Fatigue
Fatigue in general health terms relates to being tired or 'run down', the concept of being tired can differ from person to person as people react differently to feeling tired.
Fatigue effects everyone, you may feel a liitle tired due to lack of sleep or a long hard days work or in sports science terms you may be fatigued due to your body's inability to meet energy demands and therefore are unable to maintain exercise at your current rate.
Other factors that can cause fatigue include illnesses and diseases, fatigue is also related to stress. In cases such as these, seeing your GP can help to clarify and treat the problem. Another factor can be a poor diet, ensuring you eat the right foods containing the right balance of carbohydrates, protein, vitamins and minerals can help to ensure you're perfectly fuelled throughout the day.
Fatigue in sport
In sport, fatigue is the inability to sustain a given power ouput or speed. What this means is that the body is unable meet the energy demands of the exercising muscles. Runners will experience fatigue when their muscles are unable to continue at their current speed; a footballer experiencing fatigue will become slower when sprinting for the ball and their technical ability will also suffer; in weight training the weight lifter can no longer lift the weight or perform further repetitions. In summary, the exercise becomes much harder to perform, you may experience an emptiness feeling in the legs and it beomes much harder to push yourself.
Why does fatigue develop?
Fatigue develops due to a problem in the muscles and is described as muscle fatigue. However, muscle fatigue occurs differently depending on the type of activity or exercise, the type of exercise being either anaerobic or aerobic.
In anaerobic exercise such as sprinting or activities involving explosive power, fatigue develops due to ATP and PC depletion. Basically the demand for ATP exceeds the available supply.
Activities that last between 30 seconds and 30 minutes, fatigue occurs due to a different factor. When performing activities such as high repetition weight training the body produces lactic acid which is released into the blood stream, this then has to be broken down and removed from the blood stream. The higher the intensity of the exercise the more lactic acid is produced which in turn has to be removed from the blood stream. Fatigue occurs when the rate of lactic acid production exceeds the rate of lactic acid removal from the bloodstream. So during high intensity exercise lasting upto 30 mins there is a gradual increase in muscle acidity. This is not good for the muscles as an acidic environment hinders muscle contraction, continued muscle contraction in such an environment can cause cell death therefore the high intensity exercise cannot be maintained. A burning sensation can normally be felt in the exercising muscles due to the build up of lactic acid and is the body's way of preventing you from causing muscle cell destruction.
When performing moderate to high intensity aerobic exercise lasting longer than 1 hour fatigue occurs when muscle glycogen stores are depleted. Liver glycogen stores can help to maintain blood sugar levels and supply carohydrate to the muscles, however their is only a limited supply of liver glycogen and it will eventually run out resulting in fatigue.
During aerobic exercise that is of low to moderate intensity lasting more than three hours, fatigue is caused when first muscle glycogen stores are depleted and then when the body switches to your fat stores as an energy source (aerobic lipolytic system), eventually your body is unable to convert the fat into energy fast enough to supply your muscles. Other factors also come into play as even if you reduce your intensity, concentration levels of chemicals such as serotonin rise which effect the body causing tiredness and muscle damage.
Delaying fatigue
As we know, fatigue develops due to a problem in the muscles such as depletion of muscle glycogen. In all types of activity, glycogen is used as an energy source, therefore ensuring you have sufficient stores of glycogen in your muscles, and in certain events, your liver, before you exercise can ensure you delay fatigue, thus improving perormance.
The greater the muscle glycogen stores the longer you will be able to exercise at a given intensity, so obviously low muscle glycogen will encourage earlier fatigue and reduce endurance or result in smaller training gains. Ensuring you have the correct diet, made up of sufficient amounts of carbohydrate willl ensure you delay fatigue and help to improve performance and training gains.
Another way of delaying fatigue is by reducing the rate in which your body uses up muscle glycogen. One way of doing this is through pacing yourself during exercise, another way is through regular training. Endurance training such as long distnace running can improve your body's efficiency, resulting in you being able exercise for longer.