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What is Muscular Failure: Definitions

Muscular Activation

Last time I took a look at the concept of training volume, what it is, what it represents along with some of the different ways it can be defined, tracked or measured (along with their often major limitations).     Among other topics, I described how one research group suggested that sets per workout or per week can be a valid way of tracking volume but “…only if those sets are taken to the point of muscular failure.”

Effective Reps

Towards the end of that piece I described the concept of effective or stimulating reps.  These are the repetitions during a set which are done under conditions of full muscular recruitment.  The idea being that it is these reps under conditions of high tension that turn on the growth process.

Related: Why is Muscular Tension Important for Muscle Growth?

Among other ideas, this model is an attempt to rationalize how different types of training ranging from heavy work (5-8 reps to or near failure or not) to low-load sets of 25-35 to failure to Blood Flow Restriction (BFR) sets to failure or not can all generate the “same” growth.

The premise being that each set generates the same number of effective reps by the time the set ends.  Hence they can generate the “same” growth despite varying drastically in total reps and volume load.

Rounding out the last article, I offered the opinion that conceptualizing training volume in terms of effective reps might not only give us a better language to talk about training but also to compare distinct training styles.  As well, it would allow us to more meaningfully compare the training studies on volume and hypertrophy that are out there.

This is NOT to say that effective reps and training to failure are synonymous.  Sets that are near failure can and will generate some number of effective reps depending on the intensity and how near to failure the set is.  So while a set of 8 to failure might generate 3-4 effective reps, a set of 6 with the same weight might only generate 2.

The Relevance of Muscular Failure

Which is why using the endpoint of muscular failure may be useful at least in terms of discussion or research.  Because it’s at least consistent.  And apparently it’s consistent across sets of drastically varying intensity and duration.  If 3 sets of 8 to failure, 3 sets of 15 to failure and 3 sets of 30-35 to failure generate the same growth then the commonality is going to failure.

Whether or not it’s the best way to train per se, training to the point of muscular failure standardizes the training stimulus.  Only by doing so can meaningful comparisons between studies or training styles be made or discussions of the same be had.

Because it should be clear that a set of 8 reps to failure and a set of 10 at 4 reps from failure can’t be considered equivalent on most levels.    To compare 4 sets of 10 to failure and 4 sets of 10 at 4 reps from failure is meaningless on every level.   Physiologically they are not the same and they are unlikely to have the same training effect.

Within the effective reps framework this is especially true.  The first set may be generating 3-5 effective reps while the second may be generating only 1.  Meaning that it would take 3-5 times as many sets at 4 reps from failure to generate the same number of effective reps as the one heavy set to failure.  Suddenly you “need” triple the volume due to each set being so far from failure.  Hmm…..

So in the sense of being able to meaningfully compare studies or volumes or workouts, as the paper linked above suggested, muscular failure is likely to provide the most objective endpoint to use.  Yes, I there are concepts such as RIR and RTF that can be used in this regard and I’m not denying that.  But those can have their own set of problems that I’ll get to in a while.

For the time being, I will be working from the assumption that muscular failure provides the most objective endpoint to use in a research sense in terms of comparing volumes within any given study or between two different studies.

And that means actually defining and understanding what muscular failure is (or isn’t I suppose).  Because if you’re going to use a definition as an endpoint, it needs to be carefully defined or you end up trying to compare apples and oranges again.

And as is often the case, it’s going to take me a little while to get to the point.   Probably three articles to be honest.  In this one I just want to look at the general concept of muscular failure.  What it is and what it represents in the most general sense.   I’ll apply that to the weight room in the followup article.  And well, I’ll leave the topic of the third part of this for a surprise.  There will be videos and you’ll finally get to see “If Lyle even lifts”.

Let’s Be Clear on the Topic

I want to make two points clear.  First, nothing I’m going to write today should be taken as a suggestion or recommendation to train to failure.  Or not to train to failure.   In fact, I’m not even going to look at the studies on the topic, some of which suggest failure is superior and others that do not.  Many of which use designs so moronic as to be painful and pointless.

That’s not the point of this article in the least.  Because, briefly, the question of “Should I train to failure or not?” is “It depends”.  What does it depend on?  Well the goal of the training, the exercise being done, training frequency, the training age and experience of the trainee, the goal volume and there are assuredly other variables that go into that decision.   So I can’t give a simple answer.  But that’s not the point of this article to begin with.

Rather, the point of this article is to simply examine what muscular failure actually represents.   As I said above today is just about general concepts.  But the next article will focus on the weight room.  And especially in terms of the different ways that failure has been defined in practice or research.  And this is super important because not all definitions of muscular failure in the weight room are identical.

Research studies on training are variable enough but when you start trying to compare studies using different definitions of muscular failure, you run into even more problems.   And as I mentioned last time, when I talked about the potential for effective reps to help clarify this, I suspect that taking into account those differences in the definition of failure will make many of the apparent contradictions in terms of optimal training volumes disappear.     That’ll take a couple of articles to get to but I will…eventually.

Related: How Many Sets Should I Do for Muscle Growth?

Now, today I’m going to look at muscular failure in a very general sense and not even talk about the weight room specifically.  Rather I want to look at a smidgen of muscular physiology before defining the concepts of muscular fatigue, muscular/task failure and muscular exhaustion.  Basically this is just background for the next two articles that will focus on the weight room.

A Primer on How Muscles Work

So how do muscles work?  Very well, thank you.  Ha ha, I’m hilarious.  Anyhow…

Ok, let’s say that you want to do a biceps curl.  What happens, at least in a general sense?  First the motor cortex in your brain sends a signal that travels down the nerve until it reaches what is called the neuromuscular junction (NMJ) at the biceps.

Then a whole bunch of other involved shit happens and the muscles (technically motor units or MUs) contract, generating force.  That forces is translated through the tendon to the bone causing the elbow to flex and you hopefully lift the weight, impressing everyone in the gym.

It looks very schematically like this.

Muscular Activation

If you’re wondering why I’m skirting on the “involved shit happens” step it’s because it doesn’t matter for the purpose of this article.  I’m too wordy as it is, I’d have to look up a bunch of complicated shit and it simply doesn’t matter in any practical sense to the topic of this article.

All you need to know is that for any voluntary (i.e. non-reflex) activity, your brain sends a signal to the muscle which contracts to generate force.   None of the intermediate steps matter practically.

Types of Muscle Fibers

The human body has two primary types of muscle fibers: Type I and Type II.  Type II are usually divided into IIa and IIx (only animals have IIb).    And this gets all kinds of complicated when you consider the host of host of subtypes, mixed and intermediate fibers.  And none of this really matters for the purpose of this article either.

In trained individuals almost all of the IIx fibers will have converted to IIa fibers to begin with so we can sort of ignore them.  So for the sake of a simplified discussion I’ll simply talk in terms of Type I and Type II fibers and ignore the rest of the details.

The key things to know is that Type I fibers are smaller, more aerobic and generate less force than Type II fibers.  Type II fibers are larger, more anaerobic and generate more force than Type I fibers.  There are other differences but they don’t matter here.

Do realize that the different physiological aspects of the fiber types are more of a continuum than some sort of discrete cutoff.   There’s a gradation of fibers from smallest, most aerobic, weakest to largest, most anaerobic, strongest.  So you go from the smallest Type I fibers up through the largest Type II fibers.

Note: for the pedants, it’s more accurate to talk in terms of Motor Units (MU’s) than muscle fibers per se.  Where a MU is the motor nerve and bundle of muscle fibers that it activates.    The distinction doesn’t matter practically here so I’m keeping it simpler for once.

Fiber Recruitment and Rate Coding

Now when muscles are recruited to generate force, the body can use two general methods to increase force production.

  1. Increase muscle fiber recruitment
  2. Increase rate coding

Type I and Type II fibers differ in how readily they are recruited and you will see low and high-threshold used to describe them.  Don’t get hung up on these terms, it has to do with nervous system function which you just don’t want to get into the weeds on.

And with a few exceptions (most of which aren’t relevant to training to begin with), fibers are recruited in a fairly orderly fashion according to something called Henneman’s Size Principle (not to be confused with Henneman’s Wife’s Size Does Matter Principle).

And all it says is that fibers will recruit from smaller, weaker, lower threshold (Type I) fibers to larger, stronger higher threshold (Type II) fibers.  And this recruitment is determined by the force requirements of the task.

So if you’re just doing brisk walking, you will only need Type I fibers.  It’s not a high force activity, energy production is completely aerobic.  You can go until you get bored.  As you move to a slow jog, you’ll get some Type II fibers but only as many as you need to generate sufficient force.  Since the intensity is still low they will operate aerobically and you won’t fatigue too quickly.

As you start running faster, you get more and more Type II fibers being recruited.  As you approach what is variably called the lactate threshold, OBLA, anaerobic threshold and a host of other stuff, you’ll get more Type II fiber recruitment.     Those muscles will be generating various waste products such as H+ and ammonioa but they aren’t accumulating and you can keep going for a while.

Related: What Are the Determinants of Endurance Performance?

As you go above that level, you get further recruitment at least up until maximum recruitment occurs.  If you were to go all out, you’d reach full muscular recruitment.  In fact you’d probably reach full recruitment before hitting an all out speed.  But waste products would be accumulating very quickly and you’d eventually shut down and have to stop (or at least slow down).    More on this below.

Let me note that even if full recruitment isn’t achieved at a given intensity, it may over time at that intensity.  As some muscle fibers fatigue they will “fall out” of the movement meaning that the body will recruit currently un-recruited fibers.

It’s even more complex than this as the body can cycle muscle fibers in and out of the movement depending on the intensity.   So assuming full recruitment hasn’t been achieved, the body will de-recruit some fibers to let them rest while activating other fibers to take up the slack.

But if any activity is taken to failure (defined below), essentially full recruitment will be reached.

Rate coding refers to how fast signals are sent from the brain to the muscles.  Above a certain point, there are no more muscle fibers to recruit so the body will send signals faster to the muscles to generate force.  This aspect of neural function is much much much much (much) more complicated than this but no way am I getting into the weeds here.

In the Weight Room

Translating the above to the weight room, what you see in the muscle groups of relevance is that the body uses fiber recruitment up until about 80-85% of maximum force production (some put this at 90% but it’s in that range).  Above that level it uses predominantly increased rate coding to generate further force.  I’ve shown this schematically below.

Muscle Fiber Recruitment and Rate Coding

Note: The above values only hold for major muscle groups.  In what are called constrained muscle groups such as the eyes and finger, the body uses recruitment up to about 50% of maximum and rate coding after that.  This gives finer motor control.  But it’s also why studies on the fingers or thumbs have no relevance to training outside of training the fingers and thumbs (grip I guess).  And why citing them to attempt to make arguments about bigger muscle groups shows a complete non-understanding of human physiological function.

So if you start lifting at 80-85% of maximum (about 5-8 repetitions to failure), the body will recruit essentially all fibers from the first repetition.  As fatigue sets in, you will see rate coding drive further force output.

Related: How Many Reps Should I Do for Muscle Growth?

In contrast, if you start sub-maximally, say at 65%, you will not see full fiber recruitment initially.  But as fatigue sets in throughout the set, you will towards the end (i.e. the last few reps of an all out set of 15 reps).

Basically, there are two ways to get to full recruitment:

  1. Start at a sufficiently high intensity
  2. Take a low intensity task to fatigue/failure

So let’s look at those now.

Fatigue, Failure and Exhaustion

Finally we get to the real point of this article which is to define the concepts of muscular fatigue, muscular failure and muscular exhaustion.  These three concepts are inter-related (and many use the terms interchangeably although this isn’t correct) but are not identical.  So let me look at each.

Muscular Fatigue

Muscular fatigue describes the idea that, over time, a muscles ability to generate force will decrease from its starting point.  And this will occur during any exercise above a certain effort.   How quickly this occurs will depend on the intensity of the activity.  The relationship of the muscle’s force production and the intensity of the activity will, in a conceptual sense, determine its difficulty.

So say that a given muscle has an initial force production capacity of 100 (arbitrary) units of force.  If someone were walking and that only required 10 units of force it would feel very easy.   Realistically the muscle will never fatigue and the person could go essentially forever.  Boredom or dehydration are more likely to stop the activity before muscular fatigue.

Now they are slow jogging which requires 30 units.  It’s a little bit harder and fatigue might accumulate gradually over time.  The initial 100 units of force will drop to 99 then to 98.  But given the relatively low intensity of the activity, this process could take hours.

Now they are running near their threshold requiring 70 units of force.  This is pretty hard, there is only a 30 unit reserve.  As fatigue occurs, 100 units becomes 99 units, etc. and the decrease will occur more quickly than when only 30 units of force was required.

Now they are running all out which requires 90 units of force.  This will very very hard and rapidly occurring fatigue will drop the muscle’s force production from 100 units of force very rapidly.     They might have a few minutes at this pace.

And the person then goes to an absolutely all out sprint, this might require nearly the full 100 units of force right off the bat.  The effort will be all-out/maximal and fatigue will occur extremely rapidly.

Note: I am using the term muscular fatigue very generally above.  Here’s why: muscular fatigue is unbearably complicated as it can occur anywhere at like 7 steps in the chain from motor cortex to force production.      And each may play a relatively greater or smaller role depending on the activity being done.  But the differences aren’t relevant here so I’ll ignore them.

But that’s all muscular fatigue represents: a decrease in the muscle’s ability to produce force over time.

Muscular or Task Failure

Now fatigue is just a consequence of activity, occurring over time due to the things that cause fatigue that I’m not getting into.  But this is different than failure or what is usually described as task failure in research.  What that means depends on what is being tested.

Let’s say that you’re put on a bike and told to pedal at 200 watts for as long as possible.   As soon as you can no longer maintain 200 watts you have reached task failure.    It might be even more specifically defined by an inability to maintain some specific pedal cadence (i.e. 60 RPM’s).  Once you can no longer maintain your pedal speed, you have achieved task failure.

A lot of studies measure things like isometric muscular endurance.  Usually with the finger muscles probably because it’s easy to control even if it has no relevance to major muscle groups.  So the subject is supposed to maintain 20% of their maximum force or whatever for a maximal amount of time.  When they can no longer do that, muscular/task failure has occurred.

Now, any task we might test or care about in this sense is going to be related to muscular force production.  The muscle has to be able to generate enough force to meet the requirements of the task.

That means that task failure will fundamentally occur when fatigue has reached a point where the muscles cannot match or exceed the force required by the task.

So back to the example above.  Your rested muscles can generate 100 units of force.  And riding at 200 watts requires 60 units of force.    Early in the activity it’s pretty doable since you have 40 units of force in reserve making the effort a mere 60%.  Over time, fatigue starts to occur, the muscle’s force production capacity is dropping.

At some point perhaps your muscles can only generate 80 units of force, compared to the 60 required by the task.  The task will be proportionally harder although it can still be achieved.  More time passes and more fatigue generates.  Now your muscles can only generate 65 units of force compared to the 60 units that are required.  This will be very hard but not quite an all-out effort.  Once muscular force falls to 60 units of force, it will take an all-out effort to maintain the 200 wattage goal.

And as soon as your muscles can only generate 59 units of force, you will no longer be able to maintain the goal power output.   You have reached muscular/task failure defined again as an inability to generate enough force to match or exceed what is required by the task in question.

I’ve shown this schematically below.

Force Output vs. Force Requirements

The graphic should be fairly clear, red is the force required to achieve the task, green is current force output.  As time passes and fatigue sets in, force output drops and drops.  When force output falls below the red line, task failure occurs.  Once again, the actual cause of fatigue is irrelevant here which is why I didn’t get into details.

When the subject is no longer able to generate as much force as needed, muscular task failure will occur.  And please pay attention to that phrasing carefully.  Because what I said was when the “subject is no longer able to generate as much force” NOT “when the muscle is no longer able to generate as much force.”   And there is a difference of some relevance to this topic although I won’t get into that until the next article.

Note: you will often see the phrase “momentary muscular failure” used.  This refers to the fact that task failure is only occurring at the moment.   If you had hit task failure and could no longer maintain 200 watts but rested for 5 minutes so that your muscles could recover some of their force production capacity, you could go back to 200 watts for at least some period of time.  The muscular failure was momentary.

Muscular Exhaustion

Finally is muscular exhaustion and it’s critical not to confuse this with muscular fatigue or failure.  Task/muscular failure only occurs when the muscles can’t generate enough force to meet the current requirements.  That doesn’t mean that the muscles are completely exhausted.

So even if you can no longer maintain 200 watts on the bike, that doesn’t mean you can’t continue pedaling at 180 watts.  Or 160 watts.  You can keep going, just at a slower speed.   The runner who can’t maintain 10 mph can still run at 9 mph or 8mph.   In the exciting thumb isometric studies, even if a 20% maximum force can’t be maintained, that doesn’t mean a 15% force can’t be.

Fatigue eventually leads to task failure but that isn’t synonymous with exhaustion.

True muscular exhaustion would only occur when someone’s muscle could generate zero force.  When you see marathoners drop near the finish line, unable to walk or even crawl, that’s true exhaustion.

On an exercise bike, exhaustion would represent not being able to turn over the pedals against no resistance.  This can’t occur on a moving bike because it will tip over below a certain speed.  In swimming, the person would drown when true exhaustion was reached.  But I’ve long described swimming as “failed drowning” so…

In animal studies, to test true exhaustion, they have rats run on a treadmill with a shock plate on the back of it.   As the rat fatigues and can’t maintain the pace, he’ll hit the shock plate and pick up the pace.  When he lies on the shock plate unmoving, he’s exhausted.  I think this might have some potential for human exercise programs.

In swimming studies, as rats start to fatigue, they will sink.  And that will get them right back into swimming for their lives.  When the rat starts to drown and makes no effort to survive, it’s exhausted.  This probably has slightly less potential for human exercise programs.  Though see above: failed drowning.

By and large, humans do not reach true muscular exhaustion outside of the most extreme conditions.  While I’ll focus on the weight room in the next article, true muscular exhaustion would mean continuing an exercise until the muscle couldn’t move the limb at all.  It would be the ultimate drop set I guess.


Muscular fatigue, failure and muscular exhaustion are not the same thing.   Fatigue describes the muscle’s loss of ability to generate force, occurring over time depending on the intensity of the task.

At the point that muscular force falls below what is required to complete a task, muscular or task failure can be said to have occurred.

In the rare situation where a muscle is no longer able to generate any force, that would be muscular exhaustion.

And that’s where I’m going to cut it today.  Next time I’ll take the above general information and show how it applies in the weight room.  Where, as always, things get much more complicated.

Read What is Muscular Failure: The Weight Room Part 1


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