The anal sphincter is a muscle, and it is one of a unique group in the body because it is a sphincter.

Most muscles are attached to bones, via tendons, at one or both ends. They act to produce movement in a particular direction across a joint.

Sphincters are little circles of muscle that are tethered in place by tough connective tissue, and control the access between one part of the body or another, as is the case with the anal sphincter.

When we speak of tone in muscles in a medical sense, it refers to what proportion of the muscle fibres are active at a given time.

Even when relaxed, a few fibres will be working, especially in sphincter muscles. All muscles have a resting tone, which is a measure of how many fibres are active when the muscle is otherwise at rest.

Most sphincters are controlled by the autonomic nervous system, meaning they are not consciously controlled, but the anal sphincter is an unusual case as we have developed a certain amount of control over it when we are potty trained as children.


One cause of hemorrhoids is a high resting tone in the anal sphincter.

If it is more tense than usual, this will increase the pressure it exerts on the blood vessels surrounding the rectum.

This pressure makes it harder for blood to flow easily through those blood vessels. This, in turn, can promote the development ofhemorrhoids – the more blood there is in the veins around the rectum, the more swollen the veins will tend to get.

Over time, as the particular nature of the spongy tissue around the rectum makes the veins especially vulnerable to such damage, the veins get distended out of their normal shape and become hemorrhoids.


Causes of the increased resting tone

There are many causes of an increased resting tone in the anal sphincter, which depends mainly on how many nerve signals the muscles receive from the autonomic nervous system.

This will vary particularly with stress or local trauma, but also can be affected by other conditions like pregnancy, thyroid problems, anything that causes chronic or acute pain and so on.

Posture will also have an influence on the resting tone of muscles throughout the body.


What can you do to reduce resting tone?

That’s hard to say, although conscious relaxation will be important.

Warm baths will also help a tense muscle unwind, and you should avoid putting extra stress on the area.

Regular bowel movements will aid this, so if you are suffering from constipation or diarrhoea, it’s worth getting that sorted out too.

But above all, not worrying too much about it – anal sphincter tone has a relatively small effect on hemorrhoid development.

The more you fret over it, the higher it’s likely to get, so you’ll be much better off focusing on other causes of hemorrhoids that are easy to influence.


Main write by Dr. James D. Hogg, (BSc Oxon, MBBS & BA Hons), medical doctor, and minor rewrite by D S Urquhart.

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