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While the terms used to describe it aren’t completely uniform (burnout, overtraining, overreaching, “fried CNS,” etc…), most people seriously involved in lifting and training for sports know that fatigue is important, and is something that needs consideration in a training program. However, while most can agree that fatigue is important, there is some lack of clarity about the concept, as well as some downright misconceptions and fallacies. Let’s dig through the topic and see if we can’t agree on some specifics.

Defining Fatigue

First of all, what does “fatigue” mean? In sport science, fatigue is the term used to describe the inhibition of maximal performance that comes about as a result of stressors imposed on the athlete. Fatigue can come about from both training and other factors (relationship stress, schoolwork, lack of sleep, poor nutrition), but in this discussion we’ll just stick to training-induced fatigue. Fatigue, at a basic level, means you can’t put the shot as far, can’t squat as much, and can’t move as smoothly on the balance beam as usual, and for added bonus points, it makes you feel like crap, too!

Generally, the literature shows that the primary cause of training-induced fatigue is the total volume of a training program, and not nearly as much its intensity. This is likely because volume represents the amount of physical work being done, and thus energy expended and damage sustained by the body. So if the volume (and to a lesser extent, intensity) of a training is the ultimate cause of fatigue, what are the proximate causes? That is, what actually happens in the body to cause fatigue to rise?

Training-induced fatigue has 3 primary proximate causes: substrate depletion, neuroendocrine alterations, and microtrauma. They each play a role in adding to fatigue, and must of course each be addressed if fatigue reduction is a goal.

When you train hard and heavy, you use up fuel (substrate) to do so. If you perform a heavy squat, your levels of ATP right after the squat are lower than they were before you began, which is the primary reason that the next rep will be harder. Resting for several seconds usually replenishes acute ATP stores. If your set lasts for multiple reps, your creatine phosphate (CP) levels will drop, and that will cause acute fatigue as well, this time taking several minutes to recover from, as levels of CP return to normal. Lastly, if you train for multiple sets and reps, glycogen is used to generate energy to recover your ATP and CP stores. Now, glycogen doesn’t replete in just minutes, and a voluminous workout can require up to several days of eating carbs to fully replete the glycogen used up.

The interesting point is that if you train hard enough (with enough volume, intensity, and frequency) to get good adaptations, you are unlikely to replete 100% of all your glycogen each week, particularly if you’re on a lower carb or hypocaloric diet. Oftentimes, this means that week after week of hard training leads to lower and lower glycogen levels, which have been repeatedly shown to be linked with decreases in performance AND muscle growth. Kind of a bad deal!

Neuroendocrine alterations describe the change in the nervous and hormonal systems of the body with hard training over time. Over weeks of hard training, testosterone production tends to drop, while cortisol production rises. Sympathetic activity begins to predominate (fight or flight), while parasympathetic activity begins to wane (recovery and regeneration). The nervous system begins to display poorer synchronicity in operation, and even intracellular signaling pathways (neither neural nor hormonal but very important) begin to operate in such as way that promotes catabolism and discourages anabolism. There are a multitude of other documented and postulated changes in various other hormones and signaling molecules that are also likely contributing players to this form of fatigue.

Last but not least, the third contributor to fatigue is microtrauma sustained during training. Hard training, and especially heavy and high volume sessions, literally create small rips in the muscle and connective tissues (tendons, etc.). These rips are inconsequential in the short term, but they each take a little while to heal. However, if hard training continues unabated, some of the rips will be re-ripped and become even larger. Large enough rips can cause pulls and strains, so this summation of microtears is not a process that can be allowed to continue unabated. Just like with glycogen repletion, most tears heal within the week. However, the small fraction of tears that didn’t heal becomes a greater and greater fraction with each week of hard training.

While acute fatigue from training is a good thing (it lets us know we trained hard!), the type of fatigue that can cause problems is cumulative fatigue. Properly defined, cumulative fatigue is the fatigue that sums up over the weeks of hard training. It’s all the glycogen that went unrepleted, all the hormonal axes that were thrown off, and all of the microtears that added up. And while we don’t stress over acute fatigue too much, it pays to keep cumulative fatigue in check for 3 distinct reasons.

What’s Wrong with Cumulative Fatigue?

Cumulative fatigue interferes with training in 3 distinct ways. First of all, it lowers the athlete’s ability to perform maximally, both in force production and in technique execution. Cumulative fatigue makes the athlete less able to display good technique, less powerful, and weaker, probably in that order of onset. This is a bad deal for two reasons. Firstly, having crappy technique can lead to poor technical practice, and essentially makes the athlete learn how to do important movements wrong, as well as increasing the chance of them getting hurt. The last part is especially true if your technique breaks down under a heavy squat or with a heavy deadlift! Secondly, because fatigue weakens you, it does not allow you to present a very strong stimulus in training. If you’re training for strength, you had better be able to lift heavy, and if you are impaired due to fatigue, then you simply cannot present as much of an overload as needed to stimulate the best gains.

The second reason fatigue interferes with training is by its direct effects on hormonal and intracellular adaptive pathways. As fatigue levels rise, so does the catabolic hormone of cortisol. With its combination of lowering testosterone, fatigue-mediated cortisol rise literally interferes with growth and recovery. Additionally, rising fatigue levels tend to bias cellular signaling in favor of catabolic pathways (AMPk for example) and away from anabolic pathways (mTOR for example). This means that you might even be able to push through being tired (our first problem with cumulative fatigue), only to realize that most of your hard work did not turn on adaptive pathways nearly as much as usual, and your hard work was partially for naught.

Lastly, by summing up microtears and expanding their size over the weeks and months of hard training, cumulative fatigue increases the risk that such tears will lead to injuries. Now, I’m not gonna bullshit you into the idea that if you don’t deload every 3 weeks, your pec and quad are going to fly off the bone at the same time during a bench press. Fatigue-mediated injuries usually take much longer than several weeks of hard training to become likely, but what is almost certainly true is that many months of hard training without a chance for microtears to heal completely is going to increase injury risk.

How to Manage Fatigue

So now we know that cumulative, training-induced fatigue is bad, evil, and probably cheats on its taxes as well. The question is, how do we get rid of it? First of all, we’ll only be looking at training-related modalities to reduce fatigue in this article, as other modalities (nutrition, supplementation, lifestyle variables) are worthy of an article of their own. Secondly, it’s probably better to use the term “fatigue management” rather than just “fatigue reduction.” Why?

Well, while cumulative fatigue is bad, it is also 100% inevitable from hard training. We can always prioritize fatigue management above all other training principles, but if we go too far down that road, we end up highly inhibiting the training process by interfering with still other very important training principles. Most important of these is the overload principle, which states, in a general sense, that training must be TOUGH if you’re going to get much better. If we try to bring down fatigue at all costs, we risk reducing the overload of the program so much as to lead to poor gains. Since we don’t want to do that, we need to find a balance between fatigue accumulation and reduction, and this balance is called “fatigue management.”

As fatigue rises through the days, weeks and months of training, we have 4 distinct training methods to use in order to bring it down to reasonable levels. The first method employs the principle of variation within the week or week to week, particularly in exercise selection. Because some microtrauma, intracellular signaling, and nervous system changes are motor-unit specific, you can allow one set of motor units to rest a bit more while another is overloaded simply by alternating exercises ever week or half-week. Some of the motor units involved in high bar squatting are less involved in front squatting, so if you front squat later in the week instead of doing high bar squats again, some of those motor units get a chance to drop some fatigue for next week’s squats. I suspect that the high variation in exercise selection is a reason that Louie Simmons has stepped away from advocating deloads in recent years.

We must be careful to keep variation in check, however, as we want to keep our training, and thus our adaptations, directed towards a specific goal. If you train chest a ton one week, and then only train shoulders and triceps the next week, then you’re going a bit too far, and your chest will not likely grow much, even though its fatigue will be quite low! Thus, keep your exercises varied, but keep that variation limited, and stick to the movement patterns and muscles you want to work on during that phase of training.

The second method of fatigue management employs what are called “light days.” After the first half or 2/3 of the week of hard training, you can come to the gym and only use a certain smaller weight as the beginning of the weeks, so long as the reps are the same or lower as well (to prevent doing more work on your light day than on your heavy day!). It has been shown that lighter training can bring down fatigue EVEN FASTER than no training at all under certain circumstances, and using light days is a great option for doing just that. In fact, light days do one better by conserving your gains from earlier in the week AND bringing down fatigue, which means they don’t cost you the back-tracking that complete rest can cause.

The third method of fatigue management is the “deload,” which is usually about 1 week long and is likely the most commonly recognized form of fatigue management. During a deload week, training volumes must be brought down, probably by half or more of the normal training volumes in order to really bring down fatigue. In the words of prominent fatigue researcher Dr. Andy Fry; “when you hit the breaks, hit ‘em hard.” Otherwise, if you train almost normally during this week and bring volume down just a bit, you end up neither dropping fatigue nor getting the overload to make gains! A worst of both worlds for sure! Intensity can actually stay up in the usual range during this week, as it’s not a huge contributor to fatigue if volume is kept in check, and also helps conserve the gains made in earlier training. However, in order to heal completely (especially from microtears), intensity must be brought down at some point as well, and dropping it by 50% for the last half of the deload week may be a good start.

The final and most long-term method of fatigue management is termed “active rest.” It’s a form of fatigue management that is characterized by 50% reductions in volumes AND intensities, often for around 2 weeks or so. This type of fatigue management should be performed about once a year, and can allow the fatigue accumulated over months of hard meet prep, contest prep, or game season to be extinguished almost completely.

While the above guidelines give you a basis from which to start, remember that individual differences must always be considered. Many people don’t accumulate fatigue very quickly, and can go entirely without light days. While the average trainer should probably deload every 4-6 weeks or so, some can go longer and some will break before they get to 4 weeks! Make sure you pay attention to your body and play around with various schemes, finding what works for you.

Common Myths

Of course no article on fatigue would be complete without a list of common myths with brief refutations, so here we go:

Myth 1: “I don’t need to deload.”

Refutation: If you never need to deload, YOU’RE NOT TRAINING HARD ENOUGH. Do 2 days a week of 10 sets of 5, heavy in the squat and let me know how not deloading works out for you. (The author of this article is not responsible for hospital visits and gym-related dismemberment.)

Myth 2: “Keep the volume high, bring down the weights.”

Refutation: Because volume is the primary contributor to fatigue and intensity (weight on the bar) is the primary savior of training gains in a deload, dropping the weights and upping the volumes is exactly the OPPOSITE of what you want to do! Deloading like this gets your MORE fatigue and drops some gains as well. Yikes!

Myth 3: “My CNS, is fried bro!”

Refutation: While the CNS has been demonstrated to be a likely culprit in contributing to fatigue especially when very high intensities are used with high volumes, it is BY NO MEANS the only, or even the primary cause of fatigue. And how have you ruled out the PNS or the intracellular sources of fatigue? Just say “I’m fatigued,” and don’t bother with the pseudo-Russian bro-science.


I had a lot of fun writing this (yes, this is my idea of fun, go to hell for judging me), and I hope readers learned some valuable information. Fatigue management is important and needs our attention in designing and running a training program, but like with all things, personalization and attention to individual responses is important as well.

Dr. Mike Israetel

Mike is a professor of Exercise Science at Temple University in Philadelphia, PA and was previously a professor at the University of Central Missouri, where he taught Exercise Physiology, Personal Training, and Advanced Programming for sports and fitness. Mike’s PhD is in Sport Physiology, and he has been a consultant on sports nutrition to the U.S. Olympic Training Site in Johnson City, TN. Mike has coached numerous powerlifters, weightlifters, bodybuilders, and other individuals in both diet and weight training. Originally from Moscow, Russia, Mike is a competitive powerlifter, bodybuilder, and Brazilian Jiu-Jitsu grappler. He used to hold a bunch of state, national, and world records in raw powerlifting back when everyone was in equipment, so that’s cool!

READ MORE BY Dr. Mike Israetel

3 Responses to “Fatigue Explained”

May 14, 2016 at 8:25 am, Onizuka's penis said:

thanks this helped a ton, i’ll be implementing deloads. Went back to my journal and realized the week i took off while sick, and then the week it took me to get back into my training after much muscle and neurological detraining was followed by a massive peak in my sport!! This is mind blowing to me, so instead of unintentionally being brought out of training, i’ll definitely be deloading once a month to be safe.


December 30, 2016 at 10:52 pm, Per Lichtman said:

I was wondering if you could clarify the question of intensity from the “How To Manage Fatigue” section? I was a little unclear at what point were suggesting that intensity should be dropped versus maintained in regards to a deload.

“During a deload week, training volumes must be brought down, probably by half or more of the normal training volumes in order to really bring down fatigue.
…Intensity can actually stay up in the usual range during this week, as it’s not a huge contributor to fatigue if volume is kept in check, and also helps conserve the gains made in earlier training. However, in order to heal completely (especially from microtears), intensity must be brought down at some point as well, and dropping it by 50% for the last half of the deload week may be a good start.”

In other words, are you saying that reducing volume alone is enough to help somewhat, but a decrease in intensity is required for full recovery? Or are there certain circumstances where reduction in volume is sufficient but that in more acute cases a reduction in intensity is also called for? Or was there yet some other meaning you intended?

Looking forward to more of your thoughts!


December 30, 2016 at 10:58 pm, Per Lichtman said:

Also in regards to reducing volume alone in the context of “heavy, medium, light” day weekly programs, like the Texas Method, is it your experience that reducing weekly volume by reducing the number of sets on 2 of the days is enough to make a difference? Or do you find that volume must reduced every training day, so that on the third day (single sets) it is necessary to reduce the number of reps as well?


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