What Is Exercise Intensity?

Intensity is not a thing in and of itself, but a measure of something. When we’re talking about “intensity” in the context of exercise we’re really talking about the measure of things like relative effort, relative load, heart rate elevation, etc., and rather than just saying “intensity” we really ought to be more specific, saying “intensity of effort” or “intensity of load”.

In High Intensity Training (HIT) “intensity” is usually used to mean intensity of effort, or how hard you are working relative to how hard you are capable of working at the moment, and that is what I usually mean when I say or write “intensity”. For example, if you perform an exercise using a resistance that is eighty percent of your one repetition maximum your intensity of effort at the beginning of the exercise will be eighty percent, but as you fatigue the resistance requires an increasing percentage of your decreasing strength, until your strength has been reduced to the point where it equals the resistance and your intensity of effort is one hundred percent.

Intensity of effort is often criticized as being imprecise, since it is not accurately measurable during exercise except at the point of momentary muscular failure, but since relative effort appears to be the most important factor for stimulating improvements in muscular strength and size it is the most relevant, and most useful (1).

Most of the rest of the fitness industry uses “intensity” to mean intensity of load, the percentage of your one repetition maximum used for an exercise. There are several problems with this, however, including problems with accurately testing one repetition maximums. Although all types of exercise intensity are meant as a measure of how hard you are working during an exercise there are numerous factors which affect exercise difficulty and it is possible to work easier or harder with the same percentage of your one rep max by changing one or more of these factors, such as whether the exercise is performed to the point of momentary muscular failure. Also, as long as an exercise is performed to the point of momentary muscular failure (maximum intensity of effort) most people do not perceive any difference in difficulty using different relative loads (2) or even perceive greater difficulty with lighter loads (3).

Regardless of the relative load used, if you are not training to momentary muscular failure you are not working as intensely as possible.

It could be argued that despite these problems intensity of load can be used to estimate the average intensity of effort of an exercise. Assuming a relatively constant rate of fatigue, and assuming the exercise is stopped immediately after momentary muscular failure occurs, the average intensity of effort during the exercise would be halfway between the percentage of one-rep max used and one hundred percent.

This is a lot of assumptions, however, and several things can throw this off. Fatigue during exercise is the result of numerous factors, which can vary depending on protocol, and probably does not occur at a constant rate. When training people on Ken Hutchins’ iMachines with real-time force measurement and feedback (displayed as a line on a graph) it was not unusual for the rate of fatigue to increase towards the end of the exercise. Also, if you continue to contract isometrically for several seconds after achieving concentric momentary muscular failure you increase the relative time spent working at maximum intensity of effort, increasing the average. I suspect this is why many set extension techniques (forced reps, drop sets, finishing negatives, etc.) seem to make greater demands on recovery.

In addition to all these problems there appears to be little difference on average in muscular strength and size increases with different relative loads and repetition ranges as long as exercises are performed to momentary muscular failure, which throws a huge wrench into the claims that load equals intensity, or that ultra-heavy ultra-brief sets are the way to go (1, 4).

The biggest problem with using load to define intensity is thinking that mechanical definitions of work and power or force or load over time accurately describe what is happening in the muscles during exercise.

This is easy to disprove, and Arthur Jones wrote about this decades ago in The Metabolic Cost of Negative Work,

A great deal of confusion exists on this point because of attempts to compare metabolic work with mechanical work, and, secondly, because of a failure to consider several related factors.

By definition, work require movement… no movement means no work; and while this is undoubtedly true in regard to mechanical work, it certain is not true in relation to metabolic work.

Muscles produce force, and it is easily possible for a muscle to produce a high level of force without producing movement; logically, it appears that the metabolic cost of muscular force production would be related to the level of force produced and the time that the force is maintained… rather than the amount of mechanical work performed.

If, for example, a 100-pound barbell is held motionless at the halfway position of a curling exercise, then the muscles will be required to produce a certain level of force to prevent the downward movement of the barbell. Providing that force will certainly entail metabolic cost… yet no work is involved.

Slowly, curling a 100-pound barbell also requires a greater metabolic cost than curling the same barbell at a more rapid pace; even though the amount of mechanical work involved is exactly the same in both cases.

Many other examples could be given to illustrate the same point, but it should now be obvious that attempts to relate metabolic cost to mechanical work are doomed to failure.. there is no meaningful relationship. We must have another standard for comparison.

The only meaningful standard, I can think of is force/time… the amount of force produced by the muscles multiplied by the time that the force is maintained.

Force multiplied by time is a much better measure of exercise difficulty than mechanical work, but still flawed since it is possible to have the exact same force output and duration and even the same mechanical work and still have different levels of difficulty if the repetition cadence, range of motion, or other factors are varied. For example, if you perform ten repetitions of an exercise each taking eight seconds with the same load, over the same range of motion your mechanical work, power, and force multiplied by time would be the same, but the difficulty will be different if you lift in two seconds and lower in six, lift in four and lower in four, or lift in six and lower in two since your muscles are stronger and fatigue more slowly when lowering than lifting and since the force your muscles are capable of producing concentrically varies with contraction velocity.

Differences in strength and resistance curves complicate this further. Although it is possible to get a resistance curve that very closely approximates your strength curve using machines with well designed cams with adjustable timing like Ken Hutchins SuperSlow Systems and RenEx machines or lifting weights with correct body positioning and movement during some free weight exercises, it is impossible to match your strength perfectly over the full range of an exercise using weight-based machines. Because of this there will be harder and easier portions of the range of motion. Depending on the amount of time you spend in each during both the positive and negative the exercise may be more or less difficult. For example, during most compound pushing movements the target muscles are working against a much larger lever at the start point than the end point. Performing the lower turnaround very slowly, or even holding briefly at the start point makes an exercise more difficult, while performing the upper turnaround any more slowly than necessary or holding there makes an exercise easier.

Imagine performing two sets of barbell squats with the same weight, same number of repetitions, and same cadence, one set in the top half of your range of motion, the other in the bottom. The force multiplied by time and mechanical work would be the same, but the effort would be much different; the set performed in only the bottom half of your range of motion would be much harder.

This is because the force you produce when contracting against the resistance is the product of both muscular force and leverage. As your muscles contract and cause your bones to move their angle of pull and leverage changes, so the same amount of force applied to a machine or weight through those bony levers can require more or less muscular force input depending on the part of the range of motion you are in. This is the reason some exercises are harder in some portions of the range of motion and easier in others, the reason you are capable of holding much more weight at or near lockout on pushing exercises, and the reason that load multiplied by time is not a valid measure of exercise difficulty.

It is the reason leg pressing something as heavy as a small car over a range of motion of only a few inches near lockout is not more intense—does not require more effort—than performing a full-range leg press and avoiding lockout with much less weight. This is not impressive to anyone with a basic understanding of physics. It’s just stupid, and a good way to wreck your knees.

Pete Sisco leg presses a car two inches and claims he's working harder than someone squatting hundreds of pounds over a full ROM, demonstrating his ignorance of physics

This is extremely important, so I’m going to say it again. The resistance your muscles contract against during an exercise is the result of both the load and the lever you are working against. Two thousand pounds lifted with a two inch lever requires the same muscular force input as only two hundred pounds lifted with a twenty inch lever:

2000 pounds x 2 inches = 4000 inch-pounds of torque

200 pounds x 20 inches = 4000 inch-pounds of torque

Since the muscular force input required is the same it makes little difference to your muscles which you do, but it makes a huge difference to your joints in many exercises, which is why it is just as effective but much safer to perform an exercise over a greater range of motion against a larger average lever using less weight.

Also consider your muscles are capable of producing different amounts of force at different lengths (length-tension curve), with their force output being lowest when fully stretched and fully contracted and highest through the middle of their range of motion. As you approach lockout most of the muscles being worked will be closer to their fully contracted length and capable of producing less force than in the middle of your range of motion, so the muscular force input may actually be slightly lower when doing very short-range partials in the so-called “strong range” of an exercise.

I’m not saying load isn’t important, it appears you need at least a heavy enough load to be able to achieve momentary muscular failure within about two minutes. However, working as hard as possible relative to your momentary ability has a much bigger impact on your muscular strength and size gains than the load you use, and it is safer and easier on your joints in the long run to use more moderate loads.

References

1. James Fisher, James Steele, Stewart Bruce-Low, Dave Smith. Evidence-Based Resistance Training Recommendations. Med Sport 15 (3): 147-162, 2011 DOI: 10.2478/v10036-011-0025-x

2. Vitor L. Silva, Arthur P. Azevedo, Joctan P. Cordeiro, Michael J. Duncan, Jason M. Cholewa; Mário A. Siqueira-Filho, Nelo E. Zanchi, Lucas Guimarães-Ferreira. Effects of exercise intensity on perceived exertion during multiple sets of bench press to volitional failure. J of Trainology August 2014; Vol. 3, No. 2: Pages 41-xx

3. Shimano T, Kraemer WJ, Spiering BA, et al. Relationship between the number of repetitions and selected percentages of one repetition maximum in free weight exercises in trained and untrained men. J Strength Cond Res 2006; 20: 819-23.

4. N.A. Burd, C.J. Mitchell, T.A. Churward-Venne, and S.M. Phillips. Bigger weights may not beget bigger muscles: evidence from acute muscle protein synthetic responses after resistance exercise. Appl. Physiol. Nutr. Metab. 37(3): 551-554, 2012.

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  • Darren Aug 26, 2014 @ 17:49

    I lock out on bench presses, overhead presses etc. Not locking out doesn’t seem like a real lift to me. If you don’t lock out in a strongman contest, it counts as a failure. Lots don’t lock out because the weight is too heavy for them to lock out

    • Drew Baye Aug 26, 2014 @ 19:21

      Darren,

      The problem is not locking out, but only using the last few inches of the range of motion on pushing movements. That being said, you should not lock out on compound leg exercises to avoid hyperextending the knee.

  • Chad G Aug 26, 2014 @ 21:03

    This just kind of touched on a realization I had recently. I noticed while documenting my workouts that how effective the workout would be depended much more on my focus and intensity than on my TUL or the weight or any other factor. And while it is good to document what I am doing as I get stronger my level of intensity increases even though my TUL may even decrease a little. The toughest part of the HIT training is staying focused on long term gains and not sweating the details of each workout. Just staying on it and giving each workout the most you have at that time. (*sometimes that is not my best but…). What also helps is documenting my opinion of my effort, from meh to YEAHH! As they say do not let the perfect be the enemy of the good.

    • Drew Baye Aug 27, 2014 @ 8:37

      Hey Chad,

      This is why I am always telling clients how you do each rep is more important than how many you do.

  • David Lewis Aug 27, 2014 @ 7:02

    Great explanation Drew! I just started doing the big five workout this week. I have been cycling 150-250 miles per week. Do you think it is advisable to for go the leg press?

    • Drew Baye Aug 28, 2014 @ 9:10

      Hey David,

      Yes, leg pressing and other lower body exercises will help your cycling.

  • Donnie Hunt Aug 27, 2014 @ 8:51

    Great article Drew. Looks like it’s already started some great comments. Looking forward to what else you and everyone else has to say.

  • Darren Aug 28, 2014 @ 9:33

    I had bad knee trouble in the 90’s. I think leg extensions where your legs start at deeper than 90 degree angle caused it. That along with forced and negatives. Bob Kennedy mentioned in Beef It that leg extensions to failure are dangerous to the knees

    • Drew Baye Sep 1, 2014 @ 0:40

      Darren,

      Leg extensions to failure are not dangerous to the knees if you are positioned and aligned correctly and use proper form. What degree of flexion you can safely start at depends on the individual.

  • Thomas Aug 29, 2014 @ 10:48

    Regarding leverage:

    This is why I question the time honored idea that compound exercises are better for mass building in a specific muscle group BECAUSE they allow one to use more weight (I’m not taking into account the fact compound exercises tend to use more muscles in general). This is still commonly repeated.

    An example would be a lateral raises vs. a press for the deltoids. It’s true you can’t use as much weight in the lateral raise, but because the lever arm is so much longer, you are in fact working against a much higher resistance than what is in your hands. By the above standard, the lateral raise IS a good exercise for building mass, even though the actual hand held weight is much lighter. This is also why bending the elbows (shortening the lever arm), ala Larry Scott, to use more weight in the lateral raise isn’t necessarily going to result in more muscle stimulation.

    • Drew Baye Sep 1, 2014 @ 0:37

      Thomas,

      The main advantages of compound exercises are they allow you to effectively target more muscle groups with fewer exercises, have a greater metabolic and cardiovascular effect due to the larger amount of muscle involved, and are easier for people to learn and perform correctly. The ability to use more weight doesn’t really matter, since what counts is the resistance encountered by the targeted muscles, not the total weight moved. You could probably build muscular strength and size just as effectively performing only simple exercises, but it would be more time consuming and probably be less effective for cardiovascular and metabolic conditioning.

  • Bruce Feltus Aug 29, 2014 @ 16:47

    Drew,
    I’ve been on Project Kratos for a little over a year. I’m 60.
    I am not geared to be as analytical about my workouts but do jot down what I do in a notebook that I’ve kept for 3 years. I find that I can’t follow a specific schedule of days off but it runs from 4 days to 10 or 12 depending on how I feel mentally and physically. But I’ve been consistant to workout within this framework. Sometimes I mix and match and I substitute free weights for bodyweight because it just feels good to put weight over my head.
    My interpretation of the intensity is based on how I feel at the time of the exercises coupled with how I feel in a few days afterward. Sometimes when I have a seemingly intense workout I’ll recover fast and feel I didn’t hit it hard enough. Other times I will feel like I could have done more and I’ll be toasted in the recovery phase. So for me intensity is just a feeling based on thousands of variables. All I need to know is that it works, period.
    Thanks Drew I appreciate your hard work.

    • Drew Baye Sep 4, 2014 @ 10:11

      Hey Bruce,

      You’re welcome, and thanks for the feedback. A lot of factors affect how we feel and perform and what might feel like it takes a maximum effort on a bad day can feel relatively easy on a good day, and all we can do is put forth the best effort we can whatever the circumstances.

  • Lifter Aug 29, 2014 @ 19:24

    I like Sisco, always had a good rapport, but unfortnately a lot of his claims are grandiose. Power Factor Training, as Mike (Mentzer) personally told me.resulted in a slew of injuries in Golds Gym…one guy blew his knees out, another his lower back. For longevity…NO THANKS!

  • Donnie Hunt Aug 30, 2014 @ 18:07

    Here is something that has been talked about regarding momentary intensity. If one is keeping rest between exercises to a minimum. One may need to use a lighter load on certain exercises that are done further into the workout right? Some feel that this diminishes the stimulus due to less load. Others would say that this enhances the stimulus due to greater “body as a whole inroad”. I can’t comment much on this due to lack of experience. Any thoughts?

    • Drew Baye Sep 1, 2014 @ 0:31

      Donnie,

      Based on the recent pre-exhaust study, rushing doesn’t appear to positively or negatively effect improvements in muscular strength.

      • Donnie Hunt Sep 2, 2014 @ 18:30

        Thanks a lot Drew. I had thought about that when I posted that comment. I’ve never been a hard core rusher personally. If you were to instruct me during a workout I’d probably have a rude awakening of muscle failure and intensity even without the rush factor.

        I think I’ve gained a better understanding from a combination of your writings and some Ren-Ex stuff with regards to the end or stopping point of a set. Putting it into practice with certain exercises is another story, lol. I use to not understand why doing drop sets was not advocated by you or Ren-Ex. I thought that since we weaken as we progress into a set that resistance level should follow. I don’t think was taken into consideration the weakening that occurs from “attempting” to keep moving with good form, although movement will eventually cease. It’s funny because I had read stuff talking about this but the light bulb didn’t quite turn on for awhile, lol. This is where one can get into runaway negatives right?

        • Drew Baye Sep 3, 2014 @ 11:09

          Hey Donnie,

          I don’t do or teach planned drop-sets, but if a person doesn’t complete more than half their lower repetition count target because the initial weight selection was too high I will drop the weight so they can perform a few more reps.

          When you understand the real objective of exercise is to place a demand on your muscles, not simply to perform work or get reps, the importance of proper form becomes obvious, as well as the problems with all the ways people cheat to get more reps. How you perform each repetition is far more important than how many you perform.

          • Donnie Hunt May 29, 2016 @ 11:00

            There is a definite, different feel to reaching a point in an exercise where more concentric movement is not possible. I also strongly agree about how each rep is performed vs. how many.

  • Darren Sep 1, 2014 @ 9:58

    Its just squats, leg extension and stiff legs or leg curls now. I chucked out leg presses, and don’t go to failure on squats or leg extension. My legs are down from a bulky 26 3/4 inch to around 25 inch but look better with no knee trouble at age 46

  • jack Sep 6, 2014 @ 10:37

    Hi Drew,

    Would you recommend doing a seated hamstring curl and a leg extension then jumping on the leg press or is that overdoing it?

    Thank you

    • Drew Baye Sep 13, 2014 @ 10:24

      Hey Jack,

      This depends on the individual. This is fine for most people, but too much for some.

  • Lifter Sep 23, 2014 @ 17:36

    Hey Drew,

    Great article as always.

    With hypertrophy specifically in mind, would you recommend staying in a weaker range of motion (near active insufficiency rather than passive due to the nature of cross-bridging) in an isolation movement because the relative intensity on the muscle being worked will be higher? Or, would you err on the side of being able to handle a greater load (potentially greater tension) made possible by using either full range and/or strong range of motion repetitions?

    The difference, as an example, would be doing a static contraction with a fully contracted bicep (supination, elbow flexion, shoulder flexion) against a lesser resistance vs. doing a standing barbell curl static contraction with the arms flexed to 90 degrees against what would ultimately be a greater resistance.

    Generally speaking, of course, if the primary objective is to provide the muscle with the highest possible relative intensity, it would seem to make sense to load relatively weaker positions of movement. This of course takes some weight out of your hand, but would make the movements potentially easier on the joints at the same time.

    Furthermore, if the strength gained at any portion of the range of motion will increase strength throughout the entire range, would not training the “weaker” ranges (w/ proper joint and muscle function in mind) be most advantageous for strength overall?

    My apologies if this was already touched on in earlier writings.

    Thanks!!

    • Drew Baye Sep 24, 2014 @ 12:23

      Hey Lifter,

      The problem with this is that, although it would make sense for strength gains to transfer over the range of motion for a single muscle, research shows the majority of people have strength increases specific to the range being trained. This may be because most exercises involve multiple muscles whose relative contribution varies over the range of motion.

      I recommend using as much range of motion as you can while maintaining relatively consistent tension on the target muscles. How to best accomplish this depends on the exercise and the equipment used.

  • Oscar Apr 28, 2015 @ 7:52

    Excellent insights, as always. Thanks so much.

    I normally do my reps fast, but this time I did a little experiment. I made each rep last 10 seconds, so I was only able to complete 4 reps. I hit failure while attempting the 5th rep. So I am assuming the intensity of effort is high in cases where each rep is hellish. TUL or weight we use doesn’t matter, but only the nature of EACH rep. Is this right?

    • Drew Baye Apr 28, 2015 @ 10:05

      Hey Oscar,

      Any repetition speed can be effective if training is done hard, consistently, and progressively, however slower movement speeds allow for greater tension in the muscle, longer time under tension with a given load, and lower acceleration during the turnarounds which reduces risk of injury, just to name a few. There are a lot of effective ways to get bigger and stronger, but in the long run how big and strong we can get is limited by genetics so the potential end point is the same so it is a good idea to use a training method or style of exercise that also minimizes wear and tear on the joints.