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The science behind why your muscles get tired

It's not just because of lactic acid

In the world of running, we often blame a buildup of lactic acid when our muscles feel like they want to give up, or we simply say we’re just running out of energy, but those two factors aren’t 100 per cent responsible for muscle fatigue. There is still one missing piece to the puzzle, and that is our muscles’ ability to respond to signals from the brain.

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Your brain tells your muscles to contract by sending signals down long cells called motor neurons. Instead of actually touching your muscle, there is a tiny gap between the end of your motor neuron and your muscle cell. The motor neuron contains a neurotransmitter on one side of the gap called acetylcholine, and on the other side of the gap, the muscle cell is lined with charged ions — potassium on the inside and sodium on the outside. When your brain sends a signal down the motor neuron, acetylcholine is released. This causes the pores of the muscle cell to open up, allowing sodium to flow in and potassium to flow out.

The change creates an electrical signal called an action potential that spreads through the whole muscle cell, causing it to release the calcium stored inside it. This flood of calcium is what finally makes your muscles contract. In order to power the contraction, your muscle uses a molecule called ATP, which also helps reset the balance of sodium and potassium once the contraction is finished. Every time you contract your muscles, you use up more ATP, you create waste products like lactic acid, and some of the sodium and potassium ions drift away. This leaves fewer and fewer ions available for every subsequent muscle contraction.

Your muscle cells are always making more ATP, so even when they’re very, very tired, they still have some ATP kicking around. In other words, you will never completely run out of ATP (a.k.a. energy). Likewise, even at the end of a very long or hard run, when your legs are burning from lactic acid, your tired muscles will still have pH within normal limits, which means that your body is still clearing lactic acid, even when you’re fatigued. But, as your muscles contract over and over again, you eventually start to run low on potassium, sodium and calcium ions, which are needed in order to reset the muscle properly. When this happens, even if the brain sends a signal, the muscle can’t create an action potential in order to contract. This is why many runners take in some form of electrolytes during a long run, particularly when it’s hot and you’re losing a lot of sodium through sweat.

The good news is, there are plenty of sodium, potassium and calcium ions in other areas in your body, and usually all you need is a short rest to allow them to flow to the areas where they are needed (in this case, the muscles) in order to get going again. Even better, as you continue to run regularly, it will take longer and longer for muscle fatigue to set in, because the brain doesn’t have to signal the muscles as often to get the same force of contraction. This allows you to work at the same intensity (i.e., run the same speed) for a longer amount of time before fatigue sets in. In other words, the more you run, the better you get at it (shocking, we know).

Of course, with distance running, there are many factors that contribute to muscle fatigue, but if you’ve ever had your legs completely cramp up at the 30K mark in a marathon, or felt like you could hardly walk at the end of a hard 5K, this process is partly to blame.

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