We can’t exercise at the same intensity every session, we won’t see progress, we may burn ourselves out, injure ourselves or even de-motivate yourself. This is why a good fitness program will have a variety of intensities you have to work at throughout.
For example if you worked at a high intensity every session on the rowing machine, your body would struggle to recover properly for your next session. Fast interval after fast interval the first week would feel ok but then you’d start failing to hit your targets, this would be demotivating and play on your mind which in turn would knock your confidence. So not only will your health and fitness start to struggle so with the mental side of coping with exercise.
The same can be said for having a fitness program where there is no challenge. To get adaptations in your muscles they have to be put under some stress, such as lifting weights. When you lift your muscles tear and then when you go through your recovery they repair themselves and become even stronger. So in order for that process to happen you have to perform exercises that are a challenge and feel physically difficult and a little uncomfortable. Obviously never work through pain but feeling ‘the burn’ is a different type of pain that I actually enjoy! When I start feeling the burn I know the good it's doing and how after the session and during my recovery my muscles will become faster and stronger through adaptations these will help me improve my lifting.
So to get the best out of your body you need to have a balance of low/medium and high intensity. The only exception to this rule is when elite athletes train at low intensities for a major part of their program and then a small piece of it contains the higher intensity interval sessions, these are mainly aimed at endurance elite athletes not your average gym goer and heart rate monitors and zones come into the equation. Although I do think that if your interested in fitness you should explore different ways of training-which I do for myself and my clients - this had become a bit of an obsession for me and I suppose I am even more of a fitness anorak now as my heart skips a beat - excuse the pun - when I read about another training mode that could benefit me, albeit getting me stronger or faster!!
The thing to remember is the high intensity sessions need to be separated by lower intensity sessions so your body has time to recover from the stresses you have put it under, and by doing the lower intensities it breaks up the mental intensity too of having to push your body to the limit on every session, even the fittest athletes would find training solely at high intensities tough! High intensity sessions could be a higher number of reps, a quicker movement, heavier load in a lifting session. In cardio sessions higher intensity sessions normally constitute shorter intervals at speed and then vice versa on the lower intensity sessions.
So embrace the lower as well as the high intensity workouts it may not feel like your pushing yourself enough but let your body recover, and also benefit from a mental rest and refocus during your lower intensity workouts. Both of these kinds of workouts have an important role in all fitness programs and will help improve your aerobic AND anaerobic systems in time making you fitter strength and cardio-wise and helping you to achieve those all important targets/goals!