Muscle Weakness: What Causes It and What to Do

True weakness vs fatigue

Many people say "weak" when they mean tired. Doctors separate them:

• True weakness: the muscle cannot generate normal force. You cannot rise from a chair without using your arms, lift your arm above your head, or walk on your heels, even when rested.
• Fatigue: you feel exhausted or lack energy, but strength is normal when tested. Common with poor sleep, anemia, depression, infections and chronic illness. See fatigue.
• Fatigable weakness: muscles start strong but tire quickly with repeated use and recover with rest. This is typical of myasthenia gravis.

Patterns of muscle weakness and what they suggest

Muscle weakness in the legs

Symptoms of weakness in the legs include difficulty climbing stairs, rising from a low chair or the floor, tripping, a waddling walk, and legs giving way. Common causes:

• deconditioning after illness or bed rest, and age-related muscle loss (sarcopenia)
• spinal stenosis or a herniated disc pressing on nerve roots
• peripheral neuropathy from diabetes, alcohol or vitamin B12 deficiency
• peripheral artery disease, causing weakness and cramping on walking that eases with rest
• myopathy from statins or steroids

Weak ankle and foot drop

A weak ankle makes the foot slap or catch when walking. Foot drop, being unable to lift the front of the foot, usually comes from compression of the peroneal nerve at the outside of the knee (from crossing legs, a cast or weight loss) or an L5 nerve root problem in the lower back. Ankle instability after repeated sprains can feel like weakness too.

Muscle weakness in the arms and elbow

Weakness at the elbow, such as difficulty bending the arm or pushing, can come from a C5 to C7 pinched nerve in the neck, a biceps or triceps tendon rupture, or ulnar nerve compression at the elbow, which mainly weakens grip. Arm weakness on one side that starts suddenly should be treated as a stroke until proven otherwise.

Back pain and muscle weakness

Back pain with leg weakness suggests a nerve root is being compressed, most often by a herniated disc or spinal stenosis. Weakness in specific movements, such as lifting the big toe or standing on tiptoe, along with sciatica, helps locate the level. Back pain with weakness in both legs, numbness in the saddle area or bladder changes is cauda equina syndrome and needs emergency surgery. Weak core and back muscles can also contribute to mechanical back pain, which improves with strengthening.

Common causes of muscle weakness

Lifestyle and aging

• inactivity, bed rest and deconditioning
• sarcopenia: muscle mass falls by roughly 3% to 8% per decade after 30, faster after 60
• malnutrition and low protein intake

Medicines

• Statins: muscle aches and sometimes weakness
• Steroids: weakness of the hip and shoulder muscles with long-term use
• colchicine, some HIV medicines, and alcohol

Electrolytes and hormones

• Low potassium: from diuretics, vomiting or diarrhea, causing weakness and cramps
• low or high calcium, low magnesium, low phosphate
• Hypothyroidism and hyperthyroidism
• Cushing syndrome and Addison's disease
• vitamin D deficiency

Nerve and muscle diseases

These are covered in detail in muscle weakness disorders: myositis, muscular dystrophies, myasthenia gravis, ALS, multiple sclerosis and neuropathies.

Infections and illness

Viral illnesses (including flu and COVID-19), sepsis and prolonged intensive care can cause significant weakness that recovers over weeks to months.

How doctors find the cause

A doctor will ask when the weakness started, how fast it progressed, which tasks are hard, and about pain, numbness, medicines, alcohol, family history and other symptoms. They will grade strength in each muscle group, check reflexes, sensation, muscle bulk and tone, and watch you walk.

Initial tests often include:

• blood count, electrolytes (potassium, calcium, magnesium, phosphate), kidney and liver function
• creatine kinase (CK), a muscle enzyme that rises when muscle is damaged
• thyroid function, blood sugar, vitamin B12 and vitamin D
• inflammatory markers

Further tests depend on the pattern: nerve conduction studies and EMG, MRI of the brain or spine, antibody tests for myasthenia gravis or myositis, and muscle biopsy.

Treatment

Treatment depends on the cause: correcting electrolytes, changing a statin or steroid dose, treating thyroid disease, surgery for nerve compression, immune treatments for Guillain-Barré, myasthenia or myositis, and emergency stroke care.

For almost every cause, physical therapy and progressive strength training improve function. Even older adults gain meaningful strength within 8 to 12 weeks of resistance exercise two to three times a week. Adequate protein, vitamin D and fall-proofing the home also help.

Frequently asked questions

What are the symptoms of muscle weakness?

Muscle weakness causes difficulty lifting, gripping, climbing stairs, rising from a chair or walking. You may trip, drop things or tire very quickly. Depending on the cause, there may also be pain, cramps, numbness, twitching, muscle wasting or drooping eyelids.

What causes sudden muscle weakness?

Sudden weakness can be caused by stroke, Guillain-Barré syndrome, spinal cord compression, very low potassium, or a myasthenia crisis. Sudden weakness on one side of the body with face droop or speech trouble is a stroke warning sign. Call 911 immediately.

What causes muscle weakness in the legs?

Common causes include inactivity, aging, pinched nerves in the lower back, spinal stenosis, diabetic neuropathy, poor circulation, low potassium, thyroid problems and statin or steroid side effects. Weakness that rises from the feet over days, or comes with back pain and bladder changes, is an emergency.

Can back pain cause muscle weakness?

Back pain itself does not, but the same problem can. A herniated disc or spinal stenosis can compress nerve roots and cause leg or foot weakness with sciatica. Weakness in both legs with numbness in the groin or bladder problems needs emergency care.

Can vitamin deficiency cause muscle weakness?

Yes. Vitamin D deficiency causes weakness of the hip and thigh muscles and aching. Vitamin B12 deficiency damages nerves, causing weakness and numbness. Low potassium, magnesium or phosphate also cause weakness. A blood test can check these.