Merck Manual

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Michael C. Levin

, MD, College of Medicine, University of Saskatchewan

Last full review/revision Feb 2019| Content last modified Feb 2019
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Weakness refers to loss of muscle strength. That is, people cannot move a muscle normally despite trying as hard as they can. However, the term is often misused. Many people with normal muscle strength say they feel weak when the problem is fatigue or when their movement is limited because of pain or joint stiffness. Muscle weakness can be a symptom of nervous system malfunction.

For a person to intentionally move a muscle (called a voluntary muscle contraction), the brain must generate a signal that travels a pathway from

  • The brain

  • Through nerve cells in the brain stem and spinal cord

  • Through nerves from the spinal cord to the muscles (called peripheral nerves)

  • Across the connection between nerve and muscle (called a neuromuscular junction)

Using the Brain to Move a Muscle

Moving a muscle usually involves communication between the muscle and the brain through nerves. The impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to determine what something feels like, as when they feel the texture of fabric or reach in their pocket to find a nickel in an assortment of coins. This information is sent to the brain, and the brain may send a message to the muscle about how to respond. This type of exchange involves two complex nerve pathways:

  • The sensory nerve pathway to the brain

  • The motor nerve pathway to the muscle

Using the Brain to Move a Muscle
  • When sensory receptors in the skin detect a texture or shape, they transmit an impulse (signal), which ultimately reaches the brain.

  • The impulse travels along a sensory nerve to the spinal cord.

  • The impulse crosses a synapse (the junction between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord.

  • The impulse crosses from the nerve cell in the spinal cord to the opposite side of the spinal cord.

  • The impulse is sent up the spinal cord and through the brain stem to the thalamus, which is a processing center for sensory information located deep in the brain.

  • The impulse crosses a synapse in the thalamus to nerve fibers that carry the impulse to the sensory cortex of the cerebrum (the area that receives and interprets information from sensory receptors).

  • The sensory cortex perceives the impulse. A person may then decide to initiate movement, which triggers the motor cortex (the area that plans, controls, and executes voluntary movements) to generate an impulse.

  • The nerve carrying the impulse crosses to the opposite side at the base of the brain.

  • The impulse is sent down the spinal cord.

  • The impulse crosses a synapse between the nerve fibers in the spinal cord and a motor nerve, which is located in the spinal cord.

  • The impulse travels out of the spinal cord along the length of the motor nerve.

  • At the neuromuscular junction (where nerves connect to muscles), the impulse crosses from the motor nerve to receptors on the motor end plate of the muscle, where it stimulates the muscle to move.

Also, the amount of muscle tissue must be normal, and the tissue must be able to contract in response to the signal from the nerves. Therefore, true weakness results only when one or more part of this pathway―brain, spinal cord, nerves, muscles, or the connections between them―is damaged or diseased.

Weakness may develop suddenly or gradually. Weakness may affect all of the muscles in the body (called generalized weakness) or only one part of the body. For example, depending on where the spinal cord is damaged, spinal cord disorders may cause weakness only of the legs.

Symptoms depend on which muscles are affected. For example, when weakness affects muscles of the chest, people may have difficulty breathing. When weakness affects muscles that control the eyes, people may have double vision.

Complete muscle weakness causes paralysis. People may have other symptoms depending on what is causing the weakness. Weakness is often accompanied by abnormalities in sensation, such as tingling, a pins-and-needles sensation, and numbness.


Because malfunction in the same part of the signal pathway causes similar symptoms regardless of cause, the many causes of muscle weakness are usually grouped by the location of the cause (see table Some Causes and Features of Muscle Weakness). That is, causes are grouped as those that affect the brain, spinal cord, peripheral nerves, muscles, or connections between nerves and muscles. However, some disorders affect more than one location.

Common causes

Causes differ depending on whether weakness is generalized or affects only specific muscles.

For generalized weakness, the most common causes are

  • A decrease in general physical fitness (called deconditioning), which may result from illness and/or a decrease in physical reserves (frailty), such as muscle mass, bone density, and the heart's and lungs' ability to function, especially in older people

  • Loss of muscle tissue (wasting, or atrophy) due to long periods of inactivity or bed rest, as occurs in an ICU

  • Damage to nerves due to a severe illness or injury, such as severe or extensive burns

  • Certain conditions that damage muscle, such as a low level of potassium (hypokalemia), consumption of too much alcohol, or use of corticosteroids

  • Drugs used to paralyze muscles—for example, to keep people from moving during surgery or while on a ventilator

For weakness in specific muscles, the most common causes are

Less common causes

Many other conditions sometimes cause weakness (see table Some Causes and Features of Muscle Weakness). For example, electrolyte abnormalities (such as a low level of magnesium or calcium) can cause weakness that sometimes comes and goes, as well as muscle cramping and twitches.

In people with a seizure disorder, one side of the body may become weak after a seizure stops (called Todd paralysis). The weakness usually subsides over several hours.

A low blood sugar (hypoglycemia) can also cause weakness, which resolves when hypoglycemia is treated.


Many people report weakness when their problem is actually fatigue. Common causes of fatigue include a severe illness, cancer, a chronic infection (such as HIV infection, hepatitis, or mononucleosis), heart failure, anemia, chronic fatigue syndrome, fibromyalgia, and mood disorders (such as depression).


First, doctors try to determine whether people are weak or simply tired. If people are weak, doctors then determine whether the weakness is severe enough or worsening quickly enough to be life threatening. Doctors also try to identify the cause.

Warning signs

In people with weakness, the following symptoms are cause for concern:

  • Weakness that becomes severe over a few days or less

  • Difficulty breathing

  • Difficulty raising the head while lying down

  • Difficulty chewing, talking, or swallowing

  • Loss of the ability to walk

When to see a doctor

People who have any warning sign should go to an emergency department immediately. Immediate medical attention is crucial because weakness accompanied by a warning sign can worsen quickly and cause permanent disability or be fatal.

If people have symptoms suggesting a stroke (see table Some Causes and Features of Muscle Weakness), they should seek medical attention immediately because early treatment can help limit loss of function and sensation.

People without warning signs should call their doctor. The doctor can decide how quickly they need to be seen based on their symptoms and other disorders they have.

If the weakness worsens gradually (over months to years), people should discuss the problem with their doctor at their next visit.

What the doctor does

Doctors first ask questions about the person's symptoms and medical history. Doctors then do a physical examination. What they find during the history and physical examination often suggests a cause and the tests that may need to be done (see table Some Causes and Features of Muscle Weakness).

Doctors ask people to describe in detail what they are experiencing as weakness. Doctors ask

  • When the weakness began

  • Whether it began suddenly or gradually

  • Whether it is constant or is worsening

  • Which muscles are affected

  • Whether and how the weakness affects the ability to do certain activities, such as breathing, brushing their teeth or hair, speaking, swallowing, standing up from a seated position, climbing stairs, and walking

  • Whether they have other symptoms that indicate malfunction of the nervous system, such as speech or vision problems, loss of sensation or memory, or seizures

  • Whether any activity or condition (such as heat or repetitive use of a muscle) makes the weakness worse

What seems to be sudden weakness is sometimes gradual weakness, but people do not notice it until they can no longer do something, such as walking or tying their shoes.

Based on the description of weakness, doctors can often identify the most likely causes, as for the following:

  • A muscle disorder: Weakness beginning in the hips and thighs or the shoulders (that is, people have difficulty standing up or lifting their arms overhead) and no effect on sensation

  • A peripheral nerve disorder: Weakness beginning in the hands and feet (that is, people have difficulty lifting a cup, writing, or stepping over a curb) and loss of sensation

Doctors also ask about other symptoms, which may suggest one or more possible causes. For example, if people with back pain and a history of cancer report weakness in a leg, the cause may be cancer that has spread and put pressure on the spinal cord.

People are asked about symptoms that suggest fatigue or another problem, rather than true muscle weakness. Fatigue tends to cause more general symptoms than true muscle weakness, and fatigue does not follow a particular pattern. That is, it is present all the time and affects the whole body. People with true muscle weakness often report difficulty doing specific tasks, and the weakness follows a pattern (for example, becomes worse after walking).

Doctors ask about recent or current disorders that commonly cause fatigue, such as any recent severe illness or a mood disorder (such as depression).

Doctors ask about past and current use of drugs, including alcohol and recreational drugs.

Whether family members have had similar symptoms can help doctors determine whether the cause is hereditary.

During the physical examination, doctors focus on the nervous system (neurologic examination) and muscles.

Doctors observe how the person walks. How people walk may suggest the disorder that is causing symptoms or its location. For example, if people drag a leg, do not swing one arm as much as the other when walking, or both, their symptoms may be caused by a stroke. Doctors also check for other signs that the nervous system is malfunctioning, such as loss of coordination or sensation.

Cranial nerves (which connect the brain with the eyes, ears, face, and various other parts of the body) are tested—for example, by checking eye movements, the ability to speak clearly, and the ability to rotate the head (see table Testing Cranial Nerves).

Muscles are checked for size and unusual unintended movements (such as involuntary twitches and shaking). Doctors note how smoothly muscles move and whether there is involuntary resistance to movement (detected when doctors try to move a muscle that they have asked the person to relax).

Reflexes are checked. Reflexes are automatic responses to a stimulus. For example, doctors test the knee jerk reflex by gently tapping the muscle tendon below the kneecap with a rubber hammer. Normally, the knee then jerks involuntarily. This evaluation helps doctors identify which part of the nervous system is probably affected, as for the following:

  • The brain or spinal cord: If reflexes are very easy to trigger and are very strong

  • The nerves: If reflexes are hard to trigger and are slow or absent

Muscle strength is tested by asking the person to push or pull against resistance or to do maneuvers that require strength, such as walking on the heels and tiptoes or standing up.

A general physical examination is done to look for other symptoms that may suggest a cause, such as shortness of breath (possibly caused by a heart or lung disorder).

Generally, if the history and physical examination do not detect specific abnormalities that suggest a brain, spinal cord, nerve, or muscle disorder, the cause is likely to be fatigue.


Some Causes and Features of Muscle Weakness


Common Features*


Brain disorders

Headaches, personality changes, confusion, difficulty concentrating, drowsiness, loss of balance and coordination, and paralysis or numbness

Sometimes seizures

MRI or CT of the brain

Multiple sclerosis (affects the brain, spinal cord or both)

Usually other symptoms of nervous system malfunction (such as loss of sensation, loss of coordination, and vision problems)

Weakness that

  • Tends to come and go

  • Sometimes affects different parts of the body

  • Is worse in hot weather

MRI of the brain and sometimes of the spinal cord

Sometimes a spinal tap (lumbar puncture)

Symptoms that occur suddenly:

  • Weakness or paralysis, usually on one side of the body

  • Abnormalities in or loss of sensation on one side of the body

  • Difficulty speaking, sometimes with slurred speech

  • Confusion

  • Dimness, blurring, or loss of vision, particularly in one eye

  • Dizziness or loss of balance and coordination

A doctor’s examination

CT or MRI of the brain

Spinal cord disorders

Acute transverse myelitis (sudden spinal cord inflammation), often due to

  • Multiple sclerosis

  • Inflammation of blood vessels

  • Certain infections such as Lyme disease or syphilis

Tingling, numbness, and muscle weakness that

  • Occur rapidly (over hours to a few days)

  • Start in the feet and move upward

Usually a bandlike tightness around the chest or abdomen

Often difficulty passing urine

When an injury is severe, loss of bowel and bladder control and/or reduced sexual response, including erectile dysfunction in men

MRI or CT of the spinal cord

A spinal tap

Blood tests to help identify the cause

Cauda equina syndrome, caused by pressure on several spinal nerve roots, as may result from

Weakness in both legs

Loss of feeling in the upper inner part of the thighs, the buttocks, bladder, genitals, and the area between them (saddle area)

Usually pain in the lower back

Loss of bowel and bladder control and/or reduced sexual response, including erectile dysfunction in men

MRI or CT of the spinal cord

Compression of the spinal cord that develops suddenly (acute), as may result from

  • Abscesses (pockets of pus)

  • Hematomas (pockets of blood)

  • Injuries of the neck or back

  • Some cancers

Symptoms that develop in hours or days

Weakness or paralysis of the legs and sometimes arms and loss of sensation

With abscesses or tumors, tenderness to the touch over the compressed area

MRI or CT of the spinal cord

Compression of the spinal cord that develops slowly (chronic), as may result from

  • Cervical spondylosis (degeneration of the vertebral disks due to arthritis)

  • Spinal stenosis (narrowing of the passageway for the spinal cord) due to arthritis

  • Some tumors

Symptoms that are present for weeks to months

When an injury is severe, loss of bowel and bladder control and/or reduced sexual response, including erectile dysfunction in men

MRI or CT of the spinal cord

Compression of a spinal nerve root by a ruptured disk

Weakness, numbness, or both in one leg or arm

Usually back or neck pain that shoots down the leg or arm

Usually MRI or CT of the spinal cord

Usually electromyography (stimulating muscles and recording their electrical activity)

Sometimes nerve conduction studies (measuring how fast nerves transmit signals)

Multiple sclerosis (affects the brain, spinal cord, or both)

Usually other symptoms of nervous system malfunction (such as loss of sensation, loss of coordination, and vision problems)

Weakness that

  • Tends to come and go

  • Sometimes affects different parts of the body

  • Is worse in hot weather

MRI of the brain and spinal cord

Sometimes a spinal tap

Disorders that affect the peripheral nerves and the brain or spinal cord§

Progressive muscle weakness that

  • Often starts in the hands

  • Sometimes affects one side more than the other

Clumsiness, involuntary muscle contractions, and muscle cramps

Drooling and difficulty speaking and swallowing

As the disorder progresses, difficulty breathing and eventually death

Electromyography and sometimes nerve conduction studies

Often MRI of the spinal cord to rule out spinal cord disorders that can cause similar symptoms

Muscles that tire easily and progressive muscle weakness

Sometimes muscle twitching and loss of muscle tissue

In people who have had polio

Electromyography and sometimes nerve conduction studies

Disorders that simultaneously affect many nerves (polyneuropathies)

Weakness and often loss of sensation that

  • Usually begin in both legs

  • Then progress upward to the arms

When severe, difficulty swallowing and breathing

Electromyography and nerve conduction studies

A spinal tap

Nerve damage caused by

Muscle weakness that

  • Often begins in both feet

  • Then affects the hands

  • Then progresses up the legs and arms

Loss of sensation, typically before muscles become weak

Loss of reflexes

Electromyography and nerve conduction studies

Other tests depending on the disorder suspected, such as

  • Urine tests to check for toxins

  • Blood tests to check for certain antibodies or to measure sugar, vitamin, or drug levels

  • Sometimes a spinal tap

Muscle wasting (atrophy)

Sometimes loss of reflexes

Loss of sensation, including the ability to sense position of the limbs, vibration, pain, and temperature

Electromyography and nerve conduction studies

Genetic testing

Disorders that affect the connections between nerves and muscles (neuromuscular junction disorders)

Botulism (due to the bacteria Clostridium botulinum)

At first, often a dry mouth, drooping eyelids, vision problems (such as double vision), difficulty swallowing and speaking, and rapidly progressive muscle weakness, often beginning in the face and moving down the body

When contaminated food is the source, nausea, vomiting, stomach cramps, and diarrhea

No changes in sensation

Blood or stool tests to check for toxins produced by the bacteria

Sometimes electromyography

Sometimes examination of a stool sample to check for bacteria

Weak and drooping eyelids, double vision, difficulty speaking and swallowing, and weakness in the arms and legs

Excessive weakness of affected muscles that

  • Occurs after muscles are used

  • Disappears when they are rested

  • Recurs when they are used again

Use of a drug (edrophonium test) to see whether it improves muscle strength after muscles are used

Blood tests to check for certain antibodies and/or electromyography

Tearing of the eyes, blurred vision, increased salivation, sweating, coughing, vomiting, frequent bowel movements and urination, and weak muscles that twitch

A doctor’s examination

Sometimes blood tests to identify the toxin

Botulinum toxin when a dose that is too high is used to treat dystonia or other muscle spasms

Weakness of the treated muscles or sometimes of all muscles

A doctor's examination

Disorders that affect muscles (myopathies)§

Muscle malfunction due to use of alcohol, corticosteroids, or various other drugs

Weakness that tends to first cause difficulty standing up or lifting the arms overhead

Use of a drug that can cause muscle damage (such as a statin)

When due to use of alcohol or certain other drugs, muscle aches and pains

A doctor’s examination

Stopping any drug that can cause muscle malfunction

Sometimes electromyography

Blood tests to measure levels of muscle enzymes that leak from damaged muscles into the blood

Viral infections that cause muscle inflammation

Muscle aches and pains that are worsened by movement, especially walking

Sometimes fever, a runny nose, cough, sore throat, and/or fatigue

Sometimes only a doctor’s examination

Sometimes blood tests that measure the amount of muscle damage

Muscle biopsy (removal of a piece of muscle tissue for examination under a microscope)

Conditions that cause generalized muscle wasting:

Loss of muscle tissue

In people with obvious evidence of the problem

A doctor’s examination

Electrolyte abnormalities due to certain disorders or use of diuretics:

Weakness that

  • Affects the whole body

  • May come and go

  • Is often accompanied by muscle cramping and twitches

Blood tests to measure the level of potassium and other electrolytes

(such as Duchenne muscular dystrophy and limb-girdle muscular dystrophy)

Progressive muscle weakness that

  • May start during infancy, childhood, or adulthood

  • Depending on the type, may progress rapidly, causing early death

In some types, an abnormally curved spine (scoliosis) and weakness of the spinal muscles, which often develop during childhood

A thorough family history to determine whether any family members have had a similar disorder

Genetic testing

Muscle biopsy

X-rays of the spine to check for scoliosis

*Features include symptoms and results of the doctor's examination. Features mentioned are typical but not always present.

Symptoms vary depending on the location (level) of the damage. Areas that are supplied by the parts of the spinal cord below the damaged part are affected (see figure Where Is the Spinal Cord Damaged?).

In the United States, MRI is usually available. However, if MRI is unavailable, myelography with CT (x-rays taken after a radiopaque contrast agent is injected via a spinal tap) can be done.

§Sensation is usually not affected.

CT = computed tomography; MRI = magnetic resonance imaging.


If people have severe or rapidly progressing generalized weakness or any problems breathing, doctors first do tests to evaluate the strength of the respiratory muscles (pulmonary function tests). Results of these tests help doctors estimate the risk of sudden, severe malfunction of the lungs (acute respiratory failure).

Other testing is done based on where doctors think the problem is:

  • A brain disorder: Magnetic resonance imaging (MRI) or, if MRI is not possible, computed tomography (CT)

  • A spinal cord disorder: MRI or, when MRI is not possible, CT myelography and sometimes a spinal tap (lumbar puncture)

  • A peripheral nerve disorder (including polyneuropathies) or a neuromuscular junction disorder: Electromyography and usually nerve conduction studies

  • A muscle disorder (myopathy): Electromyography, usually nerve conduction studies, and possibly MRI, measurement of muscle enzymes, muscle biopsy, and/or genetic testing.

Occasionally, MRI is not available or cannot be done—for example, in people who have a pacemaker, another implanted metal device, or other metal (such as shrapnel) in their body. In such cases, another test is substituted.

For CT myelography, CT is done after a needle is inserted into the lower back to inject a radiopaque contrast agent (which can be seen on x-rays) into the fluid that surrounds the spinal cord.

For electromyography, a small needle is inserted into a muscle to record its electrical activity when the muscle is at rest and when it is contracting.

Nerve conduction studies use electrodes or small needles to stimulate a nerve. Then doctors measure how fast the nerve transmits signals.

If people have no symptoms besides weakness and no abnormalities are detected during the examination, test results are usually normal. However, doctors sometimes do certain blood tests, such as

  • A complete blood cell count (CBC)

  • Measurement of levels of electrolytes (such as potassium, calcium, and magnesium), sugar (glucose), and thyroid-stimulating hormone

  • Erythrocyte sedimentation rate (ESR), which can detect inflammation

Blood tests are sometimes done to evaluate kidney and liver function and to check for the hepatitis virus.


If the cause is identified, it is treated if possible. If weakness began suddenly and causes difficulty breathing, a ventilator may be used.

Physical and occupational therapy can help people adapt to permanent weakness and compensate for loss of function. Physical therapy can help people maintain and sometimes regain strength.

Essentials for Older People: Weakness

As people age, the amount of muscle tissue and muscle strength tend to decrease. These changes occur partly because older people may become less active but also because the production of the hormones that stimulate muscle development decreases. Thus, for older people, bed rest during an illness can have a devastating effect. Compared with younger people, older people start out with less muscle tissue and strength at the beginning of the illness and lose muscle tissue more quickly during the illness.

Drugs are another common cause of weakness in older people because older people take more drugs and are more susceptible to side effects of drugs (including muscle damage and problems with nerves).

When evaluating older people who report weakness, doctors also focus on conditions that do not cause weakness but interfere with balance, coordination, vision, or mobility or that make movement painful (such as arthritis). Older people may mistakenly describe the effects of such conditions as weakness.

Regardless of what is causing weakness, physical therapy can usually help older people function better.

Key Points

  • Many people mistakenly say they feel weak when they really mean they are tired or their movement is limited because of pain and/or stiffness.

  • True muscle weakness results only when one part of the pathway necessary for voluntary muscle movement (from brain to muscles) malfunctions.

  • If weakness becomes severe over a few days or less or if people have any of the warning signs associated with weakness, they should see a doctor immediately.

  • Often, doctors can determine whether the problem is true muscle weakness and can identify the cause based on the pattern of symptoms and results of the physical examination.

  • Physical therapy is usually helpful in maintaining strength no matter what the cause of weakness is.

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