Volume 10, Number 2, January 2015

Revision

Causes of fatigue

Sue Young

There are many causes of fatigue and these depend on the intensity and duration of the activity. For example, a marathon runner will fatigue through glycogen depletion whereas a 400 metre runner will fatigue through lactic acid build-up.

Glycogen depletion

Glycogen stores are limited — the body has enough glycogen to last approximately 90 minutes. When glycogen stores are depleted athletes are said to ‘hit the wall’ as the body tries to metabolise fat but is unable to use fat as a fuel on is own.

Lactic acid build-up

An accumulation of lactic acid releases hydrogen ions. These hydrogen ions cause an increase in the acidity of the blood plasma. This inhibits enzyme action and therefore the breakdown of glucose, and irritates nerve endings causing pain.

Reduced rate of ATP synthesis

When stores of ATP and PC are depleted there is insufficient ATP to sustain muscular contractions.

Dehydration

Water is lost through sweating during exercise and if it is not replaced then dehydration occurs. Dehydration can have an effect on blood flow to the working muscles and result in a loss of electrolytes such as calcium, which help with muscular contractions. Blood viscosity increases and blood pressure reduces. There is a reduction in sweating to prevent further water loss, which in turn increases core body temperature. This results in the performer being unable to meet the demands of the activity.

Reduced levels of calcium

For muscle contraction to occur there has to be a release of calcium. When there is an increase in hydrogen ions this decreases the amount of calcium that is released from the sarcoplasmic reticulum, thus affecting muscle contraction.

Reduced levels of acetylcholine

Acetylcholine is a neurotransmitter that can help a nerve impulse to jump the synaptic cleft (the gap that separates the nerve ending from the muscle fibre) and initiate muscular contraction. When levels of acetylcholine are low the muscles become fatigued.


Thermoregulation

Key term: thermoregulation — keeping body temperature maintained within certain boundaries

During exercise, heat is generated in the body as a result of the chemical reactions (metabolic processes) that take place to produce energy. Long distance runners can sometimes experience difficulty with temperature regulation. The heat that is produced through muscle contraction raises the core body temperature, which causes blood viscosity to increase and metabolic processes to slow down. This means the performer is unable to sweat efficiently and dehydration occurs. The thermoregulatory centre in the medulla oblongata controls temperature. Heat is transported to the surface of the skin by the blood and the vessels vasodilate enabling heat to be lost through radiation, conduction, convection or through the evaporation of sweat.

During prolonged exercise or when the body is dehydrated, total blood volume can decrease as more blood is redirected to the skin. This reduces both the volume of blood and the amount of oxygen available to the working muscles and therefore affects performance. In hot conditions this situation is exacerbated so it is important to acclimatise so the body can modify the control systems that regulate blood flow to the skin and sweating.

Heat is lost through:

·  radiation — no physical contact is needed/heat is lost by infrared rays

·  conduction — heat is lost through the skin to the air

·  convection — air cools

·  evaporation — heat is lost through sweating

During exercise sweat evaporation becomes the main cooling mechanism.

Tasks to tackle

Give an example from a sports activity where the main cause of fatigue is:

a glycogen depletion

b lactic acid build-up

c temperature regulation


Offsetting fatigue

·  Train the relevant energy system using an appropriate method: for example, continuous training uses the aerobic system.

·  Spare glycogen levels: marathon runners need to pace themselves — going too fast will speed up glycogen metabolism.

·  Keep hydrated: drink fluid with a carbohydrate level of no more than 6% carbohydrate throughout a performance to boost blood glucose levels.

·  Try glycogen loading to optimise levels of glycogen before an event to enable endurance based activity to last for longer. This is a form of dietary manipulation involving maximising glycogen stores and is often used by long distance runners. Six days before an important competition the performer eats a diet high in protein and fats for 3 days and exercises at relatively high intensity so that glycogen stores are depleted. This is followed by 3 days of a diet high in carbohydrates and some very light training. Studies show that this increases the stores of glycogen in the muscle and can prevent performers from ‘hitting the wall’.

Advantages

·  Increased glycogen synthesis

·  Increased glycogen stores in the muscle

·  Delays fatigue

·  Increases endurance capacity

Disadvantages

·  Water retention, which results in bloating

·  Heavy legs

·  Affects digestion

·  Weight increase

·  Irritability during the depletion phase

·  Alters the training programme

This resource is part of PE Review, a magazinewritten for A-level students by subjectexperts. To subscribe to the full magazine go to www.hoddereducation.co.uk/pereview

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