Fatigue

Fatigue is defined as physical and or mental lethargy or exhaustion caused by stress, exercise, being overworked ,illness or disease. For exercise fatigue is described as not being able to maintain a certain intensity of output. And as a result the intensity of activity is reduced. There are three types of fatigue local fatigue which usually refers to fatigue experienced within the a particular muscle group itself. General fatigue is where the fatigue is more general and felt all over the body rather than just in a specific muscle group. Finally chronic fatigue which is long-lasting and often is physical and mental and can be due to chronic-fatigue syndrome or  to overtraining.

Muscular Fatigue

Peripheral muscular fatigue

This type of fatigue results in a decrease in intensity or power able to be produced by the individual. Peripheral meaning it is not part of the central nervous system (the brain and spinal cord) and  more so occurs in the muscles themselves.

Mechanisms of Peripheral fatigue

Fuel Depletion

Depletion of creatine phosphate stores

There is a limited supply of creatine phosphate in the muscles (around 10 seconds at maximal exertion) as a result the ATP-CP energy system can only be used for a limited amount of time before these stores run out. When the stores of creatine phosphate run out the body experiences fatigue as it must rely on either the anaerobic glycolysis energy system or the aerobic energy system which have longer more complex reaction pathways and hence cannot supply ATP at the same rate of the ATP-CP energy system. If there is a reduction in the rate that ATP can be synthesised there needs to be a decrease in intensity as energy cannot be produced fast enough to maintain a given intensity hence fatigue is said to be experienced as the individual can no longer maintain their given intensity.

Depletion of glycogen (also known as ‘hitting the wall’)

Glycogen can be used as both an aerobic or anaerobic fuel it typically last for around 90-120 minutes for sub-maximal endurance activity, as it is in limited supply within the body. As glycogen stores deplete the body must increasingly rely on fat(lipids) as a fuel source. Lipids however require higher amounts of oxygen to breakdown and require far more complex reactions decreasing the rate at which ATP can be produced, any decrease in the rate of ATP production will result in a decrease in intensity at which the activity can be performed. As the intensity cannot be maintained the person is said to be experiencing fatigue.

Accumulation of fatiguing by-products

Accumulation of lactic acid

Lactic acid dissociates in the body into lactate and hydrogen ions (H+), the lactate itself does not lead to fatigue and is usually converted back into glycogen when enough oxygen is available. Within lactic acid the H+ ions cause fatigue as they raise the acidity(lowers pH) within the muscles. pH and acidity are important for the functioning of proteins, therefore H+ ions in the muscles interfere with glycolytic enzymes and contractile proteins within the muscles. This leads to a decrease in the muscles ability to contract and decreases efficiency in the breakdown of glycogen, a decrease in both these factors in particular the efficiency of breaking down glycogen leads to a reduction in the rate that ATP can be produced resulting in a reduction of intensity.

Accumulation of inorganic phosphate(Pi)

Inorganic phosphate is produced by both the breakdown of ATP to form ADP and Pi(inorganic phosphate) and the breakdown of creatine phosphate (CP) in the ATP-CP energy system. The build up of large amounts of Pi can result in the Pi interfering with calcium’s(Ca2+) role in the muscle contractile process. A decrease in the muscles ability to contract will lead to a decrease in the intensity at which the body can work at, causing fatigue.

Mechanisms of Central Fatigue

Is the fatigue that occurs as a result of decreased signalling or transmission in the spinal cord or brain.

Fatigue in the neuromuscular junction

This refers to the transmission of signals between neurons across the synapse which is the gap between neurons, small chemical messengers called neurotransmitters are required to pass the message across this gap. Acetylcholine(ACh) is a neurotransmitter involved in muscle contraction and may decrease as a result of fatigue.

Central Governor theory

Is a theory that suggests the existence of a ‘central governor’ in the brain the can induce fatigue and the experience of fatigue in order to prevent the body from damage due to over exertion. It works by reducing the amount of muscle recruitment if less muscle fibres are recruited there will be a decrease muscle contraction and therefore in intensity.

Thermoregulation

As a result of exercise core body temperature can increase this is due to increased muscle contraction,as  heat is produced as a by-product of energy production added to by an increase in blood flow.

Hyperthermia

Although a small increase in body temperature can increase the action of the muscles, if it rises to high an individual might suffer hyperthermia. In this case when the body’s temperature is too high it will need to sacrifice some energy from exercise into cooling the body so it does not overheat and cause damage. To do this the body redirects blood flow from the muscles to the skin to allow cooling. This also has the effect of reducing the amount of blood getting to the working muscles so that the body must rely more so on anaerobic pathways resulting in the production of fatiguing by-products such as hydrogen ions. Hyperthermia can also result in dehydration(lack of body fluids) which can be incredibly dangerous and will result in decreased performance.

Hyperthermia is likely to occur in hot/humid conditions and dry and hot conditions.

Dehydration

Which can occur as a result of hyperthermia is a loss of body fluids. It can cause a thickening of the blood due to a decrease in blood plasma. Thickening of the blood results in a decrease in the oxygen carrying capacity and pumping of the blood leading to fatigue.

Hypothermia

Is when the body temperature drops too low beyond the optimal temperature. It will result in blood be redirected away from muscles to internal organs which will result in fatigue.It will also result in a decrease in fine motor skills and will ultimately lead to the body shutting down if heat is not regained.