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This leaflet looks at types of muscle weakness and discusses the wide range of problems which may cause them. These range from the very common to the very rare. Some are mild and reversible; others are not. Most can be improved by exercise and physiotherapy.

Muscle weakness is a common complaint but the word weakness has a wide range of meanings, including tiredness, reduced power and failure to work at all. There is an even broader range of possible causes.

The term muscle weakness can be used to describe several different things.

Primary or true muscle weakness

This shows itself as an inability to perform what you want to do with a muscle, even the first time you try. There is a reduction in the force which the muscle can exert, however hard you try. The muscle is not working properly - it has become abnormal.

When this kind of weakness occurs the muscles are often floppier than usual and reduced in bulkiness. It can happen, for example, following stroke. It is also seen in a condition called muscular dystrophy (discussed further below). Both of these conditions result in weakened muscles which cannot move the usual load. It's a real change in muscle power.

Muscle tiredness

This is sometimes called asthenia. It is a sense of weariness or exhaustion that you feel when using the muscle. The muscle isn't genuinely weaker, it can still do its job but it takes you more effort to manage it. This type of weakness is often seen in people who have chronic fatigue syndrome, sleep disorders, depression, and chronic heart, lung, and kidney disease. It may be due to a reduction in the speed with which the muscle can get its energy supply.

Muscle 'fatigability'

Some muscle tiredness is mainly muscle 'fatigability' - the muscle starts off normally but tires very quickly and takes longer than normal to recover. This often goes with muscle tiredness but is particularly seen in some uncommon conditions such as myasthenia gravis and myotonic dystrophy.

The difference between these three types of muscle weakness is often vague and you can have more than one of them. You can also progress from one to another. However, working out which one is the main problem can help doctors work out what is causing it. This is because some conditions tend to cause one type of weakness rather than another.

Muscle weakness is commonly due to lack of exercise, ageing, muscle injury or pregnancy. It can also occur with long-term conditions such as diabetes or heart disease. There are many other possible causes, which include stroke, multiple sclerosis, depression, fibromyalgia and chronic fatigue syndrome (ME).

Lack of use

Lack of muscle fitness (deconditioning) is one of the most common causes of muscle weakness. It may occur as a result of an inactive (sedentary) lifestyle. If muscles are not used then the fibres within the muscles are partially replaced with fat. Muscle wasting will eventually occur: muscles become less chunky and more floppy. Each fibre is just as strong but there aren't so many of them and they don't contract so effectively. When you feel them they may be floppy and a little reduced in bulk. This leads to easy tiring when you try to do things that would have been easier when the muscles were fit. The condition is reversible with sensible, regular exercise regimes. It gets worse and more marked with increasing age.

Muscle power is greatest and recovery times are shortest in our 20s and 30s. This is why most great athletes are in this age range. However, building of muscles through regular exercise can be done at any age. Many successful long-distance runners are aged over 40. Muscle tolerance for prolonged activity such as marathon running remains high for longer than the powerful, short-burst activities like sprinting.

It's always good to stay fit, whatever your age. Recovery from muscle and tendon injury, however, also becomes slower with increasing age. At whatever age you decide to improve your fitness, a sensible training regime is essential. You need advice from trainers or physiotherapists, to prevent injury to muscles which, at least at first, may not perform as well as you hope.


As we age, our muscles tend to lose strength and bulk and they become weaker. Whilst most people accept this as the natural consequence of age - particularly great age - it is frustrating to be unable to do the things you could manage when younger. However, exercise is still beneficial and it is still possible to increase muscle power and strength with a careful and safe exercise routine. Injury recovery times are much longer with great age, balance is often impaired and thinner bones are easily broken. So, supervision is sensible, especially at first.


Infections and illnesses are amongst the most common causes of temporary muscle fatigue. This is usually through muscle inflammation. Even though recovery is usual, if inflammation is severe (such as a bad bout of influenza), the weakness can last quite a while. This can sometimes trigger chronic fatigue syndrome (CFS). Any illness with generalised temperature and muscle inflammation can be the trigger. However, some illnesses are particularly prone to cause it. They include flu (influenza), glandular fever (Epstein-Barr virus), HIV, Lyme disease and hepatitis C. Other less common causes, in the UK at least, are tuberculosis (TB), malaria, syphilis, polio and dengue.


During and just after pregnancy, high levels of steroids in the blood, together with a tendency to be relatively lacking in iron (anaemia), can cause a feeling of muscle tiredness. This is normal in pregnancy and whilst some exercise is still sensible, when pregnant you need to be particularly careful when exercising. Remember you are carrying a weight on the front. This can lead to lower back pain if you don't adapt what you do to take account of your altered posture.

Persistent (chronic) diseases

Many chronic diseases commonly cause muscle weakness. In some conditions this is due to reduced blood and nutrient supply to muscles:

Peripheral arterial disease: this is caused by narrowing of the arteries, usually due to the build-up of cholesterol. It's linked to smoking and diet. The blood supply to the muscles is slowed and this becomes particularly noticeable with exercise, as it can't keep up with the demand. Pain is often more noticeable than weakness but both can be a problem.

Diabetes: this can cause weakness and loss of fitness. Having raised blood sugar levels (and sometimes altered salt levels) puts muscles at a disadvantage too and they don't perform so well. In addition, as diabetes progresses, the blood supply to small nerves is lost. When the nerve serving a muscle fibre dies then the muscle fibre will also stop working. Finally, people with diabetes also have an increased tendency to narrowing of the arteries (see 'Peripheral arterial disease', above).

Heart disease - particularly heart failure: this can cause easy tiring of the muscles through a reduction in blood supply at times of high demand. This is because the heart is unable to keep up with the increased requirements of the exercising muscles, as it can't pump blood as effectively as it should.

Chronic lung disease: lung conditions such as chronic obstructive pulmonary disease (COPD) cause a reduction in the ability of the body to take in oxygen. Muscles require a fast supply of oxygen from the blood, particularly when exercising. Reduction in oxygen intake leads to easy tiring. Over time chronic lung disease may lead to muscle wasting, although this is mainly seen in advanced cases when blood oxygen levels start to drop.

Chronic kidney disease: this affects the whole body environment, both through an imbalance of the salts in the body and through a possible effect on calcium and vitamin D levels. Kidney disease also causes a build-up of harmful substances in the blood because the poorly functioning kidneys process them more slowly. This can lead to true muscle weakness as well as muscle tiredness.

Anaemia: this is a shortage of red blood cells. It has many causes, including heavy periods, poor diet, blood loss, pregnancy, genetic conditions, infections and cancers. It reduces the ability of the blood to carry oxygen to muscles, so they tire more easily. Anaemia often 'creeps' on quite slowly, so that significant muscle tiredness and breathlessness develop before the condition is diagnosed.

Conditions affecting brain 'drive' to muscles

Anxiety: generalised tiredness can be caused by anxiety. This is due to overactivity of the body's adrenaline (epinephrine) system.

Depression: general weariness and a feeling of generalised tiredness can also be caused by depression.

Note: anxiety and depression are both conditions which tend to cause a sense of tiredness and 'fatigability' rather than true weakness.

Chronic pain: a general effect on energy levels can result from chronic pain. Like anxiety, it stimulates the production of chemical substances (hormones) in the body which respond to pain and injury. These chemicals lead to feelings of tiredness or fatigue. In chronic pain, true weakness can result, as muscles may not be used due to pain and discomfort.

Muscle damage through injury

There are many ways in which your muscles can be directly damaged. The most obvious is injury or trauma such as sporting injuries, pulls and sprains. In any muscle injury, bleeding from damaged muscle fibres occurs inside the muscle, followed by swelling and inflammation. This makes the muscle less strong and also painful to use. Localised pain is the primary symptom but weakness also results.


Many medicines can cause muscle weakness and muscle damage as a side-effect or an allergic reaction. Usually this begins as tiredness or fatigue. It can progress to permanent changes if the medicines are not stopped.

Medicines in common use which can occasionally do this include statins (used to lower cholesterol levels), some antibiotics (including ciprofloxacin and penicillin) and anti-inflammatory painkillers (such as naproxen and diclofenac).

Long-term use of prescribed oral steroids also causes muscle weakness and wasting. This is an expected side-effect which anyone on long-term treatment is likely to experience. It's one of the reasons doctors try to avoid putting patients on long-term steroids if possible.

Less commonly-used medicines which can cause muscle weakness and damage to muscles include:

Other substances

Prolonged use of alcohol can cause weakness of shoulder and hip muscles.

Smoking can indirectly weaken muscles. Smoking causes narrowing of the arteries, leading to peripheral arterial disease.

Cocaine misuse causes marked muscle weakness, as can other recreational drugs.

Sleep disorders

Problems that disturb or reduce sleep lead to a generalised increase in tiredness, including muscle 'fatigability'. This can include:

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Chronic fatigue syndrome (CFS)

CFS is sometimes triggered by certain viral infections such as glandular fever (Epstein-Barr virus) and flu (influenza) but it is poorly understood. Muscles are not inflamed but tire very easily. Patients often feel enormous effort is needed for muscle activity which they would previously have found easy.

In CFS, muscles are not usually wasted and they may have normal strength on testing. This is reassuring, as it means the chance of recovery to completely normal function is very high. CFS also causes weariness, with other activities like reading and socialising also becoming exhausting. Patients often show signs of depression and poor sleep.


This condition resembles CFS. However, in fibromyalgia the muscles also become very tender to touch and they tire extremely easily. They are not usually wasted and can demonstrate normal (although uncomfortable) strength on formal testing. People with fibromyalgia tend to complain more of the pain than the tiredness or weakness.

Underactive thyroid gland (hypothyroidism)

In this condition a shortage of thyroid hormone leads to generalised tiredness. In untreated hypothyroidism, muscle degeneration and wasting can develop. This can be severe and difficult to reverse. Hypothyroidism is a common condition but it is usually picked up early and treated before long-lasting muscle problems can result.

Electrolyte disorders and lack of fluid in the body (dehydration)

Problems of the balance of salts in the body, including dehydration through not drinking enough, cause muscle tiredness. This may be severe in extreme cases, such as dehydration during a marathon. Muscles work less well when there is an imbalance in the salts in the blood.

Conditions of muscle inflammation

Inflammatory diseases of muscle typically affect older adults and include conditions such as polymyalgia rheumatica (muscles of the shoulders and thighs become tender and weak), polymyositis and dermatomyositis. Some of these conditions respond well to steroids (which need to be taken for many months before the condition resolves). Unfortunately, as explained above, steroids also cause muscle wasting and weakness.

Conditions of generalised tissue inflammation such as systemic lupus erythematosus and rheumatoid arthritis can cause muscle weakness. In a small proportion of cases of rheumatoid arthritis, muscle weakness and tiredness may be the only sign of the disease for some considerable time.


Cancers can cause muscle damage directly but the presence of cancer anywhere in the body can also cause generalised muscle tiredness. In advanced disease, general weight loss will also lead to true muscle weakness. Muscle weakness is not usually the first sign of a cancer but occurs later in the condition.

Nerve conditions which damage muscles

Conditions affecting nerves tend to lead to true muscle weakness. This is because if the nerve to a muscle fibre stops working, the muscle fibre can't work either and it will become floppy and eventually shrivel.

Neurological conditions: muscle weakness can be caused by cerebrovascular disease such as stroke and brain haemorrhage and spinal injury. Tumours in the brain can also lead to muscle weakness. Muscles which become partially or completely paralysed lose their normal strength and will eventually waste. Some recovery is possible but it will be slow and may not be complete.

Spine-related conditions: when nerves are damaged as they emerge from the spine (such as when you 'slip' a disc in the lower back or neck), weakness can result. When the discs slip out, they press on nerves headed lower down in the body. The weakness affects only the muscles served by the irritated or compressed nerve.

Other uncommon nerve conditions:
Multiple sclerosis (MS): this is caused by damage to nerves in the brain and spinal cord and can cause sudden paralysis. This can recover partially but does not always do so.

Guillain-Barré syndrome: this is a post-viral paralysing disease which causes weakness and loss of muscle function from the fingers and toes upwards. It may last many months, although complete recovery is usual.

Parkinson's disease: this is a progressive disorder of both movement and mood, mainly affecting people aged over 60. In addition to muscle weakness, people with Parkinson's disease notice tremor and stiffness. They often have difficulty in starting and stopping movements and they are often depressed.

Genetic conditions affecting muscles

Muscular dystrophies: these are inherited diseases which affect muscles. They are rare disorders but the best known and most common is Duchenne muscular dystrophy. This occurs in children and leads to gradual loss of muscle power from toddler-hood.

Some rare muscular dystrophies: these can present in adulthood and include Charcot-Marie-Tooth syndrome and the facioscapulohumeral dystrophies. They also cause gradual loss of power and function in muscles. Those who have these conditions may become wheelchair-bound.

Sarcoidosis: this is a rare disease in which clumps of cells (granulomas) form in skin, lungs and soft tissues, including muscles. The condition usually goes away after a few years.

Amyloidosis involves deposits of an 'unhelpful' abnormal protein called amyloid throughout the body, including muscles and kidneys.

Other rare causes: direct damage to muscles can occur in rare inherited metabolic conditions. Examples include:

  • Glycogen storage diseases (in which glycogen, a carbohydrate, infiltrates muscles as well as other organs).
  • Even rarer, mitochondrial diseases which occur when the energy systems inside muscle cells don't work properly.

Myotonic dystrophy: this is a rare genetic muscle disorder in which muscles become extremely tired. Myotonic dystrophies are passed down through families and they tend to occur earlier and become worse as they move through the generations.

Conditions affecting nerves

Motor neurone disease (MND): this is a progressive disorder of the nerves which affects all parts of the body. Most forms of MND begin at the outer extremities, hands and feet and gradually move inwards. The condition can take months or years to progress but people with MND often quickly develop profound muscle weakness and wasting.

MND is most often seen in male patients over 50 years of age but there have been many notable exceptions to this, including the scientist Stephen Hawking.

Myasthenia gravis: this condition is an uncommon muscle disorder in which muscles tire rapidly with a very long recovery time. This can be so extreme that patients can't keep their eyelids lifted and speech can become slurred.

Poisons: poisonous substances also cause muscle weakness and paralysis through their effect on nerves. Examples are organophosphates (used in farming and sadly as chemical weapons) and botulinum toxin (used in Botox®, both for cosmetic and for therapeutic reasons). In the case of organophosphates, the weakness and paralysis symptoms may be permanent.

Addison's disease

Addison's disease is a rare condition of underactivity of the adrenal gland, leading to a shortage of steroids in the blood and to abnormalities of the blood's salts. It tends to come on gradually. Patients can also develop unexpected tanning (pigmentation) of the skin. Weight loss is common but the symptoms are often vague. Muscle fatigue may be mild and is often an early symptom. The disease can be very difficult to spot and special tests are needed to confirm it.

Other rare hormonal causes of muscle weakness include acromegaly (excessive levels of a hormone called growth hormone), underactivity of the pituitary gland (hypopituitarism) and severe vitamin D deficiency.

If you have muscle weakness and visit your doctor, they will first need to know the following:

  • How it began and how long you have had it for.
  • Whether it is getting worse, getting better or staying the same.
  • Whether you are otherwise well, are losing weight or have travelled abroad recently.
  • What medicines or other drugs you have been taking and whether there are any muscle problems in your family.

Your doctor will need to examine you to see which muscles are affected and whether you have true or perceived muscle weakness. They will check to see whether your muscles are tender to touch (which suggests they are inflamed) or unusually 'fatigable'. They may want to watch you walk.

They will then need to test your nerves to see whether the muscles are getting the right signals to act. Your doctor may need to test your central nervous system, including your balance and co-ordination. They may need to perform blood tests to look for abnormalities of hormones, salts and blood cells.

Depending on the results of these tests your doctor may also order the following:

  • Nerve studies to make sure the nerves are conducting properly.
  • A muscle biopsy to see whether the muscles themselves show signs of inflammation or damage. A biopsy is a procedure where a small sample is taken to look at under the microscope.
  • Body scans such as CT or MRI to look for conditions elsewhere in the body which may affect muscle power and function.

The pattern and severity of weakness, associated symptoms, medication use, and family history help the doctor to determine the cause of your weakness.

In the physical examination, the doctor should objectively note down your loss of strength, conduct a neurological survey and search for patterns of weakness and other abnormalities.

There are many possible causes of muscle weakness. These range from common to rare, serious to minor, temporary to permanent. Fortunately, most cases of muscle weakness which lack obvious cause are reversible. Muscle weakness is rarely the only sign of serious underlying disease.

If you have persisting muscle weakness, particularly if it severe, localised, painful or present for more than two to three weeks, you should discuss this with your doctor.

Once your doctor has talked to you about your symptoms, they will be able to offer some guidance as to the likely cause. They will also be able to tell you if there are any serious concerns about your symptoms and will be able to refer you for further testing if required.

Dr Mary Lowth is an author or the original author of this leaflet.

Addison's Disease

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