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.
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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
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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 magazine written for A-level students by subject experts. To
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