Why do we get muscle cramps after heavy exercise?

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📓 The short answer

The two most common reasons for muscle cramps after heavy exercise are overworking the muscles, which disrupts the balance of contraction and relaxation signals, and losing too much sodium through sweat, which affects the electrical signals needed for muscle contractions.

📚 The long answer

We've all been there. You've got your feet kicked up after a long day when suddenly, out of nowhere, a muscle painfully starts to seize up. Ouch!

A track athlete lying on the ground with a medical staff holding his leg up in the air. His facial expression shows pain, suggesting his leg is cramping. — "Bishop Loughlin Games - Armory - Track & Field" by Steven Pisano is licensed under CC BY 2.0.

A muscle cramp is an involuntary, spontaneous, chaotic muscle contraction. While they're painful and no fun, they're also common and go away on their own after a minute or so.

As it turns out, there are a LOT of different reasons why people experience muscle cramps, including (but not limited to):

over-exercise
nerve compression
low blood flow
hypothyroidism
getting kidney dialysis
diabetes
physical inactivity
certain medications
menstruation
pregnancy
alcoholism
older age

For this newsletter, I'm going to focus on exercise-associated muscle cramps, since those seem to be the most universal muscle cramps.

Let's start with a brief and over-simplified explainer on how muscles contract and relax.

How do muscles contract and relax?

Muscles either contract or relax depending on what your body needs to do.

A diagram showing a man doing a bicep curl next to a diagram of a muscle contracting above a man with the curl in the relaxed position with the muscle elongated — "1015 Types of Contraction new" by OpenStax is licensed under CC BY 4.0.

There are essentially five basic steps to activating a skeletal muscle (voluntary muscles attached to bones that help you move) to contract or relax:

Your brain sends an electrical signal through your spinal cord.
The electrical signal travels along a nerve cell called a motor neuron.
Once the signal reaches the end of the motor neuron, it triggers the release of a neurotransmitter, which binds to the muscle fiber.
The binding opens channels in the muscle fiber's membrane, allowing sodium ions to rush in. This sudden flow of sodium changes the electrical charge inside the muscle fiber.
This electrical change triggers a series of events inside the muscle fiber, causing it to contract. When the signal stops, the muscle fiber relaxes.

If you're more of a visual learner, this video does a great job explaining this process.

Why do we get muscle cramps after heavy exercise?

The exact causes of exercise-related muscle cramps aren't fully understood, but here are the two most common reasons why you might cramp after a period of heavy exercise.

Reason #1: You've overworked a muscle.

When you exercise intensely or for a long period, your muscles can become overworked, especially if you push them beyond their usual limits or don't have enough training.

When your muscles are tired, they send more "contract" signals to the spinal cord to protect themselves from overstretching. At the same time, the signals that usually tell the muscles to relax become weaker. This imbalance, with more contraction signals and fewer relaxation signals, causes the muscles to stay contracted, leading to cramps.

So, when your muscles are overworked, they become tired and send more "contract" signals while reducing "relax" signals, resulting in muscle cramps.

Reason #2: You're low in sodium.

Electrolytes like sodium, potassium, calcium, and magnesium are essential for transmitting the electrical signals necessary for muscle contractions. Sodium is particularly important because it helps maintain the nerve's baseline electrical charge, which is crucial for triggering muscle contractions.

During exercise, sweating leads to the loss of sodium and other electrolytes. If sodium levels drop too low, it can disrupt these electrical signals, making the nerves more likely to fire off electrical signals improperly. This instability can lead to muscle cramps.

🧠 Bonus brain points

Does pickle juice help with muscle cramps?

3 jars of pickles — The things I do for this newsletter...

As it turns out, pickle juice is an effective antidote to muscle cramps. A 2010 study explored whether drinking pickle juice would be effective at shortening the duration of muscle cramps. The results showed that drinking about 2.5 fluid ounces (74 ml) of pickle juice relieved a cramp 45% faster than when no fluid was consumed.

The researchers ruled out that simple hydration was the reason why (drinking water alone didn't impact cramp duration), and the effect happened too quickly to be from the juice's electrolytes. Therefore, the researchers speculated that pickle juice's effectiveness is likely caused by the acid triggering a reflex in your mouth and throat which might reduce motor neuron activity at the muscle cramp.

So when you're in a pickle (with a cramping muscle), grab a jar of pickles. 🥒

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Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :)

All my best,

Caitlin

Click here to read five facts about Caitlin

Sources for this week's newsletter

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📖 Book of the week

The Doctors Blackwell: How Two Pioneering Sisters Brought Medicine to Women and Women to Medicine by Janice P. Nimura

This is a super interesting biography about Elizabeth and Emily Blackwell, two sisters who broke barriers and became the first two women to receive medical degrees in the United States. The sisters, especially Elizabeth, were noteworthy for their fierceness and determination to practice medicine, despite ample laughs, ridicule, and outright hostility. The Doctors Blackwell is a great book if you're interested in the roots of medicine in the United States or just want to learn about some eccentric historical characters.

Check out the full list of books I've recommended here.