One of the longer standing arguments in the field of strength training has to do with the definition of intensity with various camps essentially defining intensity in their own way and everybody talking past one another when they have debates about it.
In my opinion, most of the arguments are simply an example of people using the same words to describe different concepts and I don’t see any real reason for there to only be a single definition of intensity that can be valuable in the weight room. In fact, by using various definitions, I think that training can be more accurately described.
Intensity of load
Arguably the first definition of intensity came from sports scientists and coaches (especially Olympic lifting coaches) trying to define and measure things relevant to them. In this case it meant defining intensity as the percentage of maximum load that was being used.
In this scheme, a 75% load (e.g. if you can use 100 lbs maximum and are lifting 75 pounds), is a lower intensity than a 95% load (you’re using 95 pounds of your 100 pound maximum).
The pros of this method are that it’s fairly easy to measure and makes concrete comparisons simpler: the guy lifting 95% of his maximum is working at a higher intensity than the guy lifting at 85% or 75% of maximum. This is especially true in research where it’s relatively easy to test someone’s maximum and then determine what percentage of maximum they are working at based on what weight is on the bar.
Unfortunately, only looking at the percentage of maximum tends to miss some crucial aspects of the training load. In both Olympic lifting and powerlifting, it’s not uncommon for lifters to perform a sub-maximal number of repetitions at a given load. That is, in theory, a load that is 85% of maximum will allow a lifter to get 5 repetitions although it will be pretty grindy at the end.
Many lifters, and this is especially true in Olympic lifting would be more likely to do repeat sets of doubles at that same 85% of maximum to ensure that technique and bar speed stay high; some powerlifters train this way as well.
It should be obvious that performing 2 repetitions at 85% of maximum and 5 repetitions at 85% of maximum are going to be a very different level of effort/difficulty even though the intensity of load is identical.
There is an additional problem in that true maximum strength can be variable on a day to day basis. Basing training around percentages can get misleading when what should be a 90% maximum load is actually lower or higher due to changes in fitness or fatigue state.
Which brings me to the second most common definition of intensity.
Intensity of effort
Groups that are usually associated with the HIT (high intensity of training) theory tend to define intensity in their own way which has to do with relative closeness to failure or simply the effort expended during the set. You might simply look at this definition of intensity in terms of ‘difficultly’. The harder the set is to complete, the higher the intensity and vice versa.
A set taken to the point of concentric failure is generally defined as 100% intensity and while individuals in this camp usually argue that anything less than 100% intensity can’t be reliably measured, others will use methods like rating of perceived exertion (RPE) or simply reps short of failure to gauge intensity of effort.
Clearly an all out set to the lifter’s absolute limits would be 100% intensity and an RPE of 10 (on a 10 point scale) with no reps left to failure. A lifter who stopped 1 rep short of true failure might be at an RPE of 9 and 90% of maximum intensity, a set done at an RPE of 8 might leave the lifter with 2-4 reps short of failure, etc.
I’d note that knowing how close one is to failure often necessitates a period of training where true failure is achieved. With practice, most lifters will know if they had one or two or four more repetitions in the tank. Beginners who have no conception of what true muscular failure is will not. I’d also mention that a good coach can usually tell by watching things like bar speed and effort how close a lifter is to failure; again this takes some practice and experience to do well.
Complicating things even more we might examine the issue of speed work as often done by athletes and powerlifters. Typically a load of 30-60% of maximum (low intensity of load) might be lifted for very sub maximal numbers of repetitions. But the focus on lifting the weight as fast as possible/pushing as hard as possible might actually make the intensity of effort quite high.
A Mid-Article Summary
Frankly, with only the two above definitions of intensity, intensity of load and intensity of effort, I think that training can be more accurately described than with either one alone. So while a set of 12 to failure might only be a 75% load intensity but 100% effort intensity (RPE of 10), a set of 2-3 at 85% of maximum might be an 85% load intensity but only a 50-60% effort intensity (RPE of 6-8).
In this vein, I’d note that a recent book by IPF powerlifter Mike Tuscherer called The Reactive Training Manual has a lot of very good information on the above approach to training, using RPE, fatigue cutoffs, etc. to autoregulate powerlifting training. Anyone interested in the topic would be recommended to pick up a copy.
Other Aspects of Intensity
Of course, I also think that other definitions of intensity can be useful or at least descriptive in looking at training. Nobody would argue that both a 1 repetition max (100% load intensity/100% effort intensity) and a 20 rep set of squats (perhaps 70% load intensity but 100% effort intensity) are intense but they tend to be intense in a different way. A set of 8 to 1 rep short of failure on the bench press (80% load intensity, 90% effort intensity) might also be intense but in a different way than either of the other two examples.
Given the general belief that training can have varying effects on either neural, muscular or metabolic effects of intensity, I don’t see it as too far fetched to look at training in terms of the neural, muscular or metabolic intensity. So sets of 1-3 are going to be more neurally intense than sets of 6-10 (more muscularly intensive) and sets of 20 or more might be primarily metabolically intensive (although the muscular effort is often still quite high).
For completeness, and having watched too many bodybuilders train, I might even go so far as to suggest another definition of intense in terms of focus and concentration. It’s not uncommon to watch bodybuilders using what are apparently fairly light loads focusing extremely intensely on every repetition, using slow movement speed and attempting to generate maximal muscular tension during all aspects of the movement. While the intensity of load may actually be fairly low, the intensity of effort (and concentration) certainly are both high. While impossible to quantify, I see that as certainly another potentially useful definition of intensity here.
Invariably when I see arguments about training intensity, the problem is usually that people are talking across each other, using different definitions that each thinks is the only correct definition. Rather, I think a more useful approach is to recognize that intensity in training can have different meanings all of which can have utility or value at different time points.
Alternately, as mentioned above, there are clearly cases where taking different definitions into account at the same time may give more valuable information about training than focusing on one or the other.