Exercise Fatigue: everything you need to know about what it is, what causes it and how to manage it

We’ve all been there…

A fuzzy head, seeing stars, heart pounding out of your chest and legs like lead.

With only a few reps left or just a lap to go. But you just… can’t… push… through. 

The symptoms of exercise fatigue are all too familiar. But the physiology behind it is quite complex.

Exactly why fatigue kicks in during those last couple of sets, or final few miles is all down to the energy demands of the sport. Intensity, duration and even environment can have an impact.

In this guide we’ll break down everything you need to know about exercise fatigue and how to manage it more efficiently.

Key points:

  • Fatigue can occur in both central (brain) and peripheral (muscle) areas
  • The feelings associated with fatigue are sensations. But fatigue itself is simply the inability to cope with the demands of exercise
  • Fatigue occurs during resistance and endurance exercise. However, the mechanisms can be different

Exercise and Fatigue: A bit of background

Tiredness, exhaustion, burnout. These are all terms synonymous with the feeling you suffer from exercise fatigue. If you’re a regular exerciser, you’ll know the signs all too well.

From heavy, burning legs to pounding heart rates and plummeting energy levels.

Severe cases can lead to vomiting, blackouts, muscle pain and a whole host of negative effects on the body and mind.

Exercise tolerance

Exercise fatigue - sometimes referred to as exercise intolerance - is defined as a decreased ability to perform physical exercise or tasks at a given workload or capacity.

Simply, you can’t maintain the intensity of the exercise and a reduction workload is required in order to recover.

Exercise fatigue is defined as a decreased ability to perform physical exercise or tasks at a given workload or capacity

Fatigue doesn’t just occur during hard workouts either. Even light exercise will eventually lead to neural and biochemical changes that force you to stop.

There’s also the huge psychological element too.

Some athletes are able to fight through intense muscle discomfort when striving for a Gold medal, while others will happily fight another day.

Fatigue tolerance

Research shows that the very best athletes are able to tolerate fatigue1 better than their counterparts, illustrating just how important grit and determination are.

For years, scientists have studied why exercise causes these symptoms so that strategies can be put in place to combat them.

After all, if you could put the brakes on fatigue you could perform harder for longer – which could be the difference between a podium position and a mid-table finish.

But the human body is complex.

With so many physiological systems working together to drive you through your workout, it’s not surprising that exercise fatigue is multifactorial. It not only involves the neuromuscular system, but also other areas of the body too.

You only have to look at how short-term illness affects exercise capacity to see that.

Here’s a list of factors that could be involved in exercise-induced fatigue:

  • Accumulation of hydrogen ions that result in increased carbon dioxide that impairs muscular contraction
  • Increased heat and sweating which leads to dehydration
  • Elevated ammonia levels2 that temporarily reduces metabolic and neurological functions
  • Reduces glycogen stores and lower blood glucose levels
  • Reduction in synaptic firing rate, blocked action potential leading to a decrease in muscle contraction
  • Increased release of cytokines which induce sensation of fatigue
  • Reductions in blood concentrations of BCAAs due to increased tryptophan levels

This isn’t an exhaustive list, by any means. There are many more factors involved - discussed in more detail below.

Central and peripheral fatigue

The causes of exercise-induced fatigue can be categorized by origin.

If fatigue resides in the brain it is caused by factors related to your central nervous system (CNS). And if fatigue occurs due to changes in muscle tissue it’s related to your peripheral nervous system (PNS).

A large review published in Sports Medicine suggested that peripheral fatigue can be defined as the loss of contraction power caused by processes distal to the neuromuscular junction.

With central fatigue defined as a similar loss proximal to the neuromuscular junction.

In simpler terms, central fatigue occurs in the brain and peripheral fatigue outside of it.

 

Central fatigue

Central fatigue is linked with changes to brain tissue and occurs when your CNS downregulates activation of muscle fibers.3 It will also occur when electrical impulses are pinged from one neuron to another.

Or by neurochemical alterations in serotonin norepinephrine and dopamine concentrations.

Although scientists don’t truly know why central fatigue occurs, many speculate that it acts as a safety mechanism. Reducing the strain on peripheral actions during stressful exercise situations.

This is evidenced by research4 showing higher levels of perceived effort are correlated with higher central fatigue.

Central fatigue can be triggered by endurance sports where moderate intensity is maintained for extended periods of time, as well as maximal intensity exercise.

Peripheral fatigue

If your CNS manages to ping messages across the neuromuscular junction, they’ll reach the peripheral part of your nervous system.

Peripheral fatigue occurs within the muscle (either within the fiber itself or the motor unit that controls it) and relates several factors that can negatively impact your ability to generate force.

Peripheral fatigue can be caused by changes to muscle temperature5 - contraction is a mechanical action leading to increased heat generation.

There are also several ‘local’ factors within a part of the muscle called the sarcolemma. Including decreased calcium release, or increased build-up of inorganic phosphate concentrations.

All will result in slower muscle conduction velocity – that is, how fast the signal to contract is sent across a muscle fiber.

Additionally, the depletion of glycogen and other fuels can lead to reduced performance. Much like a sports car would suffer if it ran out of fuel.

To read more on glycogen depletion and how to control it, check out our comprehensive guide.

Peripheral fatigue is defined as the loss of contraction force or power caused by processes distal to the neuromuscular junction, and central fatigue is a similar loss proximal to the neuromuscular junction

It’s worth noting that central and peripheral fatigue are not mutually exclusive. During intense exercise, CNS commands will inevitably decrease. But force production in the muscle fiber will too.

Much of the relationship between CNS and PNS has to do with the type of sport or exercise. Art least one review6 refers to this as ‘specificity of fatigue’.

 

Exercise Fatigue in endurance v strength training: Is there a difference? 

The poison is in the dosage when it comes to exercise fatigue. Not just in terms of the quantity, but the type too. Naturally, both sprinting and long-distance running can be exhausting; but the mechanisms behind fatigue will invariably be different.

During weight training or strength-based workouts, fatigue can occur both centrally and peripherally.7

During CNS fatigue, your ability to recruit muscle fibers begins to decrease because voluntary action is reduced. No matter how hard you try to grind out that last rep, your CNS just isn’t letting you.

But there are also huge peripheral factors involved as well. Ranging from depletion of energy - including oxygen deficit - to the build-up of metabolites.

And if you’ve ever tried giant sets on leg day, you’ll know exactly how that feels!

Of course, variables like load, reps and rest periods will all affect fatigue during resistance training. Less intense workouts will see fatigue creep up on you, whereas with maximal workouts it develops much quicker.

Central fatigue is characterized by the inability to produce voluntary force and therefore usually associated with heavy strength training. But even lighter loads can cause CNS fatigue if sets are repeatedly pushed to failure due to high mechanical tension in the muscle.

Higher rates of exercise fatigue in resistance training are likely to occur with longer duration sets - with therefore lighter than maximal load.

One study8 looked at fatigue rates in both traditional strength training and high-intensity resistance circuits. Measuring metabolic responses, perceived exertion and maximum voluntary contraction.

It found that typical weightlifting resulted in minimal peripheral fatigue, but tough circuit training resulted in neuromuscular, peripheral fatigue.

Central fatigue can be caused by aerobic exercise

This was thought to have been due to higher levels of metabolites negatively impacting the ability to generate force. But interestingly, neither fatigue nor metabolite build-up are necessarily correlated with strength gains.9 

Which means the traditional strength-training group in the study were not disadvantaged because they didn’t train to fatigue.

Endurance exercise is characterized by sustained submaximal contractions. So often associated with peripheral fatigue due to glycogen depletion and/or metabolite build-up.

Contrary to popular belief, CNS fatigue also occurs during aerobic exercise.

A study published in the Journal of Applied Physiology10 found that even with weak muscle contractions of <15% maximum, CNS occurred.

Which ties in with the idea that lighter resistance contractions lead to elevated central fatigue.

Another study found that prolonged muscle endurance led to fatigue not just because of central factors, but because of increased muscle breakdown. And inflammatory markers of muscle damage11 that negatively impacted contraction velocity.

Takeaway…

Exercise fatigue affects us all.

It hits when we can no longer live up to the demands of the workout we are doing.

It can occur in both central nervous system (brain) or peripheral nervous system (muscles), depending on the type of exercise involved.

The ratio between CNS and PNS fatigue varies based on your specific energy demands. 

Understanding more about the onset of fatigue and its causes allows you to plan a more effective training program.