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What causes muscle cramps during exercise?

As a cyclist and a Nutritionist I regularly hear conversations that go something like this: "Does anyone know why I get muscle cramps when I am out on a long ride?", which is answered with "you don't have enough magnesium". Today I am going to set the record straight.

Muscle cramps are a common complaint, particularly leg cramps. They will often occur after long periods of exercise or high intensity exercise. Unfortunately, we don't fully understand what causes them. Whilst dehydration and electrolyte imbalances are often touted as the most likely cause, this is likely only to be one piece of the story.

More recent research actually suggests that muscle cramps are more likely to be caused by fatigue. Over time, the muscles involved in the exercise stop responding to inhibition and begin to over contract. This is a normal protective, neurological reflex of your body. As for why this reflex is inhibited over time is not currently clear. Supporting this theory is the fact that the muscle successful treatment for muscle cramps is actually gentle stretching.

When you consider the fact that muscle cramps are isolated to your lower limbs, it does raise the question,  "if I have an electrolyte imbalance, why is only one part of my body affected?"

That's not to say that hydration and electrolyte imbalance are irrelevant - one of the first questions I ask is always around fluid intake.

The best way to avoid a cramp is to be prepared:

  • make sure you start out your activity well fuelled and well hydrated
  • stay hydrated
  • if you are out for a longer ride or will be exercising for a while, add electrolytes to your fluids
  • building up your endurance will increase your time to cramp

And finally...

A note on magnesium deficiency. This is a very uncommon occurrence in an otherwise healthy person. This is partly because your kidneys actually limit the mount you eliminate when your intake is low. However, if you are low specifically in magnesium, you will notice other symptoms before you start getting muscle cramps, and it won't be limited to exercise. Your best bet if you're unsure is to pop in and see a health professional for a chat about your diet and exercise habits.

In the meantime, some good sources of magnesium include:

  • almonds
  • cashews
  • spinach
  • avocado
  • potato
  • brown rice
  • oats
  • beans
  • salmon
  • broccoli
  • chicken
  • milk

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