Aerobic Exercise/Endurance Training
Relationship of mitochondria and aerobic exercise
Mitochondria influences activities that last longer than a minute. There are different skeletal muscles
fibers involved depending on the type of exercise being performed. With endurance training,
mitochondrial content has the ability to increase due to both motor units being involved.
Skeletal Muscle Fiber Types
3 Categories:
Slow-twitch red fibers (Type I) contain the largest amount of mitochondria. The significance of the
mitochondria in slow-twitch fibers allow for individuals to exercise for longer periods of time. These fibers
have a low sacroplasmic reticulum progression, which means they will not release much Calcium or uptake
a large amount of it. The role of calcium will be elaborated on later in this section. The slow-twitch fiber is
most involved in aerobic exercise/endurance training. Essentially, they generate low levels of power that
last over a long period of time. 4, 10, 17
Fast –twitch fibers (Type II) are the fiber type more involved in anaerobic exercise. They generate
high levels of power over a short period of time.
2 types: Fast-twitch A and Fast-twitch B. Both types of these fibers have a high sacroplasmic reticulum
progression, which means they have the ability of releasing a large amount of Calcium as well as being able
to uptake a large amount.
Fast-twitch A is the fiber type referred to as “fast-twitch red.” Type A can be recruited for aerobic or
anaerobic exercise. Fast-twitch A fibers contain a moderate amount of mitochondria in comparison with
the other fiber types.
Fast-twitch B is the fiber type referred to as “fast-twitch white.” Type B is recruited more for anaerobic
exercise. Fast-twitch B fibers contains the smallest amount of mitochondria in comparison with the other
fiber types. The small amount of mitochondria allow for the individual to perform short bursts of exercise
such as weight lifters. 4, 10, 17
Important to Note: Mitochondria are red and the muscle fibers obtain their color due to the mitochondrial
content.10
The table below categorizes each type of fiber and characteristics: 19
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Background of Mitochondrial Biogenesis
The study of mitochondrial biogenesis has expanded over the years. Advancements have been made in
relation to the effects of endurance training as well as other factors that affect mitochondrial biogenesis.
Essentially, the adaptations of mitochondria due to exercise involve molecular signaling, transcription,
translation, and the importing of proteins.
The image demonstrates the events that take place in the mitochondria when mitochondria biogenesis
occurs:10
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Aerobic Exercise/Endurance Training
The figure below shows an overall summary of mitochondrial biogenesis: 4
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Aerobic Exercise/Endurance Training
Influence of Aerobic Exercise on Mitochondrial Biogenesis
Research has shown that trained individuals have a higher capacity of mitochondria in their skeletal
muscle compared to untrained individuals.10
The diagram illustrates the differences in mitochondria capacity between untrained and trained tissues: 4
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Aerobic Exercise/Endurance Training
Figure 6-13 Eletron micrographs of mitochondria in tissues of (a) untrained and (b) trained rats.
Mitochondria in trained animals appear to be more numerous, probably because there is a more elaborate
mitochondrial reticulum. Micrographs courtesy of P.D. Gollnick.
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Aerobic Exercise/Endurance Training
The table below illustrates a study performed on rats of the effects on mitochondria in response to
endurance training. It is displayed that the mitochondria in the trained muscle doubled in capacity
compared to the untrained muscle.4
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The table below shows the effects on muscle mass and activity of enzymes with endurance training: 4
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Aerobic Exercise/Endurance Training
Endurance training enables a stimulus to occur in the mitochondria that increases proteins causing
replication. Mitochondrial biogenesis in an active muscle is thought to be integrated by local signals due to
the adaptations being able to occur in only contracting muscles. The initial amount of mitochondrial
content the individual has will determine the amount of biogenesis that occurs. Endurance training causes
an increase in mitochondrial density, size and content due to repeated exercise. The shape and size
changes have been noted in the subsarcolemmal and intermyofibrillar regions in the mitochondria after
several weeks of exercise sessions, which expands the mitochondrial reticulum. The content of
mitochondria has the ability to increase up to 100% for an individual who has been training for about a
month. The occurrence of mitochondrial biogenesis allows for an increase in mitochondrial content, which
benefits the individual by improving performance for training. There is greater allowance for going
through the TCA cycle when more mitochondria are available. 4, 10, 11, 16, 18
More information on the effect of Aerobic Exercise on Mitochondrial Biogenesis can be found here:
http://www.ncbi.nlm.nih.gov/pubmed/8818195
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Aerobic Exercise/Endurance Training
http://www.ncbi.nlm.nih.gov/pubmed/12959619
Physiologic benefit of training
The effect of aerobic exercise causes an increase in mitochondrial content. The increase of mitochondria
allows for improved oxygen consumption for tissues as well as the furnishing of ATP. Due to endurance
training and mitochondrial biogenesis, endurance performance is enhanced and the individual is less
susceptible to fatigue as fast as an untrained individual. 10, 24
More information on the Physiologic benefit of training on Mitochondrial Biogenesis can be found here:
http://jap.physiology.org/content/111/4/1066.long
Differences in types of training (i.e. interval training, etc.)
Endurance training, a continuous type of exercise being completed at a low or moderate intensity for a
long duration, has been the exercise most frequently studied in relation to mitochondrial biogenesis. The
advancements in research being made in mitochondrial biogenesis have allowed for the study of the
effects with other types of exercise. High intensity interval training, sprinting, and resistance training
have all elicited to mitochondrial biogenesis.10,15
A study conducted by Medical Science of Sports Exercise on repeated-sprint exercise investigated effects of
mitochondrial biogenesis. There were different factors that occurred in mitochondrial biogenesis that
were evaluated in the study to determine if repeated-sprint training affects the increase of mitochondria.
Signaling processes and the proteins involved were assessed as well as the effects from the repeated-sprint
exercise on mRNA expression and protein abundance. The study also focused on the effects of PGC-1alpha,
a main regulator of mitochondrial biogenesis, hypothesizing that this type of training would increase that
protein. It was determined that several proteins increased during this exercise bout including: ACC, CaMK
II, and PGC-1alpha suggesting that adaptations of mitochondrial biogenesis are apparent in short-term,
high intensity exercise.23
More information on the effect of Repeated Sprints on Mitochondrial Biogenesis can be found here:
http://www.ncbi.nlm.nih.gov/pubmed/22089482
A study conducted by The Journal of Applied Physiology on resistance training after endurance training
assessed the effects of signaling and proteins involved in mitochondrial biogenesis. The stimulation of
rapamycin (mTOR) occurs with resistance training fueling muscle growth and protein synthesis. Increases
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Aerobic Exercise/Endurance Training
of mRNA and PDK4 were noted following both bouts of exercise, suggesting initiation of mitochondrial
biogenesis. The study had hypothesized the signaling response would decrease when adding resistance
exercise to follow a bout of endurance exercise. However, the study found the signaling of expression
genes and protein activation for protein synthesis in mitochondrial biogenesis was heightened. The results
of the study suggest that resistance training is another form of exercise that also stimulates mitochondrial
biogenesis.27
More information on the effect of Resistance exercise on Mitochondrial Biogenesis can be found here:
http://jap.physiology.org/content/111/5/1335.long
PGC-1alpha protein influence on biogenesis during endurance exercise
Of the 3 PGC-coactivators, PCG-1alpha was discovered first. Expression of PCG-1alpha is synchronized by 3
kinases including: mitogen-activated protein kinase (MAPK), AMP-activated protein kinase, and calcium/
calmodulin-dependent protein kinase II (CaMKII). PCG-1alpha is pertinent to the function and regulation
of mitochondrial content. PCG-1alpha takes on important roles within the mitochondria including:
biogenesis, respiration, and function. The protein, PCG-1alpha, has the ability to coactivate several
transcription factors, which leads to the transactivation of genes involved in the Electron Transport Chain
and transcription factors of mtDNA. The encoding of an assortment of genes by the nucleus is coordinated
by PGC-1alpha, resulting in mitochondrial biogenesis. The expression of the PGC-1alpha mRNA protein is
mostly in tissues relied on for oxidative metabolism for ATP production. Single and chronic exercise
sessions have made notable effects on PGC-1alpha mRNA and protein levels. The increases of
mitochondrial biogenesis correlate with the upregulation of PGC-1alpha mRNA and protein levels.
Increased cellular respiration and the transformation of fast-to-slow fiber types are the benefits of
increasing PGC-1alpha at the protein level. A deficiency of PGC-1alpha causes a decline in mitochondrial
content, respiratory function, and a decline in endurance performance. However, endurance training is
not completely reliant on this protein. There is belief there are compensatory proteins that replace
PGC-1alpha when needed to allow mitochondrial biogenesis to occur. 10, 15
More information on the influence of PGC-1alpha-protein on Mitochondrial Biogenesis can be found
here:22, 24
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0041817
http://jap.physiology.org/content/111/4/1066.long
The diagram below offers a visual aid of the role PGC-1alpha during mitochondria biogenesis: 10
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Aerobic Exercise/Endurance Training
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Aerobic Exercise/Endurance Training
Role of Calcium and ATP
Endurance training results in a greater number of mitochondria, and therefore, a greater capacity to
maintain ATP due to an increase in oxygen uptake. During exercise, this is an important factor because if
ATP levels are not maintained, muscle cells fatigue quickly. A higher rate of ATP/ADP ratio allows for an
escalation in oxygen consumption when there is high capacity of mitochondria. 4, 10
The role of ATP is similar for untrained and trained individuals during mitochondrial biogenesis. In the
intermembrane space of the mitochondria, an abundance of protons conducts the ATPase reaction forming
ADP. ATP levels are maintained through the CPK, glycolytic flux and mitochondrial respiration, which are
activated by ADP. A myokinase reaction can be initiated by the conversion of ATP and AMP from ADP. The
pathway from this point is dependent upon the type of muscle fiber. In a trained individual, the increased
amount of ATP causes a reduction in several processes including: phosphocreatine, glycolysis and lactic
acid production, AMP and NH3 formation, and AMPK activation. The reduction of the several processes
caused by the increase of ATP results in enhanced performanced. 4, 10
In mitochondrial biogenesis, the two contributing factors that follow the exercise session include Calcium
ion and ATP turnover. Both factors can affect the mitochondrial production of ATP. The rate of ATP
turnover is elevated with activity and elicits a change in energy status within the cell. The change in
energy status can also be accomplished without exercise by mtDNA reduction. An increase of calcium is
distinguished when there is disorder in ATP production within the mitochondria. The SR releases Calcium,
activating kinases and phosphatases, which activates transcription factors. These transcription factors
impact the encoding of mitochondrial proteins by way of the nuclear genes. The mitochondrial
chemiosmotic gradient is also affected by the release of calcium due to the gradient binding and activating
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Aerobic Exercise/Endurance Training
TCA cycle dehydrogenases. Exercise causes AMP to elevate and activates AMPK-active protein kinase.
Other kinase proteins are activated causing possible activation of nuclear transcription factors.
Peroxisome proliferators-activated receptors-alpha and –y are other transcription factors involved in the
process. The receptors and other transcription factors are believed to play a role in the binding of genes in
association with coding for subunits of cytochrome oxidase and mitochondrial free fatty acid uptake.
Understanding how some of these factors affect mitochondrial biogenesis is still unclear. 4, 10
More information on the role of ATP in Mitochondrial Biogenesis can be found here: 15
http://www.ncbi.nlm.nih.gov/pubmed/19448716
The image below illustrates the pathways of ATP in mitochondrial biogenesis: 10
Conclusion
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Aerobic Exercise/Endurance Training
Mitochondrial biogenesis contributes to aerobic exercise significantly. Slow-twitch fibers are the type of
fiber recruited for endurance training. The occurrence of mitochondrial biogenesis allows for the increase
of mitochondria enabling enhanced performance for the individual. The adaptations of mitochondria are
made through molecular signaling, transcription, translation, and the importing of proteins. The influence
of the PGC-1alpha protein was found to be the main stimulator protein of mitochondrial biogenesis. ATP
levels are also relevant because the levels must be maintained to allow mitochondrial biogenesis to take
place. Studies on other types of exercises have been conducted to evaluate mitochondrial biogenesis’ role.
The studies found training other than endurance also stimulates mitochondrial biogenesis for enhanced
performance. Overall, mitochondrial biogenesis is essential in aerobic exercise because it allows the
individual’s training performance to excel.
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