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Advice :: Excercise Physiology

Energy production in the cell
Energy to power muscle contractions is released when oxygen combines with chemical compounds in the cell to produce Adenosine Triphosphate (ATP). This chemical reaction is called oxidation. The amount of energy produced is limited by the amount of oxygen available within the cell and the chemical compounds (carbohydrates, fats, and protein) available to be oxidised (the "fuel").

The foods we eat provide the fuel for cell energy production. They contain three energy containing compounds: carbohydrates, fats, and protein. As you will learn, carbohydrates are the primary energy source for short, maximum performance (anaerobic) events (e.g. sprints). Fats can also serve as an energy source for the cell, but are more important in endurance (aerobic) events (usually performed at less than 50% VO2 max.) Proteins are generally used to maintain and repair body tissues, and are not normally used to power muscle activity.

The cardiovascular system delivers the oxygen necessary for oxidation. The oxygen is extracted from the lungs and transported in the blood (oxygenated) to the cells where it is utilised. The byproduct of energy production, carbon dioxide, is transported back to the lungs by the circulating blood and leaves the body in expired air.

When there is adequate oxygen to support the energy needs of the cell, metabolism is said to be aerobic. When the demand for energy outstrips the ability of the cardiovascular system to provide oxygen for oxidation, a more inefficient form of metabolism, anaerobic metabolism, occurs.

Oxidation and ATP
Food energy is released through a chemical reaction with oxygen in a process called oxidation. When this occurs outside the body, for example 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, this energy is rapidly released as heat and light. In the body however, food energy needs to be released more slowly and in a form that can be harnessed for basic cell functions and transformed into mechanical movement by the muscle cells.

This is accomplished by "refining" the three basic food materials (carbohydrate, fat, and protein) and converting them into a single common chemical compound called adenosine triphosphate (ATP). It is this ATP, synthesised as the cell metabolises (or breaks down) these three basic foods, that transfers the energy content of all foods to muscle action.

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

Production of ATP - Three Pathways
There is a limited capacity to store ATP in the cell, and at maximum work levels this ATP stored in the muscle cells is depleted in several seconds. In order to sustain physical activity, the cells need to continually replenish or resynthesise their ATP. There are three pathways to accomplish this, and which one is used by the cell depends on the level and duration of the physical activity.

The first breaks down phosphocreatine (another high energy, phosphate bearing molecule found in all muscle cells) to directly resynthesise ATP. But it is also in limited supply and provides at most another 5 to 10 seconds of energy, limiting its usefulness to sprint type activities. At this point, the body must switch to either of two other biologic processes to regenerate ATP; one requiring oxygen (aerobic) and another that does not (anaerobic).

Aerobic Metabolism, which is oxygen dependent, is the name for several different chemical processes in the cell, and can produce ATP from all three food elements; carbohydrates, fats, and protein. Aerobic metabolism supplies the ATP needed for endurance activities.

Glycolysis, also known as anaerobic metabolism, is limited to the breakdown of carbohydrates (glucose, glycogen). Anaerobic metabolism is limited by the buildup of lactic acid which begins within minutes and degrades athletic performance by impairing muscle cell contraction and producing actual physical discomfort or pain. Anaerobic glycolysis is the source of energy for short bursts of high level activity lasting several minutes at most (sprints).

The Krebs Cycle
This section is detailed and has a lot of terminology. Hopefully you find it interesting to read as this is how the body produces ATP from our food substrates. Those who are studying science, fitness or are simply wishing to expand their knowledge will hopefully find that we have presented this very complex cycle in a more friendly, readable manner.

The Krebs cycle is also known as the tricarboxylic acid (TCA) cycle and as the citric acid cycle. Sir Hans Krebs worked out the details of the cycle in the 1930's. The Krebs cycle takes place in the mitochondria and consists of eight steps.

Carbohydrate (Glucose) Metabolism
All carbohydrates are first broken down to glucose. Glycolysis breaks this glucose (a six-carbon-molecule) down into two molecules of pyruvate (a three-carbon molecule). Pyruvate moves (via eukaryotes) into the mitochondria. It is converted into acetyl-CoA and enters the citric acid cycle.

The first reaction of the cycle occurs when acetyl CoA transfers its two-carbon acetyl group to the four-carbon compound oxaloacetate, forming citrate, a six-carbon compound. The citrate then goes through a series of chemical transformations, losing first one and then a second carboxyl group as carbon dioxide. Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the Krebs cycle, three molecules of NAD+ are reduced to NADH. In Step 6, electrons are transferred to the electron acceptor FAD rather than to NAD+.

In one turn of the citric acid cycle, two molecules of carbon dioxide and eight hydrogen atoms are removed, forming three NADH and one FADH2. The two carbon dioxide molecules produced accounts for the two carbon atoms of the acetyl group that entered the citric acid cycle. More hydrogen is generated by these reactions than entered the cycle with the acetyl CoA molecule as water is also added during the reactions of the cycle; the hydrogens come from this water.

Because two acetyl CoA molecules (made from three-carbon pyruvate) are produced from each glucose molecule (six carbon molecule), the cycle must turn twice to process each glucose (as only on acetyl CoA is broken down in one turn of the cycle). At the end of each turn of the cycle, the four-carbon oxaloacetate is left, and the cycle is ready for another turn. After two turns of the cycle, the original glucose molecule has lost all of its carbons and may be regarded as having been completely consumed. Only one molecule of ATP is produced directly by a substrate-level phosphorylation with each turn of the citric acid cycle. The rest of the ATP that is formed during aerobic respiration is produced by the electron transport system (cytochrome chain) and chemiosmosis.

Protein Metabolism (Catabolism - Breakdown)
In protein catabolism, proteins are broken down by protease enzymes into their constituent amino acids. These amino acids are brought into the cells and can be a source of energy by being funnelled into the citric acid cycle.

Fat Metabolism (Catabolism - Breakdown)
In fat catabolism, triglycerides are hydrolysed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose (via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate) by way of gluconeogenesis. In many tissues, especially heart, fatty acids are broken down through a process known as beta oxidation, which results in acetyl-CoA; which can be used in the citric acid cycle. Sometimes beta oxidation can yield propionyl CoA which can result in further glucose production by gluconeogenesis in liver.

The citric acid cycle is always followed by oxidative phosphorylation. This process extracts the energy from NADH and FADH2, recreating NAD+ and FAD, so that the cycle can continue. The citric acid cycle itself does not use oxygen, but oxidative phosphorylation does.

The total energy gained from the complete breakdown of one molecule of glucose by 1) Glycolysis, 2) Citric acid cycle and 3) Oxidative Phosphorylation equals 38 ATP molecules. The citric acid cycle is called an “amphibolic” pathway because it participates in both catabolism and anabolism.

The Balance of Aerobic and Anaerobic Metabolism
As one begins to exercise, the anaerobic pathway provides ATP while the body increases breathing and heart rate to deliver adequate oxygen to the cell. As more oxygen becomes available, the aerobic pathways pick up the slack and anaerobic metabolism falls off. However, anaerobic pathways continue to provide a small amount of ATP energy, and small amounts of lactic acid are still being produced. However this small amount of lactic acid is readily metabolised by liver and muscle cells, and does not accumulate to the degree that occurs in anaerobic ATP activity (as in a sprint, for example).

Aerobic pathways are used preferentially by the muscle cells until VO2max. is reached. At this point, the cardiovascular system cannot provide adequate oxygen to the muscle cell to continue aerobic ATP production, and either the phosphocreatine system, or anaerobic metabolism cover the extra energy needs. See the “Energy Production in Cells” Section. When the level of activity once again returns to aerobic levels (less than VO2max), oxygen is once again available to regenerate phosphocreatine and metabolise (clear) the excess lactic acid produced during the 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.

Energy Content of Carbohydrate, Fats and Protein and their Metabolism
The energy contained in equal weights of carbohydrate, fat, and protein is not the same. Energy content is measured in calories (denoted, C). Carbohydrates and protein both contain 4 calories per gram, 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 metabolised 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 (e.g sprint) type activities.

Carbohydrate metabolism is much more efficient than fat metabolism assuming adequate oxygen is available (aerobic metabolism). But once VO2 max 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 metabolised 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 carbohydrate calories are adequate energy for several hours of brisk cycling (80 to 100 % VO2max), and if one slows the pace to 50 - 60 % VO2max where fat calories can be utilised, there are enough energy stores to support cycling at this reduced speed for days.

How can these facts help you in designing a program to maximise your performance? If one does not supplement glucose stores in the body during training, you will run out of carbohydrate stored in your muscle and liver cells after 2 hours of aerobic activity, and fatigue resulting from muscle glycogen depletion occurs (sometimes called the “bonk”). Without adequate carbohydrate to fuel continuous high level muscle activity, it is impossible to maintain a high level of energy output and one has to slow to speeds of 50% VO2 max where fat metabolism can provide the needed calories. The bonk can be delayed by using oral glucose to supplement muscle glycogen stores.

To improve initial CV performance, we suggest the following strategy:
1) Minimise extremely energy inefficient anaerobic sprints earlier in the workout (remember they are very inefficient in terms of ATP production)

2) Whenever possible, workout closer to 50% VO2 max to take advantage of supplemental calories available from fat metabolism.

3) Supplement with glucose during training so that you have more fuel left for that final sprint to the finish line.

We hope you found this section useful. We will end this section with a final not on motivation. Motivation is something that comes from within you. Some do find it hard to motivate themselves to go to the gym or go out for a cycle, some thrive on the though and can't wait to do exercise. For those that enjoy doing exercise all we can say is “that's great”, just remember the principle of over-training; simply put, too much is counter-productive, especially if there is inadequate nutrition and recovery!

For the less motivated, firstly ask yourself “How bad do I want it?”. Then ask “why am I not so motivated?”. There could be many reasons; hard day at work leaving you tired, inadequate sleep, poor eating, ill health, dehydration, injury, girlfriend/boyfriend distraction, family duties, lack of transport or lack of money. The list can go on. The truth is, for most of us, there is always something trying to pull us away from doing exercise. And, even if there wasn't, we could always find some excuse to use so that we either didn't give it 100% effort, or worse, didn't exercise at all. This may sound harsh but the fact is, if you want it bad enough, you'll get your chosen form of exercise done; it may cause you to sacrifice something but it simply comes down to conditioning and discipline. So, when next your feeling the “can't be bothered” blues, give yourself a shake, a splash in the face with cold water and get on with it.

We wish you all the best in your training.