Seasickness and Science: Are Your ‘Sea Legs’ in Your Brain or Your Muscles?

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sea-legs

There’s nothing like the joy of being in a boat on the open sea — fresh air, wind in your hair and… oh wait, the overwhelming urge to throw up.

Sea travel predates the written word and for millennia our ancestors have suffered the ignominy of seasickness.  Even the great Charles Darwin was afflicted by it on The Beagle.

Scientists generally agree the best defence against seasickness is the ability to adapt to the motion of a boat or a ship — this is also known as getting your ‘sea legs’.

But there is debate about how we do this.

According to one school of thought it’s about your brain learning how to predict body movement in the new environment, but others argue it’s about learning how to keep an upright posture.

Conflict challenges the brain

“When standing on two legs, keeping balance is an art,” said Dr Jelte Bos, of the Dutch Organisation for Applied Scientific Research.

As you start to lose your balance your eyes, inner ears, and other parts of the sensory system alert your brain, which then tells your muscles and joints to take action to keep you upright.

But there is a limit to how fast our nerves can carry information to and from the brain, so our brains must learn a capacity to predict how our movement affects our balance, said Dr Bos.

Our brains learn this predictive power when we are toddlers, but this tends to happen on solid ground.

Boats move in a very complicated way.  They don’t just pitch and roll, but make other movements called yaw, sway, surge and — appropriately — heave.

Every trip on a boat will be different, depending on the sea, weather conditions and type of vessel.  Cruise ships tend to be more stable than small boats.

Until your brain adapts and learns the rhythm of your particular sea journey, its predictive system will be based on what it learnt on land, so your brain’s expectations will conflict with what your senses tell you.

Once your brain adapts to the movement of the boat its expectations will be more realistic, the messages it gives to the body will be in sync with your senses and voila, you get your balance — and your sea legs.

Dr Bos said his theory explains why passengers, not drivers, tend to get car sick.

Because the driver is in control and knows, for example, how tight they will take the corners, their brain is better able to accurately predict their movement.

The passenger, on the other hand, is at the mercy of the driver and so is less able to predict and is more likely to get car sick.

It’s the way we sway

On the other hand, Dr Tom Stoffregen, an expert in human movement science at the University of Minnesota, suggests the main culprit that causes seasickness is how we sway.

In the same way people differ in their ability to learn tennis, ice skating or water skiing, they differ in their ability to learn to adapt to a moving ship, Dr Stoffregen explained.

He said overcoming seasickness involves an unconscious experimentation with different muscle or joint movements to maximise postural stability and keep your balance, and his research has identified key things that can help you do this.

The first is looking at the horizon.

“Mariners have been telling passengers this for ever,” said Dr Stoffregen.

“It is now a scientific fact that looking at the horizon on a ship physically stabilises your body.”

The other is standing with your legs further apart.

Dr Stoffregen said most people get their sea legs within about 36 hours of leaving shore, although some take longer.

Land sickness and land legs

  • Just as you have to find your sea legs on the ocean, you have to find your land legs when you get back to shore.
  • Symptoms of land sickness include feeling nauseous or that the earth is moving, and walking like you are drunk.
  • While this generally lasts for a day or two after disembarking, in rare cases this balance problem develops into a syndrome called Mal de Debarquement Syndrome that can go on for years.

He said his research has found that the way someone sways on land before the ship sails can predict their likelihood of getting seasick.

Interestingly, women are more prone to seasickness than men, which may be due to their different body shapes and weight distribution that causes them to sway differently, said Dr Stoffregen.

Bottom line: keep standing!

Dr Bos agreed that looking at the horizon helps — largely because it gives your brain important information to help its predictive system to adapt to the ship’s movement.

But while swaying and motion sickness are related, he said it is possible to get motion sickness without sway, and this happens in Meniere’s disease.

And it is also possible for people who sway a lot not to be susceptible to motion sickness; this occurs in people without a functioning vestibular system.

According to Dr Bos the best way to get your sea legs is to keep standing and trying to balance — any way you can.  This will challenge your brain’s predictive system to reprogram as fast as possible.

“If you fall, that’s okay.  Stand up and try it again,” he said.

“If you continue doing that it is our experience that is beneficial and you will get over feeling sick much faster.”

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