For the purpose of this topic we are talking about Workout Intensity in regards to Building Muscle, not for other areas like fat loss, cardio vascular training although there usually is some crossover involved. So if you want to get stronger and more muscular then read on...
First off let’s talk about what Workout Intensity is not!
The Busy Body: That person that jumps from machine to machine and never stops moving, they will do bench press and then drop to the floor and rip off a set of situps immediately followed with another set of bench press and so on… This is more or less a circut training session which is in some ways comparable to CrossFit when doing Metcon activities. Great for all around functional ability but not always geared towards fully benefiting maximum muscle growth.
The Permanent Gym Resident: That person who is always at the gym, that seems to be there every time you go, they are there when you arrive and still there when you leave? Time at the gym means nothing, especially if much of it is filled with socialization, texting and long breaks. I often see these people going through the motion when they actually do workout, perhaps they are pacing themselves for a long day at the gym?
Mr. Way too Much Weight: You know that guy, the one that is using every 45 pound weight in the gym on the leg press and then only bends his legs slightly while grunting and straining? Or the guy on bench with three plates a side getting his spotter to do most of the work. You have to use correct form and you need to do the bulk of the work using your own efforts, otherwise how will you know when you are improving, perhaps your workout partner is getting stronger and doing more work for you ;)
The Cardioholic: That person that wants to lift weights but ends up doing mostly cardio since their workout starts with a bunch of cardio and ends with a bunch of cardio along with a token workout effort jammed in the middle. They probably have are good intentions but if you want to get stronger and more muscular then the weight lifting needs to take priority over everything else.
Mr. 5 Exercises Per Body Part: When your muscle is done, it’s done, if it takes you 5 different exercises to tire that muscle group out then you are for sure not being intense enough! You should be able to completely tire your muscle out with one exercise, sure it may take 5-6 sets to get there but I know it is 100% possible. And once your muscle is fully fatigued, it's time to move onto a new muscle group that's fresh and ready to go. Otherwise you are simply beating a dead horse over the head and not getting max benefits from your workout. You will need to makes sure to mix-up your exercises on follow up workouts but you should also be able to hit each muscle group several times per week if you are not doing so many exercise for each muscle group during each workout.
So what is Workout Intensity?
Workout Intensity has nothing to do with how long you workout or how many days a week you workout, or how busy you are at the gym. It all has to do with preparing yourself to mentally and physically give 100% of yourself for each exercise you do until that muscle can do no more!
This does not mean that every set is at 100%, the idea is to build the weight up for your final last 2-3 sets where you deliver 100% of your focus, strength and energy to that exercise so that when you are finished, you can say you have completely exhausted that muscle group and then move on to a fresh muscle group.
I like to do 2-3 warmup sets where I keep adding weight and build up to my heavy set, my heavy set will be the goal set where I plan to do as much weight and as many sets as I can to failure. Sometimes if I feel I had a really great set, then I will do a drop set or two to finish the muscle off even more. If I feel like that muscle group has more to give, then I will drop the weight a little and do one or two more sets with decreasing weight, again to complete failure on both follow-up sets.
You don’t need to do a ton of different exercises for each muscle group, 1-2 exercises is fine as long as you completely exhaust your muscles when you train an exercise. Once that muscle group is exhausted there is no need to continue training it, that would be anti-productive and you are so much better off switching to a completely different muscle group and giving it your full attention. This way you can hit more muscle groups during each workout and be able to train each muscle group more times per week for maximum results.
When you leave the gym, you should be completed exhausted, the idea of trying to do another exercise should at this point be physically and mentally imposable.