Muscle Metabolism

Last week, I wrote about the muscles, their locations and the naming conventions used to identify the individual muscles.  Today, I’m going to introduce you to Metabolism and how the muscles utilise the various energy systems available.

As a muscle contracts, ATP or Adenosine Triphosphate, a chemical compound breaks down to supply the energy needed by the muscle to fuel the contraction.  Muscles can only store 4 to 6 seconds worth, at the most, of ATP which is really only just enough to get you going.  Because ATP is the only energy source used directly for muscle contraction, it must be regenerated as fast as it is broken down if contraction is to continue.

Fortunately, after ATP is hydrolysed to Adenosine Diphosphate or ADP an inorganic phosphate in muscle fibres, it is regenerated within a fraction of a second by one or more pathways.  They are:

1. Direct phosphorylation of ADP by creatine phosphate or, addition of an additional phosphate molecule taken from the creatine phosphate.
2. Anaerobic glycolysis, which converts glucose to lactic acid; and,
3. Aerobic respiration.

Direct Phosphorylation of ADP by CP

As we begin to exercise vigorously, the demand for ATP soars and consumes the ATP stored in working muscles within a fraction of a second.  Then creatine phosphate (CP) is tapped to regenerate ATP while the metabolic pathways adjust to the suddenly higher demand for ATP.

Coupling CP with ADP transfers energy and phosphate molecule from CP to ADP to form ATP almost instantly.  Muscle cells store two or three times more CP than ATP.  The CP-ADP reaction is so efficient that the amount of ATP in muscle cells changes very little during the initial period of contraction.

Together, stored ATP and CP provide for maximum muscle power for about 15 seconds; long enough to power a 100 metre sprint.

Anaerobic Path Glycolysis & Lactic Acid Formation

As ATP and CP are exhausted, more ATP is generated by breaking down glucose obtained from the blood or glycogen stored in the muscle.  The initial phase of glucose breakdown is glycolysis.  This pathway occurs in both the presence and the absence of oxygen, but because it does not use oxygen, it is an anaerobic (without oxygen) pathway. 

Ordinarily, pyruvic acid produced during glycolysis then enters the mitochondria and reacts with oxygen to produce still more ATP in the oxygen-using pathway called aerobic respiration, described shortly.  But when muscles contract vigorously and contractile activity reaches about 70% of the maximum possible, e.g. running 400 metres with maximum effort, the bulging muscles compress the blood vessels within them, impairing blood flow and oxygen delivery.  Under these anaerobic conditions, most of the pyruvic acid produced during glycolysis is converted into lactic acid, and the overall process is referred to as anaerobic glycolysis.  Therefore, during oxygen deficit, lactic acid is the end product of cellular metabolism of glucose.

Most of the lactic acid diffuses out of the muscles into the bloodstream.  Subsequently, the liver, heart, or kidney cells pick up the lactic acid and use it as an energy source.  Additionally, liver cells can reconvert lactic acid to pyruvic acid or glucose and release it back into the bloodstream for muscle use, or convert it to glycogen for storage.

The anaerobic pathway harvests only about 5% as much ATP from each glucose molecule as the aerobic pathway, but it produces ATP at a rate that is almost 2 ½ times faster.  For this reason, when large amounts of ATP are needed for moderate periods (30 – 40 seconds) of strenuous muscle activity, glycolysis can provide most of the ATP needed as long as the required fuels and enzymes are available.  Together, stored ATP and CP and the glycolysis-lactic acid pathway can support strenuous muscle activity for nearly one minute.

Although anaerobic glycolysis readily fuels spurts of vigorous exercise, it has shortcomings.  Huge amounts of glucose are used to produce relatively small harvests of ATP and the accumulated lactic acid is partially responsible for muscle soreness during intense exercise.

Aerobic Respiration

Because the amount of stored creatine phosphate is limited, muscles must metabolise nutrients to transfer energy from food to ATP.  During rest and light to moderate exercise, even if prolonged, 95% of the ATP used for muscle activity comes from aerobic respiration.  Aerobic respiration occurs in the mitochondria, requires oxygen, and involves a sequence of chemical reactions that break the bonds of fuel molecules and release energy to make ATP.

Aerobic respiration, which includes glycolysis and the reactions that take place in the mitochondria, breaks down glucose entirely.  Water, carbon dioxide, and large amounts of ATP are its final products.

The carbon dioxide released diffuses out of the muscle tissue into the blood, to be removed from the body by the lungs.

As exercise begins, muscle glycogen provides most of the fuel.  Shortly after, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are the major sources of fuels.  After about 30 minutes, fatty acids become the major energy fuels.  Aerobic respiration provides a high yield of ATP but it is slow because of its many steps and it requires continuous delivery of oxygen and nutrient fuels to keep it going.

Energy Systems Used During Sports

As long as a muscle cell has enough oxygen, it will form ATP by the aerobic pathway.  When ATP demands are within the capacity of the aerobic pathway, light to moderate muscular activity can continue for several hours in well-conditioned individuals.  However, when exercise demands begin to exceed the ability of the muscle cells to carry out the necessary reactions quickly enough, anaerobic pathways begin to contribute more and more of the total ATP generated.  The length of time a muscle can continue to contract using aerobic pathways is called aerobic endurance, and the point at which muscle metabolism converts to anaerobic glycolysis is called the anaerobic threshold.

Activities that require a surge of power but last only a few seconds, such as weight lifting, diving, and sprinting, rely entirely on ATP and CP stores.  The more on and off or burst activities of tennis, football and karate kumite bouts appear to be fuelled almost entirely by anaerobic glycolysis.  Prolonged activities such as marathon runs and jogging, where endurance rather than power is the goal, depend mainly on aerobic respiration using both glucose and fatty acids as fuels.  Levels of CP and ATP don’t change much during prolonged exercise because ATP is generated at the same rate as it is used.  Compared to anaerobic energy production, aerobic generation of ATP is relatively slow, but the ATP harvest is enormous.

Now that you know a little more about how the body is constructed and the different ways in which our bodies utilise the available energy sources, we will now begin investigating physiology.  The science of movement and how everything we have looked at so far combine to make movement possible. Check in again tomorrow for another fascinating look at the human body.