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The body's network of muscles, tendons, and ligaments provides strength and flexibility. In general, most of our physical strength comes from groups of large (striped) muscles, especially those in the buttocks (gluteus), thighs (hamstrings and quadriceps), back (dorsal), shoulders (triceps), and upper arms (biceps). Weakness is usually perceived as loss of strength in any of these muscles.

As we age, some of the resilient fibers of the large muscles are replaced with connective tissue. This results in a slow but steady loss of muscle mass and, consequently, brings increasing weakness. Inactivity also results in muscle loss and weakness. In both instances, at least part of the weakness can be reversed by muscle-building exercises that use resistance. Examples include working out on progressive-resistance machines or lifting weights.

Weakness, especially if it is accompanied by other symptoms such as fever or diarrhea, is also a sign of many diseases. Depending upon the cause, the weakness may be widespread or confined to a specific part of the body, such as a limb or the back. If the cause is an infection, for example, the muscle weakness is likely to be temporary. In other disorders, however, the weakness may be progressive and irreversible.

Causes of weakness

Arthritis
Inactivity due to the pain and joint deformity of rheumatoid arthritis can result in muscle wasting and weakness in the affected areas. A regimen that combines antiarthritis medication, exercise, rest, and physical therapy usually minimizes muscle loss and weakness.

Carpal tunnel syndrome
The carpal tunnel is the narrow wrist structure that surrounds and protects the median nerve, which extends down the arm and into the fingers. In carpal tunnel syndrome, or repetitive stress syndrome as it is sometimes called, the carpal ligament and other wrist structures become swollen and inflamed, impinging on the nerve. In time, this causes tingling, numbness, pain, and weakness in the hand and fingers.

and is characterized by numbness, especially in the lower limbs and hands, and rapidly progressive weakness that may evolve into paralysis. The weakness results from destruction of the nerves' protective myelin sheath. As a consequence, nerve impulses cannot reach the muscles, causing the weakness and paralysis. In severe cases, the respiratory muscles may be affected, requiring that the patient be placed on a respirator. Most victims eventually recover, but intensive physical therapy is usually needed to regain muscle strength and function.

Hypoglycemia
Hypoglycemia occurs when blood sugar (glucose) drops below normal levels. Since muscles (and most other body tissues) require a steady supply of glucose for energy hypoglycemia results in muscle weakness. Other symptoms include dizziness, palpitations, clammy skin, nervousness, mood changes, and tremor. Hypoglycemia usually results from an overdose of insulin in the treatment of diabetes. In unusual cases, however, it may be due to a metabolic disorder or a reaction to alcohol consumption or certain medications such as chlorpromazine, a tranquilizer.

Infection
High fevers, inflammation, diarrhea, and other consequences of infection often leave a person feeling weak and lethargic. Common examples include intestinal infections, flu, mononucleosis, and endocarditis, an infection of the heart valves. Infection-related weakness usually disappears when the infection is eradicated, although it may take several weeks to recover fully.

Lou Gehrig's disease
This disorder, also called amyotropic lateral sclerosis or ALS, is characterized by

gradual destruction of motor neurons, the nerve cells that control muscles. Symptoms include progressive clumsiness and muscle weakness that eventually lead to paralysis, breathing problems, and death.

Multiple sclerosis
Multiple sclerosis is a nerve disease characterized by the gradual destruction of the myelin coating of the nerve cells that control voluntary muscles. In addition to progressive weakness, symptoms may include tremors, gait changes, speech problems, hearing loss, loss of bladder control, and possible paralysis of affected limbs.

Muscular dystrophy
Most forms of progressive muscular dystrophy are genetic diseases that affect boys and are characterized by progressive muscle weakness and wasting. Muscular dystrophy usually starts in childhood and causes progressive disability.

Myasthenia gravis
This is an autoimmune disorder — an illness in which the immune system attacks body tissue. In myasthenia gravis, the receptors between nerves and muscles are destroyed, resulting in progressive weakness of the affected area(s). Early signs usually include a droopy eyelid, difficulty eating or chewing, and weakness.