Exercise training is a stressor which we apply to our bodies, hopefully in a systematic and logical pattern. However, it is not the stressor (exercise training), it is how we respond to the exercise training that really matters and determines our 'results'. Too little stressor (not exercising) obviously leads to a weakened and flabby body. Though, too much of a stressor (too high intensity) leads to stress, injury, and psychological maladaptation. The art is applying high enough stress to stimulate an adaptation, but not overtrain. Unfortunately, the message we get from the media, internet, and our local gym offering this high-intensity exercise is that you should leave the gym quivering, exhausted, and dripping sweat or you have wasted your time. That is because we base 'effectiveness' on calories burned and muscle soreness. Two very wrong assumptions. We need to take a step back and look at our adaption over a period of time (weeks, months, and years). One training session won't make you, but one training session can break you.
High intensity exercise must be used appropriately. It has its place, but often it is used too early in the training process, too frequently, too long, or without consideration of other life events. The intensity of your training must be managed, that is cycled up and down to ensure continual positive adaptation. You can't keep the pedal to the floor or you will break down.
More importantly, adaptation takes time. You can't force it. But, it will happen if you apply the stimulus in an appropriate manner, a regular manner, a consistent manner. Consistency is paramount for any health, fitness, or body composition goal. When I work with a client during a training session, I am thinking to myself "how is this training session going to affect them tomorrow, next week, next month, next year?" I look at it as a stepping stone for future gains. Sort of a deposit in a savings account, not a withdrawal.
If you made it this far in this article, then you obviously are interested in training intelligently and not just 'killing' yourself in the gym. The most important advice I can give is to find ways to make your training as consistent as possible. Remove barriers to that consistency. Shorten sessions, err on the slightly more conservative side with how much and how hard you are training. Remember, intensity and volume accumulate. Go by your numbers and how you feel versus how it 'should' work. And most importantly, don't follow what others are doing in the gym. The majority have no idea what idea they are doing, and are just copying others.
Think about 'working' on your movements (whatever type of training you do) regularly, not 'conqering' them. You should feel good (even if you are working at a higher intensity) when you finish your session, not like you've been run over by a bus! The effects of exercise training can be evident after only a few sessions, but many other effects or adaptations take months and years.
Consistency always trumps intensity when it comes to exercise training. We are always adapting to our training (or lack of it). In order to keep the adaption positive focus on consistency. It is appropriate to vary the intensity, but this needs to be done logically, and in the context of a longer time frame (months and years). Ignore the high-intensity pundits, they are simply preying on the assumption that exhaustion and soreness are indicators of effective training. There will be many 'casualties' to this 'harder is better' mentality. And, unfortunately, these individuals will either keep getting injured, go back to only very low intensity exercise, or give up on exercise training altogether.