A Quasi-Comprehensive Definition & Explanation of Intensity

I am almost certain that 99.9% of you have come across the term "intensity." You probably first encountered it in a bodybuilding magazine or perhaps from reading various articles by other Bodybuilding.com writers. However, if asked to define and explain intensity, I'm willing to bet that most of you would probably draw a blank. If you fall into this area, don't be ashamed; intensity is a very broad expression, and can be explained in terms of many measures such as a percentage of maximum heart rate, maximum oxygen consumption, or performing until physical failure takes place.
What Is Intensity?
The definition of intensity sways depending on the exercise that you are executing. For example, the definition of intensity in terms of aerobic exercises cannot be used to set the intensity level for resistance training exercises. I'll skip over the aerobic aspect of intensity for obvious reasons (hence "Quasi-Comprehensive"). In many of my training journals, I have passively mentioned intensity, neglecting to explain what I am really talking about. In this article, I will define, explain, and give examples of intensity in terms of various measures.
In the exercise world, intensity is defined as a straight percentage of an individual's maximal capacity to do work. Having said that, let's move onward to some applications.
With resistance training, intensity can be defined in terms of blood lactate levels, the number of repetitions at a percentage of your 1 rep maximum (1RM), a percentage of maximum heart rate, or ratings of perceived exertion (RPE). Some measures are more accurate and convenient to quantify than others. Therefore, it behooves you to explore the validity and measurement issues that accompany each measure.
Anaerobic Glycolysis
Blood lactate levels rise with repetitive muscular contractions under hypoxic (low oxygen) conditions. This is a direct result of anaerobic glycolysis, an energy pathway involved in resistance training. Many elite athletes, such as olympic swimmers and runners directly measure blood lactate levels in regular intervals while training in order to assess the intensity at which they are working.
The same could be done in resistance training. However, constantly taking and analyzing blood samples during a training session is not very feasible, and it can interfere with training. Also, it is fairly easy to tell if you are producing lactate during a workout by experiencing the burning sensation during and after a set. As a result, measuring lactate levels while resistance training seems obsolete.
I definitely feel that determining the number of repetitions at a percentage of your 1 RM prior to doing a workout is silly. In order to determine your 1 RM, you have to max out on every exercise for every body part. That is simply unreasonable to do, and you put yourself at high risk for injury at the same time.
Your One Rep Max
Once you determine your 1 RM, you can use various equations such as the Brzycki Equation (1 RM = weight / (1.0278 - .0278x) where x = the number of repetitions) to determine a specific weight to use for a specific number of repetitions. However, any equation that you use will not be 100% accurate, and will not be applicable to every body part.
Most times you will be able to do a couple of reps more or perhaps less than the predicted value. Also, strength varies from day to day (intra-individual variability), so prematurely selecting the weight that you will use on a given exercise could very likely be detrimental.
Click Here To Get Your One Rep Max!
Another point to consider is with beginners. Beginners would use this percentage of 1 RM idea and initially max out on everything. If percentages were calculated from those max efforts, he/she would be shooting themselves in the foot. In the beginning of a resistance training program, strength increases dramatically due to fast neuromuscular changes (not muscular hypertrophy or hyperplasia!). As a result, the calculated weight would be grossly underestimated, leaving the set intensity as being grossly underestimated as well.
Measuring heart rate is common practice when exercising. You could probably walk into any fitness club and see some schmuck touching their neck and looking at their watch. In reality, assessing heart rate is a very good indicator of intensity.
However, certain factors need to be established before considering that statement to be actual truth. First, you need to know your maximum heart rate. The only way to obtain an accurate measure of your maximum heart rate is to perform work in which your reach your max heart rate.
This is possible if you are conducting a test on yourself in which you increase the work rate of a given continuous exercise (such as treadmill running or stationary biking) in regular intervals and periodically measure your heart rate. Increasing the work level will increase your heart rate. Once you reach your max heart rate, increasing the work level will not raise your heart rate anymore. This type of test is known as a "graded exercise test." Alternatively, you could use an equation that yields a very gross approximation of your max heart rate. The equation is age dependent and is HRmax = 220 beats/min - age. This is a highly inaccurate method and the results vary widely between individuals. Hence, this is not a very good indicator of max heart rate.
Determine The Intensity
Once you have obtained your max heart rate, you can determine the intensity at which you are working out. Presumably, your heart rate would reach its maximum rate during a given set of 8-12 reps at what you feel is max intensity. This, of course, would not apply for sets with lower reps because there is not enough time to pump up your heart rate. The only problem comes in having to measure your heart rate after every single set. This is a good way to assess intensity, but is often times inconvenient.
Finally, RPE is probably the best measure to consider when assessing intensity during a workout. RPE is a subjective measure of the intensity of work effort at a given work rate. In other words, RPE is how hard you feel your are working. Through extensive research, it has been found that RPE is a very reliable measure of intensity. RPE is measured on two scales, the Borg 6-20 scale (developed by Gunnar Borg) and the CR-10 scale.
The Two Scales - Borg 6-20 & CR-10

	Category (15-point) RPE Scale		Category/Ratio (10-point) RPE Scale
6		0	Nothing at all
7	Very, very light	0.5	Very, very weak
8		1	Very weak
9	Very Light	2	Weak
10		3	Moderate
11	Fairly Light	4	Somewhat Strong
12		4	Somewhat Strong
13	Somewhat Hard	6
14		7	Very Strong
15	Hard	8
16		9
17	Very Hard	10	Very, very strong
18		*	Maximal
19	Very, very hard
20

Of course, you are not going to carry in charts to the gym and rate your perceived exertion after every set. The idea is that the intensity that you feel you are working at is pretty close to the actual intensity that you are working at. RPE possess both convenience and accuracy, two factors that are vital in choosing a measure for assessing intensity!
In summary, intensity can be assessed through a number of measures with resistance training. Of those, subjective evaluation seems to be the best measure. If you feel that you are working close to central fatigue (see my article: The Facts on Fatigue), your intensity level is close to maximal. Like I've said over and over, high intensity workouts stimulate muscle growth better than moderate or light intensity workouts!