Glucose, glycogen and triglycerides are the three main sources of energy for muscle contraction. These compounds are broken down to form adenosine triphosphate (ATP). ATP is the immediate energy source that actually powers muscle contraction.
When the human body undergoes exercise at an intensity where oxygen can be supplied to muscle cells at a rate equal to its demand the exercise is said to be aerobic.
Glucose and glycogen are the body's carbohydrate energy molecules. Glucose is a monosaccharide, which is soluble in water and is transported in the blood throughout the body. It is used as a respiratory substrate (a molecule that energy can be released from) inside cells. Glycogen is a polysaccharide (a polymer of glucose) - it is stored primarily in liver cells - it is the body's storage form of carbohydrate (just as starch is in plants). To utilise glycogen it must first be broken down into glucose molecules (shown in the diagram below). The role of glycogen during exercise is to maintain the supply of glucose to muscle cells.
As can be seen from the diagram above glycogen is formed from the condensation of glucose molecules. This normally happens after a meal is eaten.
When we exercise there is an increased demand for ATP (to power muscle contraction). ATP is generated by respiration and glucose is a molecule that is often respired in muscle cells to release energy. The glucose in our blood is only sufficient for about 5 minutes of activity - therefore when we exercise glycogen in the liver is broken down by hydrolysis into glucose and transported in the blood to muscle cells. The branched structure of glycogen aids this process
Human bodies contain enough glycogen to last for 80 minutes of running. This explains why "the wall" is hit at about 19 miles into a marathon - the author can confirm this is not pleasant. However the glycogen store is rarely totally used up because after about 20 minutes of exercise triglycerides stored in adipose tissues are converted into fatty acids and glycerol. The fatty acids pass into the blood and can be respired to generate ATP in the muscle cells
The way in which fuels are used to provide ATP when oxygen is present as exercise continues is summarised by the diagram below.
A lot more ATP can be generated from glucose when oxygen is present so aerobic exercise is more efficient. Anaerobic respiration is less efficient but when the intensity of exercise is very high (e.g. during sprinting) the rate of oxygen supplied to the muscle cells can be insufficient to meet the demand for its use. So in order to continue producing ATP anaerobic respiration can take place.
The equation for the anaerobic respiration of glucose is shown below
glucose lactate (+ 2 ATP generated)
Anaerobic respiration produces lactate which causes muscles to fatigue and can cause cramp or a stitch. Therefore exercise that lasts for a long duration must be aerobic to prevent the build up of lactate.
Lactate causes muscle fatigue because it increases the acidity of the muscle cells (lowers pH). The change in pH inhibits the function of muscles in a number of ways. One way is that the lower pH is further from the optimum pH for the enzymes involved in muscle contraction.
The removal of lactate from the body requires oxygen therefore it does not take place until the intensity of exercise decreases or exercise stops. Oxygen is used to oxidise lactate and convert it into glucose in the liver. This requirement for oxygen causes EPOC (excess post-exercise oxygen consumption) and it is 'paying off' what is termed the oxygen debt (the oxygen needed to oxidise lactate).