Energy is supplied to all cells including muscle cells by ATP generated from carbohydrates, fats and proteins .The unit of energy in the cells is called ATP (Adenosine Triphosphate)

The body have 4 methods to create ATP, they vary by speed and whether they burn oxygen or not. These are listed 1-4 below;

How The Body Produces Energy

1)  Aerobic Respiration (or cellular respiration) The energy your body requires just to live, do everyday activities and cardio exercise such as long distance running, is generated by a sugar burning process in the bodies cells known as aerobic respiration. While this process yields more energy than the anaerobic systems, it is also less efficient and can only be used during lower-intensity activities, such as long-distance running.

This is the process by which a cell uses oxygen to burn glucose and make ATP.

There are 4 parts to this;

  • Glycolysis – Occurs in the cytoplasm This produces 4 ATP and 2 NADH, from Glucose (from sugar foods or carbohydrates ) but uses 2 ATP to get there- so results in 2 ATP + 2 NADH and Pyruvate
  • The Krebs or Citric Acid Cycle- Occurs in the mitochondria –Involves 2 steps.
  • The conversion of Pyruvate to Acetyl CoA (See Acetyl Co A Support formula), makes 2x NADH and 2x CO2
  • Acetyl Co A enters a chain of events making 6x NADH, 2xFADH2, 2xATP and 4xCO2

    Total number of molecules = 8 NADH, 2FADH2,2 ATP,6 CO2.

    Electron transport phosphorylation (chain) Occurs in mitochondria. The purpose of this final stage is to break down the NADH and FADH2 pumping hydrogen ions into the outer compartment of the mitochondria. In this reaction 32 molecules of ATP are generated as hydrogen ions move down through its concentration gradient through an enzyme called ATP Synthase.


    Glucose  + Oxygen-Carbon Dioxide + Water + 36 ATP (Energy molecules)

    The body releases Carbon Dioxide and water This will theoretically burn the highest amount of calories.

    2)Anaerobic Respiration Occurs in the cytoplasm. This is effective for vigorous exercise of between 1-3 minutes duration, such as short sprints. If the intense exercise requires more energy than can be supplied by the oxygen available, your body will partially burn glucose without oxygen (anaerobic). The purpose is to generate NAD+ by reducing pyruvate. In the absence of oxygen it is the only way to generate NAD+ to use in glycolysis.

    Glucose - 2ADP - 2ATP - 2 Pyruvate - 2 Lactate - Lactic Acid 

    Lactic acid is toxic to animals and as it builds up in the muscles this is the “burn “ felt under strenuous activity. If more than a few minutes of this activity are used to generate ATP, lactic acid acidity increases causing pain.

    When sugar is only partially broken down, one of the by-products is lactic acid. Then, with enzymes, glucose is broken down to produce lactic acid; this process creates enough energy to couple with the energy requirements to resynthesize ATP.

    When H+ ions accumulate in the muscles causing the blood pH level to reach very low levels, temporary muscular fatigue results. Another limitation of the lactic acid system that relates to its anaerobic quality is that only a few moles of ATP can be resynthesized from the breakdown of sugar as compared to the yield possible when oxygen is present. This system cannot be relied on for extended periods of time.

    >The lactic acid system, like the ATP-PC system, is extremely important to us, primarily because it too provides for a rapid supply of ATP energy. For example, exercises that are performed at maximum rates for between 1 and 3 minutes depends heavily upon the lactic acid system for ATP energy. Also, in some performances, such as running 1500 meters or a mile, the lactic acid system is used predominately for the “kick” at the end of a race.



    3) Beta Oxidation/Gluconeogenesis or Fat Burning (aerobic lipolysis) This is the slowest system. It is too slow for energy production during exercise. Fats consist of a glycerol backbone with 2-3 fatty acids connected to it. The body absorbs fats and then breaks off the fatty acids from the glycerol.

    The fatty acids are then broken down to 2 carbon units and then converted to Acetyl Co A to enter the Krebs cycle. Fats supply a good source of energy, a short chain fatty acid (8 carbon) can supply 48ATP, and a fatty acid with 3 chains can supply 144 ATP. ! This makes them hard to burn to lose weight.

    The glycerol part of the fatty acid is converted to glyceraldehyde phosphate, an intermediate of glycolysis.

    Although not the preferred method of producing ATP, the body uses this when carbohydrate is not available.

    Lipid metabolism is one of the main ways that the body’s glycogen  (glucose stored in the liver and muscles) store is replenished after exercise. So after a workout on an empty stomach, wait for an hour after exercise before eating as your glycogen stores are more depleted after exercise and so more fat will be burnt with gluconeogenesis.



    4) ATP Phosphocreatine- the fastest source of energy. When energy needs to be generated as fast as possible- in short sharp bursts, the body relies on each cells small amount of energy which soon runs out. The fastest way to grab another phosphate to convert ADP to ATP is to grab it off the cells Creatine phosphate, creating extra ATP to make the cells energy last a little longer. Short, sharp explosive bursts of exercise (10-30 secs) use this system. It doesn’t require oxygen but is very limited to short periods of explosive exercise, such as a sprint or weight/power lifting. This is why Creatine supplementation helps this sort of exercise, ensuring there is adequate Creatine phosphate to provide those precious little phosphates. The ATP-CP system usually recovers 100% in 3 mins; so recommended rest time in between high intensity training is 3mins.


    ATP stores in the muscle last approx. 2 seconds.

    ATP is resynthesized from Creatine Phosphate and will continue until CP stores are depleted (approx. 4-6 secs) (Longer if athlete supplements with CP).

    There is some controversy about how long each energy creating pathway lasts but the general opinion is;

    1-4 secs Anaerobic ATP in muscles

    4-10 secs Anaerobic ATP+ CP

    10 to 45 seconds Anaerobic ATP + CP + Muscle glycogen

    45 to 120 seconds Anaerobic, Lactic Muscle glycogen
    120 to 240 seconds Aerobic + Anaerobic Muscle glycogen + lactic acid
    240 to 600 seconds Aerobic Muscle glycogen + fatty acids.

    The result of muscle contraction produces ADP which when coupled with CP regenerates ATP. CP is stored in the muscles. Actively contracting muscles obtain ATP from glucose stored in the blood stream and the breakdown of glycogen stored in the muscles. Exercise for longer periods requires the complete oxidation of carbohydrates or free fatty acids in the mitochondria. The carbohydrate store will last approximately 90 minutes and the free fatty store will last several days.

    Image Credit:

    Alison ClaireGoogle+