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There is much misinformation regarding training intensity and what it really means. I just want to shed some light on what it means from this training point of view. I have no doubt that every bodybuilder and trainer will have his / her own opinions but this is what I find works best.
Firstly when I say intensity it means training in a fully concentrated manner with the heaviest weight that you can lift for up to twelve reps until you cannot do another rep. That means that your body cannot physically do any more. This point is known as muscular failure.
Preferably ten reps is best but if you can do twelve that's OK, probably the weight was too light but if you went to twelve to failure that's fine. You will find that on your second main set that you cannot do the same amount of reps as the first. If you can then the first set was too light. This does not mean that when your arms begin to ache you put the weight down or if you start to sweat you do the same. What it does mean is when your arms cannot perform even one partial rep then you have reached failure. When there is no more juice left in the tank. This is what most people get wrong. Most people stop at the point that it is starting to do them some good. The further you can go beyond the intensity barrier the more your muscles will have to grow to accommodate it. When you train like this every rep should be difficult and not just with the first six reps easy with the last four starting to ache. When you train with intensity your arms for example should ache after the second rep due to the weights you are lifting. You wouldn't be able to look around the room or smile at anybody while you train like this. If that is what you used to do before that is why you haven't grown! The more you can go into muscular failure the bigger your muscles will grow. They have to, to cope with the stress they are being put under.
So intensity isn't how long you can train as that is endurance, which is a totally different thing for different goals, training intensity is lifting the heaviest weight for the most reps you can do. Do not stop if during a set your body part aches, going beyond this point is the point that your muscles will grow. Going beyond this point is the only point that you will gain benefit from.
Training intensity is training with total concentration with the heaviest weights till you have a muscle failure.