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  Latest update: 1/13/2022

Exercise Physiology

Cellular Energy Production

Energy to power muscle contractions is released when oxygen combines with carbohydrates, fats, and proteins in the cell to produce Adenosine Triphosphate or ATP. These are termed oxidation reactions (literally "a combination with oxygen") and are only a few of the many reactions (metabolism) within the cell that maintain cell viability. The amount of energy produced in oxidation reactions is limited by either the amount of oxygen available to the cell for oxidation, or limited fuel (carbohydrates, fats, and protein) to be oxidized.

The foods we eat provide these three energy containing compounds: carbohydrates, fats, and protein. Carbohydrates are the primary energy source for short, maximum performance events (sprints at or near 100% VO2 max). Both fats and carbohydrates can serve as an energy source for the cell in endurance events (generally performed at less than 50% VO2 max.) Proteins are used to maintain and repair body tissues, and are only rarely used as an energy source to power muscle activity (almost always when severe Calorie restrictions are in effect such as with starvation or malnutrition).

The cardiovascular system delivers the oxygen necessary for oxidation. Oxygen is extracted by the lungs from the air we breathe and then transported via the circulatory system (bound to hemoglobin) to the cells where it will be utilized. One of the end products of energy production, carbon dioxide, is transported from the cell back to the lungs by the circulating blood and then leaves the body in expired air.

The two limiting factors in cellular energy production are:


Food energy is released via oxidation - a chemical reaction with oxygen. When oxidation occurs outside the body - for example in the burning of oil (a fat) in a lamp, or the use of a flaming sugar cube (a carbohydrate) as a decoration in a dessert - the energy is released in an UNCONTROLLED fashion as heat and light. In the cell, this same energy from carbohydrates and fats in food is released more slowly and in a CONTROLLED fashion that makes it available to power basic cell functions and mechanical movement by muscle cells.

Controlled oxidation in the cell converts the three basic food compounds (carbohydrates, fats, and proteins) into a single common chemical compound adenosine triphosphate (ATP). It is the ATP, this intermediate synthesized from the three basic compounds, that transfers the energy content of food into muscle activity.

ATP is a molecule consisting of a base (adenosine), a sugar (ribose) and three phosphate groups. The chemical bonds between the phosphate groups contain the stored energy in this molecule, and it is the breaking of these bonds (as ATP is converted into ADP or adenosine diphosphate) that in turn provides the energy to power muscle contractions and other cellular functions.


There is a limited storage capacity for ATP in the cell, and at maximum work levels the ATP stored in the muscle cells will be depleted in a few seconds at most. Thus, to sustain physical activity, the cells need to continually replenish (resynthesize) their ATP.

There are three pathways for ATP synthesis, and the one used by the cell depends on both the degree and duration of the physical activity.


Almost all ATP is produced from the oxidation of carbohydrates and fat. Only in starvation does protein become a significant source of energy.

ATP production occurs in our mitochondria and is most efficient when adequate oxygen is available. (For those interested in evolutionary biology, the theory on the development of the mitochondrial energy pathway - presented below** - is fascinating.).

The carbohydrate used in muscle ATP production comes from muscle stores (glycogen) and glucose extracted directly from the bloodstream. Blood glucose, in turn, is from dietary sugar absorbed in the small intestine supplemented by glucose released from non-muscle glycogen storage, mainly in the liver.

Fat is stored in the muscle cells as triglycerides and transported to the muscle cell as free fatty acids (FFA) from fat cells throughout the body.

Muscle cell triglycerides contain 2,000 - 3,000 kcal of potential energy and muscle glycogen approximately 1,500 kcal of energy. Both glucose and fats are metabolized to produce ATP at modereate levels of exertion (50% VO2max) but as exertion increases, this ratio begins to shift and at >100% VO2max only glycogen can be utilized for ATP production. Here are several theories on the reason for this shift.

This graph (from Romjin et al shows the relative energy contributions of glycogen and triglycerides stored in the muscle cell compared to that available from circulating glucose and FFAs as the level of exertion increases. What I found most interesting was the plateau in fat Calories utilized as exertion (% VO2max) increased, resulting in a fall in the percent of total expended Calories. The ability of the muscle cell to extract FFA to support exercise appears limited and total fat Calories may actually go down at higher levels of exertion.

Training can modify this triglyceride/glycogen ratio. To quote: "As discussed in a recent issue of Sports Science Exchange (Terjung, 1995), one of the most functional adaptations to endurance training is an increase in the size and number of muscle mitochondria to greatly enhance aerobic metabolism, i.e., the ability of muscles to use oxygen to metabolize fat and carbohydrate for energy. The reduction in muscle glycogen oxidation as a result of endurance training was directly associated with an increase in oxidation of triglycerides derived from within muscle, but not from plasma......Therefore, it appears that intramuscular triglyceride is the primary source of the fat that is oxidized at a greater rate as an adaptation to endurance training and that it is the oxidation of this intramuscular fat that is associated with a reduction in muscle glycogen utilization and with improved endurance performance."

What points should we take away?

  1. If you want to maximize the number of triglycerides Calories being used to fuel exercise (thus maintaining glycogen stores for later, high VO2 sprint activity), the ideal rate of exertion is ~65% VO2 max.

  2. Training will modify the fat/carbohydrate energy ratio so that for any specific %VO2max the metabolism of intracellular fat (triglycerides) relative to intracellular glycogen will increase. This is the result of an increase in the number of mitochondria (where carbohydrate and fat metabolism take place) and changes in the enzyme makeup facilitating triglyceride metabolism.

  3. You can graphically appreciate the reason for "the Bonk". Once glycogen stores have been depleted, no matter how hard you try, you can only produce ~ 50 - 60 % of the total Calories possible if glycogen is still available.


As one begins to exercise, the anaerobic pathways provide for the initial ATP needs while the body shifts into gear to increase adequate oxygen to the cell - increasing both breathing and heart rates. As more oxygen is transported to the exercising muscle cell, the aerobic pathways (metabolizing both fats and carbohydrates) pick up the slack and anaerobic metabolism tapers off. However, anaerobic pathways continue to provide a small amount of ATP energy, and small amounts of lactic acid are still being produced. At this level of production, lactic acid is actually a source of cell energy as it is metabolized by liver and muscle cells. It is only as cell energy needs rise (as in a sprint, for example) and a significant shift occurs towards anaerobic metabolism as the more dominant metabolic pathway for energy production that lactic acid levels begin to rise (paralleling the rise in other intracellular acidic compounds).

Aerobic pathways are used by the muscle cells for energy production (metabolizing both fats and carbohydrates as an energy source) up to exercise levels of approximately 50% VO2 max. At that point (called the "crossover point" ), although metabolism is still oxygen based (aerobic), it shifts towards glycogen as an energy source (and fat metabolism decreases). Then, as 100% VO2max is approached, and the cardiovascular system can no longer provide adequate oxygen to the muscle cell to continue aerobic ATP production, either the phosphocreatine system, or anaerobic metabolism pathways once again cover the cells energy needs. When the level of activity drops back to less than 100% VO2max, and oxygen is once again available to the cells, metabolism once again shifts away from anaerobic pathways, and excess oxygen is available to regenerate phosphocreatine and metabolize (clear) the excess acids produced during the anaerobic sprint type activity. With training, changes occur in the cardiovascular system and muscle cells that support higher levels and longer duration of physical activity before anaerobic pathways are needed, and also clear lactic acid more quickly leading to faster recovery from anaerobic sprints.


As you exercise, a series of chemical reactions metabolizes glucose in a step wise fashion, releasing ATP at each step. When adequate oxygen is available, the glucose is completely converted into carbon dioxide and water. However, when you are exercising intensely, are anaerobic, there is not enough oxygen you need, the process stops at the production of lactic acid. It then accumulates in the muscles and the bloodstream. When adequate oxygen is again available, lactic acid is once again metablized (like any other carbohydrate) to CO2 and water with the release of additional ATP. In fact, cells require less oxygen (are more efficient) metabolizing lactic acid than other carbohydrate.

Why do we slow down when lactic acid accumulates? At one time it was postulated the acidity of the lactic "acid" impaired muscle cell contractiopns. But further studies on isolated muscle cells suggest lactic acidosis "...has little detrimental effect or may even improve muscle performance during high-intensity exercise." And it is the bloodstream acidosis from associated acidic byproducts of anaerobic metabolism that impair exercise performance by reducing the CNS drive to muscle (the central governor).

Lactic acid accumulation, an important intermediate in glycogen/glucose metabolism in the muscle, which may help to protect the cell from the excess acidity of anaerobic metabolism is often blamed for the "burn" experienced with anaerobic metabolism. However other products of ATP metabolism (free H+ ions) also increase in the anaerobic environment and are more likely to be the real the culprit. Rather than being a negative for performance, lactate (the Na+ or K+ salt of lactic acid) is a positive, fueling the exercising muscle with glycogen sparing effects.

Two relevant articles:

These 2 studies support the contention of Dr. Mirkin that the second wind phenomena is a result of this increased lactic acid metabolism with a slight slowing in the cyclists (or runners) speed (below LT, I presume).

To quote: "Your muscles get their energy from each of several successive chemical reactions, called the Krebs cycle. The Krebs cycle requires large amounts of oxygen to burn carbohydrates, fat and protein for energy.... However, if you run so fast that your muscles do not get all the oxygen that they need, you develop an oxygen debt that slows down the successive reactions of the Krebs cycle. This causes lactic acid to accumulate in your muscles to make them acidic. It is the acidity that makes muscles burn, and you gasp for air, trying to get more oxygen.

The muscle burning and shortness of breath caused by the accumulation of lactic acid forces you to slow down....Your muscles switch to burning more lactic acid for energy, you need less oxygen and then you pick up the pace. You tell everyone that you suddenly got your "second wind", but actually:

Of course when you keep on pushing the pace, you can again accumulate large amounts of lactic acid in muscles, which will make them burn and hurt again."

For those of you interested, here is another link to a Dr. Mirkin blog on lactic acid.


The energy contained in equal weights of carbohydrate, fat, and protein is not the same. Energy content is measured in Calories (note the capital C). Carbohydrates and protein both contain 4.1 Calories per gram (120 Calories per ounce) while the energy "density" of fat is more than double at 9 Calories per gram. The disadvantage of fat as a fuel for exercise is that it is metabolized through pathways that differ from carbohydrates and can only support an exercise level equivalent to 50% VO2 max. It is an ideal fuel for endurance events, but unacceptable for high level aerobic (or sprint) type activities.

Carbohydrate metabolism is much more efficient than fat metabolism assuming adequate oxygen is available (ie aerobic metabolism). But once VO2max has been reached, and anaerobic metabolism takes over, the efficiency of carbohydrate metabolism drops off dramatically. Carbohydrate will produce 19 times as many units of ATP per gram when metabolized in the presence of adequate cell oxygen supplies (aerobic) as opposed to its metabolism in an oxygen deficient (anaerobic) environment.

In the well fed and rested state, the human body contains approximately 1500 carbohydrate Calories (stored as glycogen) in the liver and muscle tissue, and over 100,000 Calories of energy stored as fat. The muscles are the main glycogen repository containing anywhere from 300 - 600 grams of glycogen (which equates to 1200 to 2400 Calories) while the liver contains 80 to 110 grams (300 to 400 Calories). The exact numbers are less important than the concept that internal carbohydrate stores are only adequate for several hours of brisk cycling (80 to 100 % VO2max). On the other hand, there is enough stored fat to continue to cycle at a reduced speed (50 - 60% VO2@max) for days.

If one is not supplementing their internal carbohydrates with oral supplements, the muscles first use their internal glycogen reserves for fuel, then call upon liver glycogen which has been maintaining a constant blood sugar level, and finally, when there is no more glucose as an energy source, the muscles switch to fat metabolism and you " bonk". This term describes the fatigue resulting from muscle glycogen depletion. Without adequate carbohydrate to fuel continues high level muscle activity, it is impossible to maintain a high level of energy output and one has to slow to speeds of 50% VO2max where fat metabolism can provide the needed Calories. Any oral supplements delay the onset of this mandatory switch in energy source and associated inability to maintain a high level of performance. The bonk can be delayed by using oral sugars to supplement muscle glycogen stores. Thus, on a long ride, a rider that snacks will have more glucose remaining in the body to fuel that final sprint than one who cuts corners on their nutrition.

Two other strategies are to 1) minimize extremely energy inefficient anaerobic sprints earlier in the ride (remember they are very inefficient in terms of ATP production) and 2) whenever possible, ride closer to 50% VO2max to take advantage of supplemental Calories available from fat metabolism. In addition to eating while riding, these two strategies will help to save a few more grams of muscle glycogen for that final sprint to the line.


Insulin is made by special cells in the pancreas, is released when they sense a rise in the blood glucose level, and works by stimulating the cells of the body to extract that extra glucose from the blood stream. Insulin is essential to the normal functioning of exercising muscle cells - and the replenishment of glycogen post exercise.

The disease of diabetes, at its most basic, is the result of a lack of glucose inside the cells to power them. The blood glucose level is usually elevated but it can't get into the cells where it is needed to supply the energy they need.

Exercise moves carbohydrates into the cell, where it is needed to produce the energy to power the cell, in 2 ways.


The traditional teaching is the stress (of aerobic training) stimulates adaptive changes within the muscle cell, very likely within the mitochondria, and improved performance is the result.

Mitochondria are the powerhouses of our muscle cells. They contain the enzymes that convert glycogen and fats into the ATP needed to power muscle contraction. With aerobic training, we know the mitochondria increase in both size and number which then translates into more ATP produced per minute (assuming there is sufficient carbohydrates and oxygen delivered to the exercising cell.)

This article suggests that the physiologic chnages behind training improvement is more complex than just adaptive changes within the muscle cell and involves gut - muscle cell/mitochondria interaction of some type.

The article makes the argument that it is our mitochondria that affect colon bacterial composition which in return produces molecules that will positively influence mitochondrial metabolism and growth. A positive feedback loop which magnifies the benefits of aerobic training.

We know that regular aerobic training changes both the types and numbers of bacteria (the microbiome) in the colon. It is not clear which came first, "the chicken or the egg". Do athletes eat healthier with a diet higher in fiber and vegetables (which we know changes the composition of the microbiome)? Or does exercise itself alter the composition of the microbiome?

This study suggests it may be lactic acid, a byproduct of high level aerobic activity, that stimulates the growth of a colon bacteria whose only carbon energy source is lactic acid - Veillonella.

And it appears that this bacteria does improve athletic performance. The researchers confirmed the link to improved exercise capacity in mouse models, where they saw a marked increase in running ability after supplementation with Veillonella.

Did the bacteria work as a metabolic sink to remove lactate from the system, the idea being that lactate build-up in the muscles creates fatigue? Probably not as the idea that lactate build-up causes fatigue is not accepted to be true.

Perhaps it wasn't the removal of lactic acid, but the generation of a product of Viillonella metabolism, the short chain fatty acod propionate, that provided a performance boost.

To investigate further, the researchers introduced propionate into mice [via enema] to test whether that was sufficient for an increased running ability. And it was.

A great example of a positive feedback loop. The athlete is producing something that this particular microbe favors, and in return, the microbe is creating something that benefits the athlete.

So what, you might ask, is the point? There are no clear training take-aways......yet. But if we can isolate the bacterial products that stimulate mitochondrial growth, we may have a cue as to how we might magnify the benefits of aerobic training. And if we can identify specific bacteria that metabolize lactic acid, it may be possible to take an oral probiotic that gives the athlete an extra boost from their own lactic acid. And as we know our diet can directly affect the microbiome, perhaps we will find helpful (or harmful) diets that to be included/avoided to provide a training benefit.

So no recommendations yet. Just a nice example demonstrating how complex physiology can be, and illustrate how a clear understanding could give us clues to provide a competitive edge.


A theory on the development of mitochondria and other cell organelles - from The Tangled Tree by David Quammen.

In 1967, a young, little-known scientist named Lynn Margulis made a daring scientific proposal that was immediately and widely rejected: namely that the small organelles inside of cells had originally been separate bacteria that had taken up residence inside larger cells, which then became useful, symbiotic partners with that cell, and eventually simply part of the cell. It was a theory that radically rewrote evolutionary theory. Today, that theory is widely accepted.

Lynn Margulis made her debut in March 1967 with a long paper in the Jour­nal of Theoretical Biology, the same journal that had carried Zuckerkandl and Pauling's influential 1965 article on the molecular clock. This paper was much different. Its author was no canonized scientist like Pauling, and its assertions were peculiar, to say the least. Put more bluntly: it was radical, startling, and ambitious, proposing to rewrite two billion years of evolutionary history. It included some cartoonish illustrative figures, funny little pencil-line drawings of cellular shapes, and virtually no quantitative data. According to one account, it had been rejected by 'fifteen or so' other journals before a daring editor at JTB accepted it. Once published, though, the Margulis paper provoked a robust response. Requests for reprints (a measure of interest, back in those slow-moving days before online access to journals, when scientists mailed one another their articles) poured in. It was titled "On the Origin of Mitosing [Dividing] Cells.

This paper presents a theory, [Lynn] Sagan [nee Margulis] wrote - a theory proposing that 'the eukaryotic cell [cells that contain a nucleus and organelles] is the result of the evolution of ancient symbi­oses. (Symbiosis: the living together of two dissimilar organisms.) She gave her theory the more specific name endosymbiosis, connoting one organism resident inside the cells of another and having become, over generations, a requisite part of the larger whole. Single-celled creatures had entered into other single-celled creatures, like food within stomachs, or like infections within hosts, and by happenstance and overlapping interests, at least a few such pairings had achieved lasting compatibility. So she proposed, anyway. The nested partners had grown to be mutually dependent, staying together as compound individuals and supplying each other with certain necessities. They had replicated - independently but still conjoined - passing that compoundment down as a hereditary condition. Eventually they were more than partners. They were a single new being. A new kind of cell.

No one could say, not in 1967, how many times such a fateful combining had occurred during the early eras of life, but it must have been very rare that the resultant amalgams survived for the long term. Later, there would be ways of addressing that question. Sagan left it open. Microscopy, which was her primary observational mode of research, couldn't answer it.

The little entities on the inside of such cells had begun as bacteria, she argued. They had become organelles - working components of a new, composite whole, like the liver or spleen inside a human - with fancy names and distinct functions: mitochondria, chloroplasts, centri­oles. Mitochondria are tiny bodies, of various shapes and sizes but found in all complex cells, that use oxygen and nutrients to produce the energy packets (molecules known as adenosine triphosphate, or ATP) for fueling metabolism. ATP molecules are carriers of usable energy, like rechargeable AA batteries; when the ATP breaks into smaller pieces, that energy is released for use. Mitochondria are factories that build (or recharge) ATP molecules. To drive the production, mitochondria respire, like aerobic bacteria. Chloroplasts are little particles -- green, brown, or red -- found in plant cells and some algae, that absorb solar energy and package it as sugars. They photosynthesize, like cyanobacteria. Centrioles are crucial too, but for now, I'll skip the matter of how. All these components, Sagan wrote, resemble bacteria by no coincidence but rather for a very good reason: because they evolved from bacteria.

The bigger cells, within which the littler cells were subsumed, had been bacteria too (or possibly archaea, though that distinction didn't exist at the time). They were the hosts for these endosymbioses. They had done the swallowing, the getting infected, the encompassing, and had offered their innards as habitat. The littler cells, instead of being digested or dis­gorged, took up residence and made themselves useful. The resulting compound individuals were eukaryotic cells.

Never mind that 'compound individuals' is oxymoronic. The whole process, as Sagan described it, was oxymoron brought to life - paradoxi­cal and counterintuitive, though supported throughout the paper by her detailed arguments.

All questions and suggestions are appreciated and will be answered.

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