"I disagree with your views on cardiovascular training - ie low level duration cardio is catabolic [to muscle]. I would say moderate level 60-80 percent on their max heart rate can be if you go past 1 hour and don't refuel properly. The problem with prescribing high intensity interval training as a fat loss strategy to non athletes is that they never approach anywhere near 85-95 percent on their max heart rate so they are probably working more in the moderate level and so not really progressing. Look at some different sports that have an endurance factor - soccer, basketball, 800 meter runners, 2000 meter rowers etc. You can't tell me these guys are burning muscle all the time. For me cardio should be an important aspect of every training plan - interval training does work for some people if they are fit. For the rest I would say high intensity interval training is a magic bullet - it's less time intensive and maybe easier mentally. I'm not saying everyone should be running like a marathoner but cardio for my moey still has a place in a weight loss program."
-Paul McQaude
Just to clarify, I would ballpark low intensity/long duration as around 55-75% of HRmax and >45 mins so I think we're in agreement there. I also agree that interval training should not be prescribed to de-conditioned people. So, if you've been generally sedentary your whole life and are looking to start an exercise program, interval training would not be an appropriate starting point. The reason is thus: although most people associate the aerobic system with supplying energy during long duration activity (and indeed this is its main function), the aerobic system plays a crucial role is facilitating recovery between bouts of maximal efforts. That means, the better shape your aerobic system is in, the better your performance on intervals and the like. For that reason, I always start new clients on a block of moderate intensity (75-85% HRmax) aerobic work if their fitness levels require it.
Your choice of sports is interesting since they all involve the crucial component of speed. Isn't soccer is a great example of training intervals since much of the game involves bursts of energy followed by periods of recovery? Meanwhile, 800m runs and 2000m rowing events, both which I'd classify as high intensity, are incredibly taxing on the body's energy system since the athletes have to produce incredible amounts of force and sustain it for several minutes. So, one cannot compare a 2000m rowing event that lasts 6-7 minutes (all of which are gruelling) to a long distance events which lasts hours. 'Muscle wasting' occurs with the release of a hormone called cortisol and cortisol secretion increases with duration of activity. Although the stress of a 2000m row certainly releases some of this hormone, it is not enough to significantly impact on the muscle tissue. On the other hand, the ultra-long duration of, say, a triathlon gives the body ample amount of time to pump out this hormone. So you're right to say that these guys aren't burning muscle during their events.
Finally, I'd argue that high intensity work is much harder on the CNS (i.e. mentally) since the neural drive to increase force output to the muscles is jacked every 90 seconds or so. With long duration activity, neural drive is low since force output is low. Is this good or bad? As always, the answer to that question depends on your goals and what you're training for.
At the end of the day, there is no form of exercise that is wrong - only a wrong time to do it.
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