Labelled diagram of the anterior muscles of the human body.

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Supply of energy for muscle contraction

Adenosine Triphosphate (ATP) is the immediate source of (chemical) energy for muscle contraction.

Very little ATP is stored in muscle fibres (= 'muscle cells'), only enough to power muscle contractions for a few seconds.
The ATP in muscles must be constantly replenished as it is used for various processes.

The basic equation to describe the release of energy from ATP for use to power muscle contractions is:

In words:

Adenosine Triphosphate Adenosine diphosphate + inorganic Phosphate + Energy

In symbols (short form):

ATP ADP + Pi + Energy

where the symbols have the meanings indicated on the right.

The process described in the above equation consumes ATP (the ATP is hydrolysed to ADP). Therefore a continuous supply of ATP is needed.

ATP is replenished within muscle fibres in three ways:

  1. from creatine phosphate (anaerobic)
  2. by glycolysis (anaerobic), and
  3. by cellular respiration (aerobic respiration),
    whose initial stage is as for glycolysis, then continues via another pathway that includes oxygen and generates much more ATP
    .


These 3 sources for ATP production in muscle fibres are explained below.

1.

Creatine phosphate

This (creatine phosphate reaction) source can typically release enough energy for muscle contraction for up to about 15 seconds, e.g. to run for a short time / distance.

This process of releasing ATP and hence energy, does not require oxygen so can be described as anaerobic.

This is known as the Alactic or Creatine Phosphate (CP) System.

The short-term reserve for replacement of ATP is creatine phosphate (CP), a molecule produced from excess ATP available in muscle cells when they are at rest.

ADP + CP ATP + C + Energy

When muscles need more ATP to release energy required for muscle contraction, the reaction represented by the above equation proceeds from left to right. Conversely, when the muscle is at rest and excess ATP is available, the reaction represented by the above equation proceeds from right to left.

When the reaction proceeds left-to-right, CP donates its high-energy phosphate group to ADP, forming ATP + creatine.

The enzyme creatine phosphokinase (CPK) catalyzes the reaction, ensuring that equilibrium is reached quickly.
However, after about 15 seconds of muscle contraction, much of the supply of CP has been used so the next source of ATP production is required until the supply of CP has been replenished.

2.

Glycolysis

Glycolysis can typically release enough energy for maximum muscle contraction for approx 30-40 seconds.

This process does not require oxygen so it can be described as anaerobic. However, unlike other metabolic pathways, glycolysis can produce ATP under either aerobic or anaerobic conditions - with different final products.

Glycolysis is the metabolic pathway (i.e. the sequence of reactions) that produces 2 ATP molecules by converting a glucose molecule into pyruvate (which is also known as pyruvic acid), water and NADH. No equation is shown here because this does not occur via one single reaction but via a series of reactions.

The glycolysis metabolic pathway occurs in the cell cytosol in cells all over the body (not just in muscle cells).

It involves a series of 10 anaerobic chemical reactions, as a result of which one glucose molecule produces:

  • 2 molecules of pyruvic acid
  • 2 molecules of ATP
  • 2 molecules of reduced nicotinamide adenine dinucleotide (NADH)
  • 2 molecules of water (H2O)

Introductory courses in biology, human biology and anatomy, physiology and pathology (e.g. ITEC) might not require detailed knowledge of the 10 stages of glycolysis.

After the 10th stage of glycolysis (in which pyruvate is formed) there are two possible routes:

  • Anaerobic glycolysis - the net effect of which is the formation of 2 molecules of ATP per molecule of glucose.
    In this case, which is also known as the Lactic Acid System:

    Glucose + Pi + 2 ADP 2 Lactate (i.e. 'Lactic Acid') + 2 ATP + 2 H2O ,

    which is simplified to "Glucose Lactic Acid" in the simple description of formation of lactic acid.

    That is, the breakdown of glucose, which could have been supplied via blood or it could have come from glycogen stored in the muscle tissue, proceeds via the glycolytic path to lactic acid so the end-product is lactic acid rather than pyruvic acid.

  • Aerobic glycolysis - the net effect of which is the formation of (initially) 7 molecules of ATP per molecule of glucose. However, in this case the pyruvate then enters a mitochondrion where it is oxidized by the tricarboxylic acid (TCA) cycle, which leads to the generation of many more ATP molecules per molecule of glucose - see aerobic cellular respiration, below.

The production of ATP via anaerobic glycolysis (to supply energy for muscle contraction) has advantages and disadvantages:

Advantages:

  • produces ATP even without the presence of O2
    (huge advantage if there is insufficient O2 available for aerobic celluar respiration)
  • happens quickly
    (2-3x faster than aerobic cellular respiration, though at only half the rate of ATP production from creatine phosphate)

Disadvantages:

  • forms only 2 ATP molecules per glucose molecule
    (v. little compared with aerobic cellular respiration)
  • forms lactic acid (which is toxic) at the same rate as it generates ATP
  • pathway limited by the build-up of lactic acid, which creates oxygen-debt.

 

Remember: Glycolysis is the break-down of glucose molecules.
Glucose molecules can reach muscle tissues via blood circulation. They can also be produced within muscle cells by the breakdown of glycogen molecules (glycogen being the form in which glucose is stored muscles).

3.

Aerobic cellular respiration

This process requires oxygen so it is described as aerobic.

Aerobic cellular respiration is especially important for muscle contractions of more than 30 seconds duration as it usually takes about 30 secs - 2 mins for aerobic metabolism to adjust to meet the increased demands of physical exercise.

Aerobic cellular respiration requires oxygen.

Although the whole process starts with glycolysis (which occurs in the cell cytosol, i.e. the fluid inside the cell membrane), many of the reactions of aerobic cellular respiration (e.g. those in the TCA cycle) occur in the mitochondria within the muscle cells.

Initially, the metabolic pathway for aerobic respiration involves the same 10 chemical reactions as anaerobic glycolysis. However, in the presence of oxygen the process of aerobic respiration continues via the TCA cycle (instead of by lactate dehydrogenase reducing pyruvate to lacate - as in anaerobic glycolysis). The aerobic respiration pathway yields more ATP molecules per glucose molecule than anaerobic glycolysis (36 molecules ATP per glucose molecule via aerobic cellular respiration vs 2 molecules ATP per glucose molecule via anaerobic glycolysis).

Aerobic cellular respiration is not only necessary to meet the ATP needs of muscles that are continually active over a long period of time, it is also needed to enable the body to re-pay oxygen debt built-up due to the accumulation of lactic acid from intense bursts of exercise during which the ATP needed to supply energy (the energy stored in the chemical bonds of the ATP) has been supplied anaerobically, i.e. without using oxygen.


More about aerobic vs anaerobic energy systems
:

The differences between aerobic and anaerobic metabolism (i.e. chemical changes that release or store energy) are particularly important for athletes and sports scientists.

Briefly, the top 2 of the above 3 sources of ATP production in muscle cells are anaerobic because they do not require oxygen. Short intense bursts of physical activity tend to involve mainly these processes (1. and 2.) whereas in lower intensity activities sustained over longer time periods, e.g. walking all day, aerobic cellular respiration (3.) is the main source of the ATP needed to release energy to muscle cells. However, in reality, physical exercise does not divide neatly into "short and intense" OR "prolonged at lower intensity". For example, in a game of football most of the players are continuously moving but sometimes they sprint or otherwise use much more energy in a short period of higher intensity physical activity. Therefore, although most of the energy they use for muscle contraction for most of the time is from aerobic cellular respiration, they also use energy from anaerobic processes when necessary.

This can also be explained by considering aerobic cellular respiration as the 'normal' or 'default' source for ordinary activities then stating that during intense bursts of strenuous exercise the systemic circulation system is not always able to supply oxygen to the muscle cells via red blood cells fast enough for all the energy to be supplied to the muscle by aerobic cellular respiration alone. In that case ATP can be supplied quickly by anaerobic processes such as the anaerobic break-down of glucose (or its stored form, glycogen).

The pathways for ATP production in muscles outlined in the table above indicate how muscle fibres source the important reactant Adenosine Triphosphate (ATP).

The basic equation to describe the release of energy from ATP for use to power muscle contractions is:

Adenosine Triphosphate (ATP) Adenosine diphosphate (ADP) + inorganic Phosphate (Pi) + Energy

Another important consideration regarding which source of ATP production is used by muscles is the type of skeletal muscle fibres of which the muscle is composed. Depending on the main function(s) of muscles, they are formed from combinations muscle fibres better suited to some forms of ATP production than others.

See also lactic acid formation, the effects of exercise on muscles and the effects of exercise on circulation.

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