TTo some, working out is simply about numbers. Three sets of eight. Four sets of 10. Rest one minute between sets. Do 20 total sets. But beneath every tangible number and finite measurement used to define the amount of work you’ve done, there’s the enigmatic — albeit ever-important — variable known as intensity. In fact, this might be the most crucial training variable of all. Intensity isn’t a number. It can’t be written down in a training log nearly as succinctly as, say, a tally of sets and reps you do for an exercise. And a set of 50 reps isn’t necessarily more intense than a set of six. Bottom line: Where training for gains in muscle size is concerned, intensity equals muscle failure.
If your muscles fatigue to the point that you can’t do another rep (aka “failure”), that’s an intense set. Stopping short of failure? Not as intense. Yet intensity goes far beyond just one set — each set affects the next set, every workout affects the next workout, every week affects the next week, and so on. How you manage your intensity from set to set and workout to workout goes a long way in dictating the effectiveness of your program. That said, there’s a certain hierarchy to training intensity, and the best way to articulate it is to start small (with a single rep) and pull the layers back until you see the big picture. Hence, the following six levels of intensity.