subject would be whether or not this protein can come from your diet. It seems to always be assumed that it comes from your own muscles being broken down. However, assuming you're burning 700 calories an hour for 2 hours for a total of 1,400... 5% of that is 70 calories. Can't you just pop a 100 calorie 20 gram whey shake in your belly beforehand to take care of these protein needs? Studies have shown that distance runners do significantly better when drinking whey shakes before a distance running workout, so would this not be why? And if so, would this not make the muscle loss negligible, especially if you're refueling properly afterwards?

Good question.

Let's say you're in good shape and you can run a pretty fast 45 minute run on the treadmill. ' Good shape 'means you run fast but still pretty much run the 45 minutes aerobically - fuel is coming from both fat and glycogen stores. If protein accounts for 3% - 5% of the calories needed to fuel this run, I'd think the first place your body would look to - from a protein perspective - would be to break down amino acids in your bloodstream. And thereafter, it turns to protein from existing muscle tissue. Again, even if it is true that the run sucks up 5%+/-of protien for fuel, I've always been unlcear on what ' triggers it ' to use protein when so much other fuel is available

So, you're in good shape, your glycogen stores are full, you've got adequate amino acids, glucose, fatty acids in your blood stream and you take off for you 45 minute run - you wonder why protien would be sourced as fuel at all. If the reason is that 45 minute run is so stressful, cortisol is released which will enhance protein catabolism......in some way, I'd find a ' balls out ' HIIT session just as stressful - if not more - than an areobic 45 run. Perhaps it's the relationship between duration and intensity that is more important when speaking of protein for fuel.


Makes sense to me.

( but I'm just guessing.......I think we need some " experts " to weigh in )

I have always been under the impression that any exercise burns a small amount of protein, and at the same ratio as most anything else. I thought you had to use some protein to be able to use any glucose. Maybe not. Due to the supplement industry, I was under the impression that this burning mainly came from the amino acid Glutamine which is stored in your muscles. Supposedly most any workout will significantly drop levels of glutamine at least according to the supplement folks. So, I thought this was unavoidable, but at the same time, I thought this was something that your muscles stored temporarily, in the same manner that it stores glucose, and the burning of it doesn't necessarily mean that your muscles are actually getting broken down.

I don't pretend to understand all of the science involved, so I can't debate anything stated so far. But, based on everyone's comments, it seems reasonable that the difference is negligible, especially at 30% bodyfat.

That brings up a second question. Is the goal to get my heart-rate up and keep it up, or is the goal to get my heart-rate up and continue exerting the same effort? I can get my heart-rate up to 170 and keep it there without working at max effort.

I dunno, I'm not sure there's an exact science to this situation. Most people who do this are doing it because of a particular sport, so basically you just participate in this sport with max performance and max fun in mind and don't worry about everything else.

One thing that dawned on me though... I guess the biggest advantage of intervals is that it makes you go anaerobic. There are some people who are in great shape and can utilize slow twitch muscles and stay in their aerobic zone for long periods of time even though their heart rate is up high. So basically, I'd still try to push yourself into going anaerobic then recover, even if you don't stop. Mountain biking kinda takes care of itself in that manner.

I am happily exiting the world of "fat-burning zone" mentality. But I have to get my body used to these higher heart rates.

So - if I'm on the treadmill or eliptical machine, I'm thinking I'll warm up for 5, then push to HR of 145. Then down to 130, push back up. Do that for a while then up to HR 150, back down and so forth, until I get used to feeling that higher heart rate. Right now, I think it makes me panic. I'm sure my body is ready for it, it is more my mind.

Does this make any sense at all??

OH MY GOD

wonderful reading but i am more confused then before

I am 39 living in Saudi Arabia.

started working out 6 weeks backs.

Have rreached a level where i work out on a bike 25 minutes at level 8 burning 200 calories and maintainign heart rate from 70 to 80%

Immediately after that treadmill for 45 minutes burning 500 calories and the heart rate is usually 70 to 85 %

When I started i was 104 kg after two weeks it jumped to 106 ( i dont know why) any way after 6 weeks I am now 100

I follow up this routine almost daily followed up by strength training for 1 hour

I am 5'11 and want to loose FAT

Do I need to change my exercise routine??

BTW thanks every one for sharing the wonderful informations

I know my questions can be random but here it goes. Can you do weight training at a pace that you heart is at your target heart rate to burn fat. now I know that if you have more muscle you burn more calories through out the day, so I hope you understand my question

my very unproffesional answer to you... based simply on what worked for me, would be get it as high as you can for as long as you can... as you start to experience fatigue, or some burning (lactic acid buildup) back down for as long as it takes for your heart rate to get back down to 65-70% and repeat. This is more or less beginning interval training. as you progress you will build a cardio base and keep going further. when i first started i could do this for maybe 20 mins... now i get hungry before im tired unless im really pushing it to the 90% range