by Lucas Ogden-Davis
The word “intensity” gets thrown around a lot in the exercise world. It’s in the definition of CrossFit: “constantly varied functional movements performed at high intensity.”
Most people think of “intensity” as something roughly equivalent to “working hard.” That’s a useful definition, in that putting forth more effort is generally a good thing in exercise, but it’s also an incomplete definition that can lead to incorrect assumptions about the best ways to achieve your fitness goals.
A more accurate definition of “intensity” is “percentage of maximal effort.” Intensity by this definition is easily quantified in terms of load on a barbell. If your 1rm back squat is 300 pounds, then squatting 270 pounds would be working at 90% intensity (270 is 90% of 300).
We can apply this specific definition of intensity in other areas of fitness, as well. If the fastest you can run 1 mile is 8 minutes (equal to an average speed of 7.5 miles per hour), then running a mile in 10 minutes (average speed 6 mph) represents an effort at 80% intensity (because 6 mph is 80% of 7.5 mph). If your record for pull-ups in a minute is 20, then performing 10 pull-ups in a minute is a 50% intensity effort. For any exercise effort which we can quantifiably measure and for which we have a measurement (or reliable estimate) of maximal effort, we can calculate a corresponding level of intensity.
So what does this mean for us in the gym? It means that if we believe that high-intensity effort is the best way to improve fitness and health (Which is what we believe at YSCF, and what the research tends to show.), then we should be spending most of our exercise time and effort in the high-intensity range.
This is where the specific definition of “intensity” (percentage of maximal effort) leads us in a completely different direction than a casual and incomplete understanding of “intensity” (hard work). While many people equate hard work in the gym with long workouts that leave you weak in the knees and soaked in sweat, high intensity, by definition, is impossible to sustain for a long time. In other words, intensity inevitably and necessarily decreases as the duration of an effort increases.
Let’s take a look at this concept in the realm of rowing. The graph below shows the average power output, in watts, that world-class rowers are capable of producing over different amounts of time:
As you can see, at an all-out effort for a very short amount of time, these rowers can produce an average of ~725 watts. We’ll call that 100% intensity. When the duration of the effort (time spent rowing) pushes out to about 1 minute, that intensity has fallen to about 625 watts (~86%). At 6 minutes, it’s down to 400 watts (~55%); and at 20 minutes, average power output is roughly 325 watts (~45%).
From the shape of the curve, we can see that intensity falls in an exponential fashion: It decreases very quickly as duration of effort increases from a few seconds to a few minutes, and falls more and more slowly as we go from a few minutes to 20-, 30-, or 60-minute efforts. In fact, the average level of intensity for a 20-minute effort (325 watts, 45%) is remarkably close to that of an hour-long effort (300 watts, 41%).
This is why we program the vast majority of our conditioning work at levels that can be completed in less than 20 minutes: It is impossible to maintain high levels of intensity for efforts longer than 20 minutes, even for highly-trained elite athletes.
The takeaway from all this is that if you’re looking to improve your fitness or push your training to the next level, the solution isn’t to work out more or to bias your work towards longer, harder workouts. By definition, doing these things will necessarily decrease the intensity of your exercise, and in the words of Greg Glassman, “intensity is the independent variable most commonly associated with maximizing the rate of return of favorable adaptation.”
A more useful technique for improving your fitness would be to focus on keeping the intensity of your efforts high, the volume (number of reps) and duration (time) of your efforts relatively low, and make sure that you recover properly (eat, sleep, chill out) between workouts. That’s what high intensity work looks like, and that’s how you most efficiently move from where you are now to where you want to be, in terms of your fitness and overall health.