Muscle Weakness: Causes, Warning Signs and Tests

01Definition

True weakness vs fatigue

MedlinePlus distinguishes two situations. You may feel weak but have no real loss of strength, which is called subjective weakness and can be due, for example, to an infection such as the flu. Or you may have a loss of strength that can be detected on a physical examination, which is called objective weakness.

Mayo Clinic’s neurologist explains that sleep problems, depression, chronic medical problems, medicines and poor nutrition are among the many causes of fatigue that can make people feel weak. Heart or lung conditions, kidney failure, arthritis and painful conditions such as fibromyalgia can also limit movement. In these cases muscles still work normally, but long periods of disuse can lead to muscle loss and genuine weakness.

02Levels

How weakness is classified: brain, nerve, junction, muscle

Mayo Clinic explains that muscle movement starts as a signal from the brain, travels down the spinal cord, then along peripheral nerves to the neuromuscular junction, where the nerve stimulates the muscle to contract. A problem anywhere along this pathway can cause weakness:

• Brain or spinal cord: for example stroke, Parkinson’s disease, multiple sclerosis or spinal cord compression; signs may include stiffness, tremor, brisk reflexes or cognitive changes
• Nerve roots and peripheral nerves: weakness may come with loss of feeling, reduced reflexes and pain along the affected nerve
• Neuromuscular junction: conditions such as myasthenia gravis cause weakness that varies with muscle use, without loss of sensation
• Muscle: muscle diseases cause weakness, sometimes with pain, but not loss of sensation unless nerves are also affected

MedlinePlus adds that weakness in one area is more noticeable and may occur after a stroke, after a nerve injury or during a flare-up of multiple sclerosis.

03Common causes

Common causes: deconditioning, medicines, thyroid, electrolytes

Mayo Clinic’s neurologist notes that being unfit is one of the most common causes of weakness: muscle mass can decline gradually over time or after short periods of inactivity, such as a hospital stay. Some loss of muscle mass and strength can occur with aging, but many medical problems also cause weakness, so concerns should be discussed with a doctor.

Other causes listed by the sources include:

• side effects of medicines, such as cholesterol-lowering statins and corticosteroids
• thyroid problems, including an overactive thyroid
• low blood levels of sodium or potassium and other electrolyte imbalances
• Addison disease and an overactive parathyroid gland
• anemia, vitamin D deficiency and other undiagnosed conditions such as rheumatoid arthritis, lupus, polymyalgia rheumatica, diabetes or infections

Do not stop a prescribed medicine or start supplements on your own because of weakness: discuss it with the prescribing doctor first.

04Neurological

Neurological causes

MedlinePlus lists neurological causes of weakness, including stroke, multiple sclerosis, a pinched nerve (for example from a slipped disk in the spine), Bell palsy (weakness of the face), Guillain-Barré syndrome (nerve inflammation causing weakness), cerebral palsy, amyotrophic lateral sclerosis (ALS) and polio. Myasthenia gravis, which affects the connection between nerves and muscles, is another cause.

Mayo Clinic explains that many types of peripheral neuropathy first affect the nerves of the hands and feet, and that blood tests may be needed to look for an underlying cause such as diabetes, inflammatory or genetic diseases, or nutritional deficiencies. The outlook depends on the cause, which is why identifying it matters more than the symptom alone.

05Muscle

Muscle diseases

Weakness can also come directly from the muscles. MedlinePlus lists muscular dystrophies, a group of inherited conditions that cause slowly worsening weakness and loss of muscle tissue, and dermatomyositis, a muscle inflammation with a skin rash. Mayo Clinic adds other inflammatory muscle diseases such as polymyositis and inclusion body myositis, genetic disorders and hormonal problems such as thyroid disease.

MedlinePlus also mentions poisoning, such as botulism, insecticides or shellfish poisoning, and cancer among possible causes. Most of these are uncommon; the pattern of weakness, its speed of onset and accompanying symptoms guide which causes a doctor considers.

07Assessment

How a doctor assesses weakness

MedlinePlus explains that the doctor will examine you and ask when the weakness began, how long it has lasted, whether it is constant or only at certain times, which medicines you take and whether you have been ill recently. The doctor may pay close attention to the heart, lungs and thyroid, and will focus on nerves and muscles if the weakness affects one area.

Mayo Clinic stresses that the medical history is critical to find out whether diseases or medicines contribute. Signs such as loss of feeling, changes in reflexes, twitching, muscle wasting or weakness that varies with use help locate the problem along the nerve pathway.

08Tests

Tests: blood tests, imaging, EMG and nerve conduction

Depending on the findings, MedlinePlus explains that blood or urine tests and imaging such as x-ray, ultrasound or MRI may be ordered. Mayo Clinic notes that MRI of the brain can show stroke damage, tumors and some inflammatory conditions such as multiple sclerosis, and MRI of the spine can identify spinal cord compression. When a nerve, nerve root, nerve–muscle junction or muscle problem is suspected, doctors may request electromyography (EMG) and nerve conduction studies.

If your doctor has requested an EMG or nerve conduction study, our clinical neurophysiology service in Bilbao performs these tests and explains what will be examined (consultations are in Spanish). It does not replace the assessment of the doctor who will interpret the results with your history.

09Sources

Where the sources differ

MedlinePlus (July 2024) groups causes by body system and gives clear advice on when to contact a doctor. The Mayo Clinic Q&A is from 2013, so it is used here only to explain how weakness is classified by level and to distinguish true weakness from fatigue, not for current data. The two sources agree that weakness has many causes and that the history and examination guide the tests.

Neither source supports self-diagnosis from a list, home strength testing or assuming ALS, multiple sclerosis or a deficiency from weakness alone. Recovery depends on the cause: MedlinePlus advises following the treatment your doctor recommends, and Mayo Clinic notes that nearly everyone can do some form of exercise, ideally once a new weakness has been assessed.

10Summary

In summary

Muscle weakness is reduced strength in one or more muscles and differs from simply feeling tired. It can come from the brain or spinal cord, the nerves, the nerve–muscle junction or the muscles, as well as from medicines, hormonal or electrolyte problems and lack of fitness. Sudden weakness, especially on one side or with trouble speaking, breathing or swallowing, is an emergency.

To prepare for an appointment, write down which muscles are weak, when it started, whether it came on suddenly or gradually, whether it changes during the day or with use, which everyday tasks have become harder (climbing stairs, rising from a chair, lifting objects, gripping) and any numbness, pain, twitching or medicines you take.