High intensity training is anaerobic exercise with either short bouts of intense training or even nearing the point of exhaustion. This type of training is considered high intensity when your heart rate is regularly at about 80% of your heart rate max or in Zone 4. When you think of high intensity training, you may think of HIIT (high intensity interval training), tabata, boot camp classes, etc. High intensity training is not to be confused with high effort. For effort we use the RPE Scale (1-10) for perceived effort.
Fun fact: I used to teach tabata and boot camp classes at my first training job.
While there is a place for high intensity training within a program, it is not everything. There's also a place for low intensity and moderate intensity training. In fact, most of your training will likely be low-moderate intensity. Most of our strength training sessions together or programmed sessions to do on your own fall in these two categories. Lifting is helpful for gaining strength, building muscle, and increasing endurance along with many other benefits. I encourage strength training to be a pathway for reaping those benefits vs working against it by increasing the intensity to a high level.
High intensity training can be beneficial when completed as conditioning 1-2x/week or within cardio sessions via intervals or similar. If high intensity training were to be performed regularly or most days of the week, it could quickly lead to burnout. Burnout is not helpful because, as we know, consistency is what carries you to results. It's hard to be consistent if you're burnt out, right?
Ultimately, high intensity training is something we want to include in moderation. Just like everything else!