The Mechanisms of fatigue, including neuromuscular fatigue and list the effects of metabolites (lactate/carbon dioxide)

Failure of neural transmission

This creates fatigue as no impulse can be transported so therefore no muscular contraction can take place. For a muscular contraction to take place the nerve impulse must pass through the synaptic cleft in order to reach the muscular fibre. The transport of the impulse cannot occur in fatigued muscles because acetylcholine cannot be replenishment and put back into the synapse. A sporting example of when this occurs is near to the end of a 5000m run.

An Increase in metabolic bi products

As a result of the body using an energy system a bi-product occurs, this is shown in table 1:

System

Fuel

Bi Product

ATP/PC

PC

None

Lactic Acid

Carbohydrates

Lactic Acid

Aerobic

Carbohydrates

Fats

Carbon Dioxide

(Table 1)

As you can see from the table the two main bi products which cause fatigue are lactic acid and carbon dioxide.

Lactic Acid is being produced all the time but the body is able to deal with these lactic acid levels as it is cleared by the blood during a process known as buffering, buffering is the removal of acid from muscles via blood transportation. During high intensity exercise the body creates high amounts of lactic acid and the body is unable to clear this high accumulation as buffering cannot work fast enough. A sporting example of when this occurs is after a 400m race. The lactic acid which is produced is not what causes muscular pain, the hydrogen ions in the lactic acid is what acts on the body to cause pain and increase in acidity in the muscles. This results in the restriction of fuels working effectively and enzyme activity and therefore causes muscular fatigue.

The other bi product which is produced all the time through the aerobic system is carbon dioxide. Whilst the body is resting carbon dioxide is usually removed by the breathing mechanics but as the body is exercising a higher level of carbon dioxide is taken in. Chemo receptors detect these higher levels of carbon dioxide and in response the body's breathing rate increases in order to clear the carbon dioxide. When carbon dioxide accumulates in the body it gets converted in carbonic acid which has a similar affect as lactic acid, the higher acidity levels restricts muscle action and therefore causes fatigue. A sporting example of when this occurs is after lots of repeated sprints.

Figure 1

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As the body exercises intensely blood lactate levels increase and the lactate begins to accumulate in the blood. This is known as the onset of blood lactate accumulation (OBLA). OBLA is the point at which lactic acid accumulates in the muscles. 'OBLA is known as the anaerobic threshold and is considered to be somewhere between 85% and 90% of your maximum heart rate'. OBLA is shown in figure 1. (Stafford Brown et al 2001).

As the athlete begins to exercise and therefore become fitter it affects the onset of blood lactate accumulation. In a fitter athlete the point of which the blood lactate level begins to increase and build up will be longer. Therefore looking at the graph in figure 1 on the point in which the blood lactate would begin to increase would be more around the point of speed level 10 and around the 4 at blood lactate level.

Due to the effects of exercise the body experiences a lack of oxygen this is known as oxygen debt. Oxygen debt occurs when body works anaerobic ally as this happens when phosphocreatine stores become depleted and therefore lactic acid builds up within the muscle as there is not enough oxygen available to help break down the lactic acid and convert it back into pyruvic acid. Oxygen debt is also known as excess post-exercise oxygen consumption (EPOC).

The roles of EPOC are to reduce pH levels, remove carbon dioxide and replace ATP via the aerobic system. For this to occur the athletes heart rate and respiratory rate should kept at a high rate than their resting level in order to put back the oxygen into the muscles so they are able to recover. After high intensity exercise for example a 1500m run, in order for the muscles to recover and work again ATP must be replaced. PCr stores must be replenished, all lactic acid build up within the muscles must be removed, oxygen and glycogen levels must be restored within the muscles. The faster these factors are completed the quicker the athlete can recover and therefore exercise again.

The oxygen debt consists of two components; alactic oxygen debt and lactic acid. Alactic oxygen debt is the recovery process after events which do not use lactic acid therefore events lasting up ten seconds use this recovery process. This process is quick as ADP is converted into PC and ATP through the aerobic system. When the ATP/PC system is used this also occurs whereas the lactic acid recovery system takes time as lactic acid has to be removed which is a slow process. The recovery time depends upon the intensity of the exercise.

These recovery processes can be helped by an active warm down by doing low intensity exercise as this buffers acid in the muscles and quickens up the removal of lactate and reduces soreness and the chance of delayed muscle soreness. To recover quicker from using the lactic acid or anaerobic systems the athlete needs to increase their heart rate, as done so in an active warm up. This will then also increase blood flow, this will result in the reduction of lactic acid and carbon dioxide levels and this will also increase oxygen levels.

Below is a table which shows sporting examples to show how recovery is different for different energy systems.

Sport

Energy System

Recovery

00m

ATP-PC

In order for an athlete to recover from 100m the alactic recovery system is used. ADP is converted into PC and ATP through the aerobic system. ATP must be replaced. PCr stores must be replenished and lactic acid build up within the muscles must be removed. This is achieved by oxygen being put back into the muscles so buffering can occur and glycogen being restored.

5000 meters

Aerobic System

After the body has worked aerobically in order for the body to recover myoglobin and haemoglobin stores must be replenished. This occurs by oxygen being put back into the body. The oxygen being put back into the muscles allows buffering to occur and glycogen being restored.

(Table 2)

Exercise Physiology

IVA Assignment

Task 4

Vikki Keeping

Exercise Physiology

IVA Assignment

Task 4

Vikki Keeping