There are two types of nerves that we use with our voice box. There are the "motor" nerves that we use to consciously control the muscles, then there's the autonomic nervous systems. These are the nerves that regulate our temperature and our heartbeat, our blood pressure, and digestion.

What we don't think about until we get nervous, is that both types of nerves affect our voice box. Suddenly our voice quavers, our throat goes tight. There are lots of anecdotes about people getting scared and not being able to use their voice at all.

It happens because we've triggered the fight or flight response, which is controlled by the autonomic nervous system. When we're under stress, our reflex is to prepare us to run to get away from a predator. We need the maximum strength in our arms and legs - that's how we beat them off.

Bio-mechanics tells us that to do that, the part of your body the limb attaches to must be as stable as possible.

To see how this works, try this simple experiment. First, write your name on a page, as small as possible. Now try the same thing, but don't let your hand touch the table. With your hand hovering over the page, the result will be a messy scrawl.

When we're in a fight or flight response, our body automatically adjusts to deliver maximum power to your arms and legs. To do that you must stabilise your core muscles - and so you hold your breath. A similar thing can happen if you put your hands together in front of you, and push as hard as you can.

When you do, your instinct is to hold your breath.

It can also happen when concentrate hard on something, even if you're not running away. We've all held our breath during the terrifying scenes in a scary movie. Or perhaps that big moment when you have to speak before an audience.

Another reason our voice goes tight is because the main job of your larynx isn't speaking - it's to stop any foreign body getting into our lungs. When under stress, your larynx can go tense, ready to close just in case.

What this shows is that staying alive is more important than holding a steady voice.

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