concerning the tens:

You hit a high point, if it is absolutely impossible for you to do one more rep in your last set, i. e. a thirteenth rep is impossible (11th or 12th would be okay either).
Intensity of "9" means for me that I am able to do 12 reps in the 5th set, but is is hard and I am breathing heavily. On the other hand the first set is very light and I keep the other sets between it. IMHO the last two sets are most important. I use the first sets to control my form and to warm up the concerned muscles. I have the feeling that this kind of pyramid training is much better for my tendons and joints than three heavy sets which are advised often.
If you did not hit a high point just increase the weight next time and so on. If you do it slowly you will be able to increase the weights almost constantly without the fear of injury or over training, since your body will adapt to the new load, especially in the beginning. Actually the happens to my legs, but unfortunatly not to my arms and shoulders in the same way since my arms and shoulders are very weak. But of course every physique is different.

Concerning HIIT. I do it in the gym using Ellipse machine and I do it by the book. At the end I count speed, level and burned calories to control my progress.
You could do the same using a polar watch with a heart monitor. If you want to train longer or on a lower intensity it is not HIIT (High Intensity Interval Training) anymore, but Interval Training or usual endurance training. This isn't bad either, but I prefer it short and tough.