Weakness refers to the loss of muscle strength. This means that the affected person is unable to move a muscle normally, although they try with all their might. However, the term is often used incorrectly. Many people with normal muscle strength say they feel weak when they are simply exhausted Exhaustion weakness refers to the loss of muscle strength. This means that sufferers are unable to move a muscle normally, even though they try with all their might. The designation. Learn more are or when their movement is restricted due to pain or joint stiffness.

In order to move a muscle intentionally (called voluntary muscle contraction), the brain must generate a signal that travels through the following pathway

Through the nerve cells in the brain stem and spinal cord

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

Through the connection between the nerve and muscle (called the neuromuscular endplate)

The muscle-brain circuit

Muscle movement usually requires communication between muscle and brain via nerves. The impulse to move a muscle often comes from the sensory organs. Certain nerve endings in the skin (sensory receptors) allow us to perceive how something feels, z. B. the texture of a fabric or different coins in the pocket. This information is sent to the brain, and the brain signals the muscles how to respond. Two complex neural pathways are involved in this type of communication:

The sensory nerve pathway to the brain

The motor nerve pathway to the muscles

When sensory receptors perceive a texture or shape, they transmit an impulse (signal) that eventually reaches the brain.

The impulse is transmitted to the spinal cord via a sensory nerve.

The impulse travels through a synapse (contact point between two nerve cells) between the sensory nerve and a nerve cell in the spinal cord.

It runs through a nerve cell in the spinal cord to the opposite side of the spinal cord.

The impulse is sent to the spinal cord and through the brain stem to the thalamus, a part of the diencephalon for processing sensory information.

The impulse travels through a synapse in the thalamus to nerve fibers that transmit impulses to the sensory cortex in the cerebrum (the area of the brain that receives and processes information from sensory receptors).

The sensory cortex perceives the impulses. Thus, conscious movements are initiated, triggering excitation for the generation of impulses by the motor cortex (the area that designs, controls, and executes voluntary movements).

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

The impulse is sent to the lower part of the spinal cord.

The impulse travels through a synapse between nerve fibers in the spinal cord and a motor nerve located in the spinal cord.

The impulse travels outside the spinal cord along the entire length of the motor nerve.

The impulse reaches the neuromuscular junction (where nerves connect to muscles), where it jumps from the motor nerve to the motor endplate of the muscle, causing a muscle contraction.

Also, the amount of muscle tissue must be normal and the tissue must be able to contract in response to nerve signals (contraction). For this reason, true weakness occurs only when one or more parts of this pathway – brain, spinal cord, nerves, muscles, or the connections between them – are damaged or diseased.

Weakness may develop suddenly or gradually. Weakness can affect all muscles in the body (called generalized weakness) or just one part of the body. Depending on the area of the spinal cord that is damaged, for example, spinal cord damage can cause weakness that affects only the legs.

Symptoms depend on which muscles are affected. For example, if the weakness affects the chest muscles, the affected person may have difficulty breathing. Weakness in the muscles that control the eyes can cause double vision in people with the condition.

Complete muscle weakness causing paralysis. Depending on the cause of the weakness, affected individuals may experience additional symptoms. Weakness is often accompanied by sensory disturbances, such as tingling, pins and needles, or numbness.

Causes of weakness

A malfunction of the signal transduction pathway at the same site causes similar symptoms regardless of the cause. The many causes of muscle weakness are therefore grouped based on the location on the pathway that causes it (see table of some causes and characteristics of muscle weakness Some causes and characteristics of muscle weakness Weakness refers to the loss of muscle strength. This means that the affected person is not able to move a muscle normally, although they try with all their might. The term. Learn more ). That is, the causes are grouped as those that affect the brain, spinal cord, peripheral nerves, muscles, or the connections between nerves. However, some disorders affect more than one area.

Common causes

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

The most common causes of generalized weakness are

A decrease in general physical condition (called deconditioning) caused by illness and/or decrease in natural reserves (weakness), such as z. B. Causes muscle mass, bone density and, especially in the elderly, the functioning of the heart and lungs

Loss of muscle tissue (muscle atrophy or atrophy) due to prolonged periods of inactivity or bed rest, as is the case in an intensive care unit (IPS)

Damage to the nerves due to a serious illness or injury, as in severe or extensive burns

Drugs used to paralyze muscles – for example, to keep people motionless during surgery or while they are hooked up to a ventilator

The most common causes of Weakness of certain muscles are

Nerve damage, as in carpal tunnel syndrome Carpal tunnel syndrome is a painful pinching (squeezing) of the median nerve in the wrist carpal tunnel. In most cases, the cause is unknown. Finger and palm. Learn more or as a result of injury

Rarer causes

In people with seizure disorder, one side of the body may become weak after a seizure (Todd paralysis). The weakness usually goes away after a few hours.


Many people report weakness, although their real problem is exhaustion Fatigue means that a person feels a strong need to lie down and rest, and has so little energy that they have a hard time getting and staying active. After physical. Learn more is. Common causes of exhaustion include serious illness, cancer, chronic infections (such as HIV infection Human immunodeficiency virus (HIV) infection Human immunodeficiency virus (HIV) infection is a viral infection that gradually destroys certain white blood cells and causes acquired immunodeficiency syndrome (AIDS). Learn more , Hepatitis Overview of Hepatitis Hepatitis is an inflammation of the liver. (See also acute viral hepatitis overview and chronic hepatitis overview.) It is common all over the world. Hepatitis can be acute. Learn more or mononucleosis Infectious mononucleosis The Epstein-Barr virus causes a number of diseases, including infectious mononucleosis. The virus is spread through kissing. Symptoms vary, but are most common. Learn more ), heart failure Congestive Heart Failure (CHF) In congestive heart failure (CHF), the heart can’t keep up with the body’s demands. This leads to decreased blood flow, congestion of blood in the veins. Learn more , Kidney failure Overview of renal insufficiency In renal insufficiency, kidney function is impaired to such an extent that the kidneys are no longer able to adequately excrete metabolic waste products. renal insufficiency. Learn more , liver failure Liver failure is a severe deterioration of liver function. Liver failure is caused by a disease or substance that damages the liver. Most sufferers exhibit. Learn more , Anemia Overview of Anemia Anemia is a condition in which the number of red blood cells is low. With the help of the hemoglobin contained in them, red blood cells transport oxygen from the. Learn more , chronic fatigue syndrome Chronic fatigue syndrome Chronic fatigue syndrome refers to prolonged severe and debilitating exhaustion with no demonstrable physical or psychological cause and no objective abnormalities. Learn more , Fibromyalgia Fibromyalgia is usually accompanied by sleep disturbances, fatigue, mental clouding, and pain and stiffness in soft tissues, including muscles, tendons, and ligaments. Learn more and affective disorders depression The disorder of depression is a feeling of deep sadness that is so intense that it interferes with functioning and/or results in decreased interest or diminished pleasure. Learn more (like z. B. Depression).

Multiple sclerosis can cause exhaustion (fatigue), which becomes more severe when people are exposed to heat and humidity.

Assessment of weakness

Doctors first try to determine if the affected person is experiencing weakness or simply fatigue. If sufferers are weak, doctors try to determine if the weakness is so severe or progresses so quickly that it is life-threatening. Doctors also try to determine the cause.

Warning Signals

The following symptoms are a cause for concern in people with weakness:

Weakness that worsens within a few days or less

Difficulty lifting the head when lying down

Difficulty chewing, speaking, or swallowing

Inability to walk

When to consult a doctor:

People who have any warning signs should go to an intensive care unit immediately. Immediate medical attention is critical because weakness combined with a warning sign can spread very quickly and cause permanent disability or be fatal.

People who do not experience such alarming features should call their doctor. The doctor can decide how soon you should be examined based on the symptoms and other disorders that may occur.

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

What the doctor does:

You ask people to describe exactly what they experience as weakness. The doctor asks

When the weakness started

Whether it started suddenly or gradually

Whether it is constant or worsens

Which muscles are affected

Whether and how the weakness interferes with the ability to perform certain activities, such as breathing, brushing teeth or combing hair, speaking, swallowing, getting up from sitting, climbing stairs, and walking

Whether they have other symptoms that indicate a nervous system disorder, such as speech or vision problems, loss of sensation or memory, or. Seizures

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

What appears to be sudden weakness may be progressive weakness, but it is not noticed by the person until they are no longer able to do anything, such as walk or put on shoes.

Based on the description of the weakness, doctors can often determine the most common causes, such as. B.:

A muscle disease: weakness that affects the hips and thighs resp. the shoulders begins (sufferers have difficulty standing up or lifting their arms above their head) but does not affect sensory perception

A disease of the peripheral nervous system: the weakness begins in the hands and feet (meaning that the affected person has difficulty lifting a cup, writing, or stepping over a curb) and is accompanied by loss of sensation

Doctors also ask about other symptoms that may indicate one or more causes. For example, when people with back pain and a history of cancer report leg weakness, the reason may be due to the spread of cancer putting pressure on the spinal cord.

Doctors ask about circumstances that increase the risk for conditions that cause weakness. For example, they ask patients if they have high blood pressure and atherosclerosis (which increase the risk of stroke) or if they have traveled to areas where Lyme disease is common.

It asks about symptoms that are more likely to indicate exhaustion Fatigue means that a person feels a strong need to lie down and rest, and has so little energy that it is hard for them to get and stay active. After physical. Experience more or indicate a different problem than true muscle weakness. Fatigue often causes more general symptoms than true muscle weakness and does not follow a specific pattern. That is, it is ubiquitous and affects the entire body. Sufferers with true muscle weakness often report difficulty performing certain activities, and the weakness follows a specific pattern (gets worse after a walk, for example).

They ask about past and current use of drugs, including alcohol and restorative drugs.

Knowing if family members have had similar symptoms can help doctors determine if the cause is hereditary.

Doctors observe the affected person’s gait. The gait may give a clue to the condition causing the symptoms or its location. For example, if the affected person drags one leg behind when walking, does not swing one arm like the other arm, or both, the symptoms may have been caused by a stroke. Doctors look for other signs of nervous system dysfunction, such as loss of coordination or sensation.

Muscles Motor nerves If a neurological disorder is suspected, the doctor usually examines all organ systems during a physical examination, but focuses on the nervous system. Learn more Are checked for size and unusual, unintended movements (such as involuntary twitching and tremors). Doctors look for smoothness of muscle movement and possible involuntary resistance to movement (which is detected by trying to move a muscle that the person is supposed to relax).

Reflexes Reflexes When a neurological disorder is suspected, the doctor usually examines all organ systems during a physical exam, but focuses on the nervous system. Learn more are checked. Reflexes are automatic responses to a stimulus. For example, doctors check the patellar tendon reflex by lightly tapping the muscle tendon below the kneecap with a rubber mallet. Normally, the knee joint then stretches involuntarily. Through this evaluation, doctors can determine which of the following parts of the nervous system is likely to be affected:

Brain or spinal cord: When reflexes can be triggered very easily and are very pronounced

The nerves: when reflexes are difficult to trigger and slow or absent

The Muscle strength Muscle strength If a neurological disorder is suspected, the doctor usually examines all organ systems during a physical exam, but focuses on the nervous system. Learn more Is examined by a doctor, asking the person to push or pull against resistance or to make movements that require strength, such as walking on the heels or tiptoes or getting up from a chair.

A general physical exam is done to look for other symptoms that might indicate a cause, such as shortness of breath (possibly caused by heart or lung disease).

If the history and physical examination do not indicate a specific dysfunction suggestive of a brain, spinal cord, nerve, or muscle disorder, the cause is generally often fatigue.


If affected individuals have severe or rapidly progressive generalized weakness or any breathing problems, doctors first perform tests to evaluate the strength of the respiratory muscles (pulmonary function tests Pulmonary function testing (PFT) A pulmonary function test measures how much air the lungs can hold, how much air can be inhaled and exhaled, and how much oxygen can be absorbed. These tests allow. Learn more ). The results of these tests help doctors assess the risk of sudden, severe lung dysfunction (acute respiratory failure Respiratory insufficiency In respiratory insufficiency, blood oxygen saturation is dangerously low and blood carbon dioxide levels are alarmingly high. Possible causes of respiratory failure. Learn more ) to estimate.

Other tests are done where doctors suspect the problem:

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

Spinal cord disease: MRI or, if MRI is not available, CT myelography and sometimes a spinal tap (lumbar puncture)

A peripheral nerve disorder (including polyneuropathies) or neuromuscular endplate disorder: electromyography and usually a measurement of nerve conduction velocity

A muscle disorder (myopathy): electromyography, usually a measurement of nerve conduction velocity 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, an implanted metal device, or another type of metal (such as shrapnel) in their body. In such cases, a backup test is performed.

When a CT myelography Myelography If disease is suspected based on the history and neurological examination, further tests can confirm the diagnosis. Electroencephalography. Learn more A needle is inserted into the lower part of the back to inject a radiopaque dye (contrast material visible on X-rays) into the fluid surrounding the spinal cord.

In the case of Electromyography Electromyography and measurement of nerve conduction velocity If a disease is suspected based on the history and neurological examination, further tests can confirm the diagnosis. The electroencephalography. Learn more a small needle is inserted into a muscle to record the muscle’s electrical activity at rest and during tension.

The Measurement of nerve conduction velocity Electromyography and measurement of nerve conduction velocity If disease is suspected based on the history and neurological examination, additional tests can confirm the diagnosis. The electroencephalography. Learn more Is done by electrodes or small needles designed to stimulate a nerve. The speed at which the nerve transmits signals is then measured.

If the affected person has no symptoms or abnormalities during the examination other than weakness, the test results are usually normal. However, doctors sometimes do certain blood tests, such as

Complete blood count (CBC)

Measurement of electrolyte levels (how z. B. potassium, calcium, and magnesium), blood glucose (glucose), and thyroid-stimulating hormone

Blood sedimentation rate (ESR), which can detect end-stage inflammation

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

Treatment of weakness

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

Important information for older people: Weakness

As people age, the amount of muscle tissue and muscle strength tends to decrease. These changes take place partly because older people may not be as active, but also because production of the hormones that stimulate muscle growth declines. Bed rest during an illness can thus have a devastating effect on older people. Compared to younger people, older people have less muscle tissue and strength at the beginning of the disease and also lose their muscle tissue more quickly during the disease.

Medicines are another common cause of weakness in older people because they take many more medicines, making them more prone to side effects (including muscle damage and nerve problems).

When checking older people who complain of weakness, doctors also focus on conditions that do not cause weakness but affect balance, orientation, vision, and ability to move or cause painful movement (such as arthritis). The effects of such conditions may be mistakenly described by older people as weakness.

Regardless of the cause of weakness, physical therapy usually improves functioning in the elderly.

Most important points

Many people mistakenly say they feel weak, when what they really mean is that they are tired or that their ability to move is limited due to pain and/or stiffness.

Real muscle weakness occurs only in the case of malfunction of a part of the pathway responsible for skeletal muscles (from the brain to the muscles).

Doctors can often determine whether a true muscle weakness is present by determining the cause based on symptom patterns and the results of the physical examination.

Physical therapy is usually helpful in maintaining muscle strength, regardless of the cause of weakness.

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