The Cause Of Muscle Fatigue And It’s Not What You Think

I’m sure many of you fitness fanatics would have felt muscle fatigue during a stressful workout. Personally, I have suffered from it just walking to the fridge and back. But what causes it? If you took GCSE PE You will probably be yelling “lactic acid” at your screen. But it is a tiny bit more complex than that. Let’s take a look.

Obvious causes

There are two very obvious culprits for the reason your muscles lose their ability to contract. The first one is the aforementioned lactic acid and then there is the fact that sometimes you just lack the energy, but neither of these factors is the primary cause of muscle fatigue.


In fact, the main reason is far more complex and needs an understanding of how the brain gets signals to the muscles. The signals that control the contraction of the muscle travel from the brain to the muscle in a fraction of a second. They travel down nerve cells known as motor neurones. If they sound familiar it’s probably because Stephen Hawking suffered from Motor neurone disease. It is why he was confined to a wheelchair throughout most of his adult life.

The muscle cells and the motor neurones are not touching one another. There is a tiny gap between them and when the electrical impulses traverse this gap this is what causes muscle contraction.

Now it gets really complicated

If you are still with us, well done. The science behind muscle movement is more complex then you may have thought. On one side of the gap between the nerve and the muscle, there is a motor neurone called acetylcholine.

Meanwhile, on the other side, there are ions lining the membrane of the muscle cells there is potassium on the inside and sodium on the outside. When the brain gives the signal the acetylcholine is released causing pores on the muscle membrane to open. Sodium flows in and potassium flows out. With us so far?

This creates a change in charge and creates what is known as an “action potential” this spreads throughout the muscle cell causing the release of stored calcium. The loss of calcium allows fibres in the muscle to lock together and this is how muscles contract.

But how is this done?

The energy to power this process comes from a substance called ATP. ATP also pushes all the ions back to where they belong after the muscle has contracted. This happens every single time a muscle contracts and when it does energy in the form of ATP is used. It is not a perfect process lactic acid is produced as a waste product and some ions are lost when contraction occurs.

Energy not the issue

Although ATP is used every time muscles contract your body constantly reproduces more of it so even if your muscles contract a lot there is never a lack of energy to complete the processes. And as mentioned before although waste products like lactic acid are acidic, muscle tissue retains its PH neutrality.

Ions are the issue

It is the dispersal of ions that become problematic after a while their numbers dwindle to points where contractions are no longer possible.

However, there will be plenty more ions around your body. Given rest time they flow back to where there is a shortage. This is why if you take a rest muscle fatigue subsides.

Good news

The great news is that the more you exercise the stronger your muscle becomes. Which means that they need fewer contractions to carry out actions, which in turn means less ion usage and therefore less muscle fatigue. So there you have it – that is why regular exercise helps to stop muscle fatigue.

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