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 Fatigue 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... read more or when their movement is limited because of pain or joint stiffness.
Muscle weakness can be a symptom of nervous system malfunction Introduction to Symptoms of Brain, Spinal Cord, and Nerve Disorders Disorders that affect the brain, spinal cord, and nerves are called neurologic disorders. Neurologic symptoms—symptoms caused by a disorder that affects part or all of the nervous system—can... read more .
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 in the brain, as when a person consciously decides to move a muscle—for example, to pick up a book. Or the impetus to move a muscle may originate with the senses. For example, special nerve endings in the skin (sensory receptors) enable people to sense pain or a change in temperature. This sensory 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:
| |
 |
|
If the sensation occurs suddenly and is severe (as when stepping on a sharp rock or picking up a cup of very hot coffee), the impulse may travel to the spinal cord and directly back to the motor nerve, bypassing the brain. The result is a quick response of a muscle—by immediately withdrawing from whatever is causing the pain. This response is called a spinal reflex. | |
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.
Causes of Weakness
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 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 intensive care unit (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 Hypokalemia (Low Level of Potassium in the Blood) In hypokalemia, the level of potassium in blood is too low. A low potassium level has many causes but usually results from vomiting, diarrhea, adrenal gland disorders, or use of diuretics. A... read more ), consumption of too much alcohol Alcohol Use Alcohol (ethanol) is a depressant (it slows down brain and nervous system functioning). Consuming large amounts rapidly or regularly can cause health problems, including organ damage, coma,... read more , 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
Strokes Overview of Stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more (the most common cause of weakness affecting one side of the body)
Nerve damage, as occurs in carpal tunnel syndrome Carpal Tunnel Syndrome Carpal tunnel syndrome is a painful compression (pinching) of the median nerve as it passes through the carpal tunnel in the wrist. The cause of most cases of carpal tunnel syndrome is unknown... read more
or results from injuryPressure on (compression of) the spinal cord Compression of the Spinal Cord Injuries and disorders can put pressure on the spinal cord, causing back or neck pain, tingling, muscle weakness, and other symptoms. The spinal cord may be compressed by bone, blood (hematomas)... read more , as can result from cancer that has spread to the spinal cord
Less common causes
Many other conditions sometimes cause weakness (see table Some Causes and Features of Muscle Weakness Some Causes and Features of Muscle Weakness ). For example, electrolyte abnormalities (such as a low level of magnesium Hypomagnesemia (Low Level of Magnesium in the Blood) In hypomagnesemia, the level of magnesium in blood is too low. (See also Overview of Electrolytes and Overview of Magnesium's Role in the Body.) Magnesium is one of the body's electrolytes,... read more or calcium Hypocalcemia (Low Level of Calcium in the Blood) In hypocalcemia, the calcium level in blood is too low. A low calcium level may result from a problem with the parathyroid glands, as well as from diet, kidney disorders, or certain drugs. As... read more ) can cause weakness that sometimes comes and goes, as well as muscle cramping Muscle Cramps A cramp is a sudden, brief, unintended (involuntary), and usually painful contraction of a muscle or group of muscles. Muscle cramps can be a symptom of nervous system malfunction. The most... read more 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 Hypoglycemia Hypoglycemia is abnormally low levels of sugar (glucose) in the blood. Hypoglycemia is most often caused by medications taken to control diabetes. Much less common causes of hypoglycemia include... read more ) can also cause weakness, which resolves when hypoglycemia is treated.
Fatigue
Many people report weakness when their problem is actually fatigue Fatigue Fatigue is when a person feels a strong need to rest and has so little energy that starting and sustaining activity is difficult. Fatigue is normal after physical exertion, prolonged stress... read more . Common causes of fatigue include a severe illness, cancer, a chronic infection (such as HIV infection Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and can cause acquired immunodeficiency syndrome (AIDS). HIV is transmitted... read more 
, hepatitis Overview of Hepatitis Hepatitis is inflammation of the liver. (See also Overview of Acute Viral Hepatitis and Overview of Chronic Hepatitis.) Hepatitis is common throughout the world. Hepatitis can be Acute (short-lived) read more , or mononucleosis Infectious Mononucleosis Epstein-Barr virus causes a number of diseases, including infectious mononucleosis. The virus is spread through kissing. Symptoms vary, but the most common are extreme fatigue, fever, sore throat... read more 
), heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more 
, kidney failure Overview of Kidney Failure This chapter includes a new section on COVID-19 and acute kidney injury (AKI). Kidney failure is the inability of the kidneys to adequately filter metabolic waste products from the blood. Kidney... read more , liver failure Liver Failure Liver failure is severe deterioration in liver function. Liver failure is caused by a disorder or substance that damages the liver. Most people have jaundice (yellow skin and eyes), feel tired... read more , anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. Red blood cells contain hemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts... read more , chronic fatigue syndrome Chronic Fatigue Syndrome Chronic fatigue syndrome, also called myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS), refers to long-standing severe and disabling fatigue without a proven physical or psychologic... read more , fibromyalgia Fibromyalgia Fibromyalgia is characterized by poor sleep, fatigue, mental cloudiness, and widespread aching and stiffness in soft tissues, including muscles, tendons, and ligaments. Poor sleep, stress, strains... read more , and mood disorders (such as depression Depression A short discussion of prolonged grief disorder. Depression is a feeling of sadness and/or a decreased interest or pleasure in activities that becomes a disorder when it is intense enough to... read more ).
Multiple sclerosis can cause fatigue that increases when people are exposed to heat and humidity.
Evaluation of Weakness
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 Warning symptoms of stroke A stroke occurs when an artery to the brain becomes blocked or ruptures, resulting in death of an area of brain tissue due to loss of its blood supply (cerebral infarction) and symptoms that... read more (see table Some Causes and Features of Muscle Weakness 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 Medical History in Neurologic Disorders Before doing a physical examination, doctors interview the person. Doctors ask the person to describe current symptoms, including those related to the nervous system ( neurologic symptoms )... read more . 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 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.
Doctors ask about conditions that increase risk of disorders that cause weakness. For example, They ask people whether they have high blood pressure and atherosclerosis (which increase the risk of stroke) or have traveled to area whether Lyme disease is common.
People are asked about symptoms that suggest fatigue Fatigue Fatigue is when a person feels a strong need to rest and has so little energy that starting and sustaining activity is difficult. Fatigue is normal after physical exertion, prolonged stress... read more 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 Depression A short discussion of prolonged grief disorder. Depression is a feeling of sadness and/or a decreased interest or pleasure in activities that becomes a disorder when it is intense enough to... read more ).
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 Neurologic Examination When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the nervous system. Examination of the nervous system—the... read more ) 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 Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more (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 Testing Cranial Nerves ).
Muscles Motor Nerves When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the nervous system. Examination of the nervous system—the... read more 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 Reflexes When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the nervous system. Examination of the nervous system—the... read more 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 Muscle strength When a neurologic disorder is suspected, doctors usually evaluate all of the body systems during the physical examination, but they focus on the nervous system. Examination of the nervous system—the... read more 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.
Testing
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 Pulmonary Function Testing (PFT) Pulmonary function tests measure the lungs' capacity to hold air, to move air in and out, and to absorb oxygen. Pulmonary function tests are better at detecting the general type and severity... read more 
). Results of these tests help doctors estimate the risk of sudden, severe malfunction of the lungs (acute respiratory failure Respiratory Failure Respiratory failure is a condition in which the level of oxygen in the blood becomes dangerously low or the level of carbon dioxide in the blood becomes dangerously high. Conditions that block... read more ).
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, if 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 Myelography Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Electroencephalography (EEG) is a simple, painless procedure in which... read more 
, 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 Electromyography and Nerve Conduction Studies Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Electroencephalography (EEG) is a simple, painless procedure in which... read more , 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 Electromyography and Nerve Conduction Studies Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Electroencephalography (EEG) is a simple, painless procedure in which... read more 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.
Treatment of Weakness
If the cause is identified, it is treated if possible. If weakness began suddenly and causes difficulty breathing, a ventilator Mechanical Ventilation Mechanical ventilation is use of a machine to aid the movement of air into and out of the lungs. Some people with respiratory failure need a mechanical ventilator (a machine that helps air get... read more 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 Warning signs 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... read more 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.
