Table 1 - The body's stores of energy and their duration of effect.
The stores of ATP never drop significantly since the rate of demand never exceeds the rate of supply. ATP like muscle creatine phosphate contains a high energy phosphate bond which provides energy for muscle contraction, especially in short, high speed contraction exercise such a 100m sprint.
Creatine phosphate + ADP ---> creatine + ATP
The other supply of energy for anaerobic exercise is the glycogen-lactate system (Figure 1). This provides ATP for muscle contraction, but because not enough oxygen is being supplied to the glycolysis occurring in the muscle, lactate is formed, at the cost of some energy. For prolonged exercise the production of lactate and therefore anaerobic exercise is undesirable since the build up in lactate causes a change in pH that leads quickly to fatigue.
Figure 1 - Anaerobic glycolysis resulting in the formation of lactate.
By careful running, an athlete undertaking a marathon run ensures that their pace does not exceed the amount of oxygen that can be supplied by the body for aerobic glycolysis. Glycolysis is activated by adrenaline, an increase in the AMP to ATP ratio and an increase in Ca2+ concentration resulting from nervous system stimulation. During prolonged exercise such as a marathon run both liver and muscle glycogen are used, however the rate of use is limited by the blood glucose concentration. It is important that blood glucose is maintained since any reduction can effect the brain which requires 5g of glucose an hour. A reduction of blood glucose by fifty percent is enough to result in brain damage.
Throughout a marathon the respiratory quotient can be measured, providing a value as to the ratio of CO2 produced to O2 consumed. It is found to be between 0.8 and 0.9 for a marathon run, and since the RQ for glycogen oxidation is 1, and the RQ for fatty acid oxidation is 0.7, this shows that both are occurring throughout the marathon run.
Adipose triglycerides are therefore used in some quantity throughout the marathon, a mechanism that occurs to maintain the stores of glycogen, helping them to last as long as possible. However the amount of glycogen oxidation and the amount of fatty acid oxidation, as well as some ketone body oxidation, are controlled firstly, by the release of hormones, and secondly, by the inhibiting action of free fatty acids and ATP on glycolysis.
During prolonged exercise variations in the plasma concentration of certain hormones influence the substrate usage by the muscles (Figure 2). An increase in adrenaline and glucagon concentrations results in an increase in triglyceride breakdown through the activation of adipose tissue lipase, as well as the activation of hepatic glycogenolysis and gluconeogenesis. An increase in plasma growth hormones also leads to an increase in adipose tissue lipolysis. The majority of fatty acid versus glycogen control is carried out by the inhibitory effect of both fatty acids and ATP on glycolysis at the fructose 6-phosphate to fructose 1,6-bisphosphate reaction. This inhibition acts to ensure that the supply of ATP for glycolysis does not exceed the demand and that some fatty acid oxidation always occurs. The problem with fatty acid oxidation is reflected by that fact that it can only achieve fifty percent of maximum power, this is because its rate of supply is limited by the transport rate from adipose tissue, limited by the amount of albumen in the blood available for transport.
Figure 2 - Hormone effect and substrate use in prolonged exercise.
As the marathon progresses, after an hour or so, most of the glycogen stores have been used up and this results in an increase in the oxidation and plasma concentration of fatty acids, acting to spare the remaining glycogen for oxidation in the brain. The marathon is in effect run on the glycogen and when most of this is used up, fatty acid oxidation becomes the major energy provider, resulting in reduce performance and fatigue.
3. FATIGUE
An increase in ATP consumption results in an increase in carbohydrate rather than fatty acid consumption, which progresses until stores of carbohydrate are reduced to a critically low level, the result being fatigue .
In prolonged exercise fatigue occurs when almost all the glycogen stores have been used up, however the actual cause of fatigue is uncertain. Non-metabolic fatigue is thought to be caused by dehydration , resulting in the loss of body fluid and therefore blood volume, or by an increase in body temperature, which requires additional cooling resulting in a decreased blood supply to the muscles. However non-metabolic fatigue can be overcome through the addition of liquids and by taking other precautions during prolonged exercise .
Fatigue in a marathon usually occurs at around the 18th mile, a phenomenon know as hitting the wall, at which point running becomes very painful and there is a great desire to stop. Up until this point the runner has been utilising 80% of their maximum aerobic capacity, but after this point there is little muscle glycogen available and the ATP requirement can no longer be satisfied by increasing the rate of fatty acid oxidation. This results in a temporary fall in ATP concentration which in turn results in a decrease in output by the inhibition of myosin ATPase, until an equilibrium is reached where the amount of ATP being produced is equal to the demand at the reduced pace. The equilibrium point occurs where most if not all of the ATP can be provided by fatty acid oxidation. It has also been suggested that exercise induced hypoglycaemia may be a major factor contributing to fatigue, however this seems to be unlikely.
The case of pain in fatigue in uncertain however it has been suggested that the release of a chemical occurs that increases in concentration with decreasing ATP, and which acts to stimulate nerve endings in the muscles, a stimulus the brain receives as pain.
Glucose can however be provided in cases of fatigue by gluconeogenesis, the action by which amino acids are converted into glucose in the liver from the break down of muscle proteins, however this is undesirable since it results in muscle damage.