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Asked by eziornic to Mark, Jenni, Helen, Martin, Stu on 14 Jun 2011. This question was also asked by limey5298, cupcake.
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Mark Burnley answered on 11 Jun 2011:
That’s a great question, as fatigue is what I study and I’m trying really hard to find out the answer to it. Fatigue can mean different things to different people. It could mean tiredness after a day in the classroom (teachers get tired too, I know from experience!), or feeling run down. In sports science we usually mean muscle fatigue. We define fatigue as “a drop in a muscle’s ability to produce force that is recoverable by rest”. If you lift a weight several times it gets harder and harder to do it. That sensation can also be called fatigue, so fatigue can be both mental and physical. Eventually, no matter how hard you try, you can’t lift the weight anymore. A physiologist would then say that you are exhausted because you have failed the task you were attempting.
I’ll briefly talk about the physical side of fatigue. The muscle can become fatigued because its internal ability to produce force is reduced (this is called “peripheral fatigue”) or because your brain cannot activate the muscle as you would like it to (this is called “central fatigue”). We know from a lot of recent experiments that both the muscle and the brain fatigue during exercise, and more importantly we now think that the muscle and brain are always communicating with each other to try and succeed in a task (like running 5 km, for example). One interesting thing about the muscle itself is that it can be very powerful (throwing and lift things, for example), or it can sustain activity for a very long time (like walking around all day). But it can’t do both at the same time. The price the muscle pays for being very powerful is that these activities cause fatigue to happen very quickly. This fatigue involves a build up of sustances that cause fatigue (such as hydrogen ions and inorganic phosphate), as well as changes in the chemical composition of nerve cells that make them harder to use.
*And so to answer the question!*:
The body deals withe fatigue by resting and recovering, which allows all of the sustances that built up to be removed so that the body can be ready to work again. This process can take a few seconds or it can take hours, especially if you have done a lot of work and the amount of sugar in your blood (glucose) and muscles (glycogen) has been used up.
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Jenni Tilley answered on 13 Jun 2011:
I don’t know much about how the body physically copes with fatigue, but I do know that it copes much better than metals and ceramics.
In metals and ceramics, fatigue is damage caused by repeated loading and unloading, and is causes huge problems in structures like aeroplanes and bridges. This damage cannot be repaired by resting like it can in the body so instead engineers define a ‘fatigue life’ – the maximum amount of time an object can be used before it has to be retired because it miht break due to fatigue.
Imagine is we did the same thing with footballers and rugby players….
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Helen O'Connor answered on 14 Jun 2011:
I don’t know the ‘science’ behind fatigue, and I think Mark’s answer did that really well. What I know about fatigue is when athlete’s experience it – like when they hit “the wall” during a marathon (between mile 20-23 usually) and they need to use mental strategies, like distraction (thinking about other things) just to push through the mental and physical fatigue. I also know that some athletes can over-train (they train too hard without having adequate recovery time) and this can lead to athlete burnout, or ‘over training syndrome’.
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Jenni commented on :
Just as an aside, I know nothing about fatigue in the body but I know lots about fatigue (damage and cracks associated with repeated cycles of loading) in metals. It causes huge problems in things like airoplane wings, because unlike in biological material, fatigue is not reversible in engineering materials. Taking time off and resting won’t help a plane’s wings so instead, most metal objects have a fatigue life – once they reach this lifespan, they have to be retired from use so they don’t break.
Helen commented on :
It’s really interesting to read Mark’s clear explanatin of central and peripheral fatigue. I only have on the job experience of athletes experiencing fatigue. One example is where marathon runners and other endurance athletes “hit the wall”: a point at which the brain and the body are both screaming at the athlete to stop. They have to try all sorts of psychological techniques to get through that (mostly thinking about other things to distract themselves – which more experienced runners are better at doing)