Patients with functional weakness experience symptoms of limb weakness which can be disabling and frightening.

  • Problems walking
  • A “heaviness” down one side
  • Dropping things
  • A feeling that a limb just doesn’t feel normal or “part of them”

It often looks as if you have had a stroke or have symptoms of multiple sclerosis. However, unlike these conditions, with functional weakness, there is no permanent damage to the nervous system which means that it can get better or even go away completely.

It is important to note the difference between weakness and fatigue.

  • An example of a true weakness is not being unable to lift something—no matter how hard you try and although you may have been able to lift it earlier.
  • Weakness usually only affects some muscle groups, while fatigue affects all muscle groups.
  • With fatigue, you can get the strength you need to lift something using your full effort, but it may feel more difficult and tiresome to do so.

This difference between weakness and fatigue is important because while fatigue can be caused by benign problems like sleep loss or mild illness, weakness can signify something more important, such as stroke or neuromuscular disease.

Like numbness, weakness is especially concerning if it comes on suddenly or just affects one side of the body. This may be a sign of a stroke or other serious problem which requires immediate attention.


When a patient is truly weak, neurologists try to figure out the exact cause of the weakness. All other subsequent treatments rely on this crucial step.

Accurately locating the source of the problem can be challenging and requires a degree of expertise. Semmes Murphey physicians are methodical about proper diagnosis of the weakness.

The first step is to compare upper and lower motor neuron findings to determine if the problem is within peripheral nervous system or central nervous system (the brain and spinal cord).

The Central Nervous System

If the physician discovers significant upper motor neuron issues during the neurological examination, he or she will further investigate the brain and spinal cord for other signs that can lead to the exact location of the problem.

For example, if someone is numb below a certain level on the neck, this suggests they have a problem with the cervical spinal cord.

If they have a problem that includes the face (especially if it's just the lower half of the face), the problem is more likely to be in the brainstem or the brain itself.

If someone's right leg is weak, it could be a problem with the right side of the spinal cord or the left side of the brain.

The Peripheral Nervous System

If the physician finds that the weakness is due to a problem with the peripheral nervous system (the peripheral nerves, the neuromuscular junction, or the muscles), the peripheral nerves could have been damaged by infection or metabolic diseases. Another common source is an impingement in small passages such as the foramina, where they exit the spine. Perhaps the most common examples include a pinched nerve, tennis elbow or carpal tunnel syndrome.

There is a wide variety of muscular disorders which can lead to weakness.

Often the weakness affects both sides of the body equally, but in other cases the muscle weakness may be asymmetrical.

In addition, neurologists use information about the course of the weakness, and how it spread in order to determine the cause.

A stroke, for example, tends to come on very quickly, whereas others can take months to develop.

The pattern of spread is also important: Guillain-Barre syndrome, for example, typically starts in the feet and spreads upwards, whereas botulinum toxin causes weakness that descends from the top of the body.

The number of medical problems which cause weakness is very large. Recognizing the location of the problem, and the pattern associated with the weakness, can help doctors to sort through the long list of potential problems to find the true culprit.

Remember that unexplained weakness should always be investigated by a qualified medical professional.

This is an area where each patient truly is unique. But with the correct diagnosis, physicians can prescribe a variety of treatment courses that can be beneficial.

For example, many patients may respond to specific types of exercise or stretching. Working alongside a physical therapist, physicians and patients can create a plan to strengthen muscles and alleviate weakness.

This information was provided by the specialists at Semmes Murphey Clinic. Readers are encouraged to research trustworthy organizations for information. Please talk with your physician for websites and sources that will enhance your knowledge and understanding of this issue and its treatment.

The physiatrists at Semmes Murphey are experienced in the proper diagnosis of weakness. Contact us if you are having any weakness issues.

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