Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision Oct 2019| Content last modified Oct 2019
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What is weakness?

Weakness is a loss of muscle strength. If you have weakness, you can’t move a muscle normally, even when you’re trying as hard as you can. Weakness is different than being tired (fatigued), having stiff muscles, or not being able to move part of your body because it hurts too much to move.

  • Muscle weakness may happen suddenly or little by little over time

  • You may have weakness all over your body or just in certain muscles

  • Weakness can be dangerous if it involves the muscles that control your breathing

  • Physical and occupational therapy can often help your muscles get stronger depending on what’s causing the weakness

When should I see a doctor?

Go to the emergency department right away if you have muscle weakness and any of these warning signs:

  • Weakness that starts over a few days or faster

  • Trouble breathing

  • Trouble raising your head while lying down

  • Trouble chewing, talking, or swallowing

  • Being unable to walk

Call your doctor if you have muscle weakness without any warning signs so your doctor can decide how quickly to see you.

What causes weakness?

Weakness all over your body usually has different causes than weakness that's just in certain muscles.

If you have weakness all over your body, common causes are:

  • Worsened physical fitness, especially if you’re over 55

  • Losing muscle strength after being on bed rest

  • Problems with nerves all over the body that control your muscles

  • Muscle problems that can happen from low potassium, taking corticosteroids, or alcoholism

  • Certain medicines, such as drugs that doctors use during surgery to keep you from moving

If you have weakness just in certain muscles, common causes are:

  • Stroke (especially if your weakness is on one side of your body)

  • Nerve damage from injuries

  • A pinched nerve such as from carpal tunnel syndrome

  • A ruptured disk in your spine

  • Pressure on your spinal cord from severe arthritis, infection, or cancer that has spread to your spinal cord

What will happen at my doctor visit?

Doctors will ask questions about your weakness. They’ll do a physical exam to check for problems with your brain, spinal cord, nerves, and muscles, and to make sure you’re breathing well.

Depending on what doctors think is causing your weakness, they may do tests (see Diagnosis of Brain, Spinal Cord, and Nerve Disorders). These tests may include:

How do doctors treat weakness?

  • Doctors will treat the cause of your muscle weakness

  • If you have trouble breathing because of weakness in the muscles needed to breathe, they may put you on a breathing machine

  • They may have you get physical therapy to help strengthen your muscles and occupational therapy to help you learn new ways to do daily activities

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Symptoms of Brain, Spinal Cord, and Nerve Disorders
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