Causes of Muscle Fatigue – Coursera Science of Exercise

This video will examine the courses for muscle fatigue during
a single part of exercise. As you’ll see there is no one
single course for muscle fatigue. Further the cause of fatigue
will be event specific. In other words what contributes to fatigue
when you’re doing bench presses will be very different from the courses of
fatigue near the end of a marathon. First, let’s begin by defining by
what is meant by muscle fatigue. Muscle fatigue is defined as the inability
to maintain a given exercise intensity or power output resulting in a decrease
in work or performance capacity. Shown here is the textbook example
of the onset of muscle fatigue. They are in the early stages of exercise. Power output or
force production is well maintained. However over time, the ability to maintain
this level of force production declines, thereby fulfilling our
definition of fatigue. For years physiologists have been studying
the mechanisms for muscle fatigue. The issue is extremely complex and,
as already mentioned, the causes will vary based upon
the type of exercise involved. Any step between when the decision is
made in the brain for muscle movement and the eventual contraction of the skeletal
muscle are potential sites for a fatigue. However generally the causes of fatigue
are located in the peripheral sites beginning with the neural
muscular junction and beyond. As such I will only focus on those sites. As a reminder, the type of muscle fiber
recruited will play a significant role in the cause of fatigue. Type 2 X fibers,
which are recruited during short-term, high intensity exercise,
are quick to fatigue. While Type 1 fibers, used during more
long-term, lower intensity exercise, are slow to fatigue as long as
there’s ample fuel for ATP production. First, let’s examine
the most common causes for fatigue when engaging in short term,
high intensity exercise. For example, when bench pressing a heavy
weight, you may be able to manage completing five repetitions, but
just cannot squeeze out the sixth. Or while sprinting 200 meters, you cannot maintain your speed
during the final 20 meters. Frequently the cause of
fatigue is the depletion or accumulation of sub substance or
metabolite. While many factors can potentially
contribute to the onset of fatigue, I will discuss the three
most common causes for muscle fatigue during a short
term high intensity exercise. These include the depletion of both ATP
and Creatine Phosphate stores in muscle. As well as the accumulation of hydrogen
ions or an increase in muscle acidity. First, the depletion of ATP
resulting from a mismatch between ATP utilization and
production will result in fatigue. As a reminder, ATP is required for cross
bridge cycling and tension development. If ATP is being consumed at
a faster rate than it can be made, then levels will drop
below resting values. At this point the individual must
reduce their exercise intensity, thereby lowering the rate of ATP
utilization, allowing ATP production to keep pace, or the individual
must stop exercising all together. Second, during short-term
high intensity exercise, creatine phosphate stores
are rapidly depleted. As creatine phosphate is the main
immediate energy source in muscle for ATP production, its depletion will compromise
one’s ability to maintain ATP levels. Shown here are the ATP and creatine phosphate levels in muscle in
response to a bout of maximal exercise. Notice that the creatine phosphate levels,
which are abbreviated PC for phosphocreatine deplete very
rapidly at this intensity. When levels are extremely low, this negatively impacts our ability to
maintain ATP production and thus ATP levels in muscles decline dramatically
leading to exhaustion or fatigue. The third common cause for muscle fatigue
during short term, high intensity exercise is an increase in muscle acidity or
an accumulation of hydrogen ions. This metabolic acidosis can contribute
to fatigue in a number of ways. The high concentration of hydrogen
ions can interfere with calcium’s role in cross bridge formation and
thus tension development in muscle. This will result in a reduction
in force output by the muscle. Also, hydrogen iron concentration
will inhibit anaerobic glycolysis. As you remember,
anaerobic glycolysis is a major source for ATP production during short-term
high intensity exercise. Thus a decline in creatine
phosphate stores, in conjunction with
a reduction in ATP production from anaerobic glycolysis will lead
to a rapid onset of muscle fatigue. As will be discussed in the video
on performance enhancing drugs, this is the concept whereby buffer loading
can potentially lead to an improvement in performance during high
intensity explosive exercise. Having a greater buffering capacity prior
to engaging in these intense activities will allow for a neutralization or buffering of the negative effects
associated with metabolic acidosis. Shown here is an example indicating that
when trained women were given sodium bi-carbonate prior to one bout of
maximal exercise, they were capable of performing at a higher peak power and
intensity for 60 seconds. When compared to when they performed
the exact same test with a placebo or no supplement at all. This is just one of many studies
suggesting that metabolic acidosis is a factor contributing to fatigue during
short term high intensity exercise. Now let’s examine the three
most common causes for fatigue during long term
lower intensity exercise. As I have discussed previously,
the depletion of carbohydrate stores is a common cause for
fatigue during this type of exercise. Additionally, over time, a decline in intramuscular calcium
levels can also contribute to fatigue. Finally, an accumulation of heat or
increasing body and muscle temperature is a potential factor
leading to the development of fatigue. Participation in distance events such as
a marathon are performed at submaximal or moderate exercise intensities
that can be tolerated or maintained over a prolonged
period of time. The depletion of muscle glycogen
frequently coincides with the unset of fatigue as a major source for
ATP production no longer exists. This is why the technique of
carbohydrate loading known to increase muscle glycogen stores prior to
competition can improve ones performance. When muscle glycogen is depleted,
the individual is left with two choices. Reduce the exercise intensity and
thus the rate of ATP utilization or stop exercising completely. Both scenarios adhere to our
definition of muscle fatigue. Additionally, the depletion of liver
glycogen will eventually result in significant reduction in blood
glucose levels or a hypoglycemia. As the muscles and brain rely on blood
glucose for fuel, the ability to continue to exercise is not sustainable once
a state of hypoglycemia has been reached. Again this is why the practice of
consuming dilute carbohydrate drinks during the course of distance events can
lead to an improvement in performance. During hours of prolonged exercise,
the cytoplasmic reticulum in muscle cells are repeatedly stimulated to release
calcium for cross-bridge formation and tension development. Over time, some of the calcium release can leak out into the extracellular fluid
and/or be taken up by mitochondria. This can result in less
calcium being available for muscle contraction,
thereby impairing force or power output. Finally, an increase in body and
muscle temperature can lead to fatigue. During prolonged exercise, these temperatures can approach 42 degrees
Celsius, or 108 degrees Fahrenheit. This will result in a greater percentage
of your blood being diverted to the skin for thermoregulation. Meaning less blood is going
to the active muscles. Further, failures to keep properly
hydrated will cause a reduction in plasma volume, reducing your cardiac output and
blood delivery to the muscles. In summary, muscle fatigue is defined as
the inability to maintain a given exercise intensity or power output, resulting in a
decrease in work or performance capacity. The most common sight of
fatigue during excercise is located within the muscle itself. The causes of fatigue are task specific,
and may result from the depletion or accumulation of key variables. Finally, causes of fatigue during
intermediate events such as a 10k run, likely involve factors mentioned for
both short term high intensity exercise as well as long
term lower intensity exercise.


  1. I thouht that ATP remained the same as Pi was increasing… but atp depletion was mortal.
    Do you recommend some paper about the topic?

  2. Hi sir, could you recommend some research papers on the topic? Especially about the calcium release relationship with muscles fatigue. Thank you.

  3. So your saying steroids eliminate the fatigue feeling? bet

  4. Thank you.

  5. Many thanks

  6. So then would a ketogenic diet lead to faster onset muscle fatigue due to the lack of glycogen from carb restriction?

  7. Hello, my left bicep is alot softer than my right. Im an Armwrestler, so my biceps are under alot of pressure. My biceps were harder before I started working out. When I start flexxing, its hard, but after 5 seconds it starts to get extremely soft, and I don't have the power to flex no more. Is this the same thing you are talking about in this video?

  8. For endurance (low intensity) exercise, the main thing that is mentioned here is glycogen depletion in muscles and liver. Why then do muscles still fatigue in a runner regardless of whether they continue to feed themselves with glucose/glycogen during an ultra marathon. Secondly, on a keto diet the body is burning minimal glucose in the blood but instead is metabolizing fat for energy production. Even in this state your muscles will still fatigue after a long enough endurance exercise, even though it has copious amount of fat for energy at its disposal. Conclusion is that there is something else at play causing muscles to fatigue aside from just available fuel for the body!

  9. I am sure you will find great workouts instructions on Unflexal Workouts page. That insane how it is good for sane 😀

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