I'm surprised no one has offered any suggestions. Your questions are simple, but the answers aren't really.
How much HIIT is best, is dependent on a lot of factors., what other exercise you do, frequency, volume, rest, intensity level, etc...
As you are aware, Intensity of effort is all important in achieving an efficient AND effective exercise "effect". The greater the intensity of effort, the less volume you can do. They are opposite sides of the same ruler.
100===========================================================0 Volume
0===========================================================100 Intensity
So you can either work long or work very intensely, or somewhere in between. But you can't work very intensely and long. And the intensity level will be the primary determiner of the efficiency and effectiveness of the exercise, ie., your improvements.
Also, working intensely makes a greater inroad into your recoverability abilities. Most don't understand that there are limits on how much and how fast your body can recuperate from any stressor (like training). More is definitely not better.Thus all the info written about "periodization" techniques to modulate intensity and volume to try to regulate recoverability and keep from over training and maximize your improvements all at the same time. Its a great balancing effort. In exercise, over training is the great satan... you want to avoid it at all costs. Its far better to rest maybe a bit too much than it is to over-train.
Also, there has been lots of discussion over the years about anaerobic and aerobic training and whether they can be done well in the same training program. Many studies in the past seem to show an "interference" effect when doing both...say weight lifting M-W-F and running on other days. Thereby making less improvements in both exercise regimes and overtraining much more easily. However, there is a recent study that showed weight training and aerobics combined on the same day doesn't appear to have an "interference" effect. That is as long as you take plenty of time to rest between sessions.
That's what I do. I combine both HIT style resistance training (barbells/dumbbells) and HIIT style aerobic training (Tabata's on an exer-cycle) on a three day per week schedule (M-W-F). Its brutally hard, but brief. Since I'm only experimenting with this style, we'll see what long term effects there maybe. I suspect though that I may be increasing the rest days between sessions before too long.
And one last thing, HIIT training can use a wide variety of exercises, not just running and swimming and cycling. In fact you can even do weight-lifting in a HIIT style like classic circuit training (though safety may be compromised If your not careful).
Sorry for the rambling response but it would take a couple books to really explain it all
by eXtremE » Wed Apr 29, 2015 10:30 am 
