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Breathing and lung function tests

Respiratory muscle tests

What is a respiratory muscle test?

Respiratory muscle tests measure how much pressure your breathing muscles can generate when you breathe in or out. There are two types of respiratory muscle test:

  • mouth pressure tests
  • sniff pressure tests

What happens during a respiratory muscle test?

Sometimes the person carrying out the test will use a handheld device with a mouthpiece. Or they might use a fixed piece of equipment in a lung function lab. They will explain how you should breathe for mouth pressure tests and sniff pressure tests. Some people find the mouth pressure test easier and some find the sniff pressure test easier.

You will usually have to repeat each test a few times to make sure the results are as good as you can get.

The lung specialist may also ask you to do a spirometry test lying down, to see if this makes your lung capacity fall by more than 30% when moving from an upright position to lying down. This is a sign of muscle weakness.

Mouth pressure tests

Mouth pressure tests measure the strength of the muscles that help you breathe in and out.

Checking the strength of the muscles that help you breathe in

You’ll be asked to breathe out for as long as you can, and then suck hard on a mouthpiece for at least 1 second. It will feel like sucking a very thick milkshake through a straw. This is the maximal inspiratory pressure (written in your results as PImax or MIP).

Checking the strength of the muscles that help you breathe out

To check how strong the muscles are that help you breathe out, you’ll be asked to breathe in deeply first and then blow out as hard as you can into a closed off mouthpiece. This is the maximal expiratory pressure (written in your results as PEmax or MEP).

Sniff pressure test

In a sniff pressure test, a small probe is placed to block one of your nostrils. It measures the pressure while you sniff as hard as you can. The sniff pressure test also looks at the strength of the muscles that help you breathe in. It may be written in your results as SNIP.

What will the results look like?

Normal results will vary from person to person. They will depend on your age and sex.  Some sample results are included in the table below. It includes the predicted (normal) values for a male and a female.

In this table, the results are shown in cmH20 (centimetres of water), a unit of measurement that describes pressure.

LLN stands for lower limits of normal. Results below these figures suggest weakness of the muscles involved both breathing in and breathing out.

The results shown below (in the first column) indicate the person tested has very weak muscles.

    Measured Pred (male) LLN (male) Pred (female) LLN (female)  
MEP (PEmax) @TLC + 17 118 61 73 57 cmH20
MIP (PImax) @RV - 9 93 52 67 29 cmH20
SNIP (Pna)  - 21 93 52 67 29 cmH20


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Last medically reviewed: January 2020. Due for review: January 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.