Weakness refers to loss of muscle strength. That is, people cannot move a muscle normally despite trying as hard as they can. However, the term is often misused. Many people with normal muscle strength say they feel weak when they feel tired (see Fatigue) or when their movement is limited because of pain or joint stiffness.
For a person to intentionally move a muscle (called a voluntary muscle contraction), the brain must generate a signal that travels a pathway from
- The brain
- Through nerve cells in the brain stem and spinal cord
- Through nerves from the spinal cord to the muscles (called peripheral nerves)
- Across the connection between nerve and muscle (called a neuromuscular junction)
Also, the amount of muscle tissue must be normal, and the tissue must be able to contract in response to the signal from the nerves. Therefore, true weakness results only when one part of this pathway―brain, spinal cord, nerves, muscles, or the connections between them―is damaged or diseased.
Weakness may develop suddenly or gradually. Weakness may affect all of the muscles in the body (called generalized weakness) or only one part of the body. For example, depending on where the spinal cord is damaged, spinal cord disorders may cause weakness only of the legs.
Symptoms depend on which muscles are affected. For example, when weakness affects muscles of the chest, people may have difficulty breathing. When weakness affects muscles that control the eyes, people may have double vision. Complete muscle weakness causes paralysis. People may have other symptoms depending on what is causing the weakness. Weakness is often accompanied by abnormalities in sensation, such as tingling, a pins-and-needles sensation, and numbness.
Because malfunction in the same part of the signal pathway causes similar symptoms regardless of cause, the many causes of muscle weakness are usually grouped by the location of the cause. That is, causes are grouped as those that affect the brain, spinal cord, peripheral nerves, muscles, or connections between nerves and muscles. However, some disorders affect more than one location.
Causes differ depending on whether weakness is generalized or affects only specific muscles.
The most common causes of generalized weakness are
- A decrease in general physical fitness (called deconditioning), which may result from illness and/or a decrease in physical reserves (frailty), such as muscle mass, bone density, and the heart's and lungs' ability to function, especially in older people
- Loss of muscle tissue (wasting, or atrophy) due to long periods of inactivity or bed rest, as occurs in an ICU
- Damage to nerves due to a severe illness or injury, such as severe or extensive burns
- Certain muscle disorders, such as those due to a low level of potassium (hypokalemia), consumption of too much alcohol, or use of corticosteroids
- Drugs used to paralyze muscles—for example, to keep people from moving during surgery or while on a ventilator
The most common causes of weakness in specific muscles are
- Strokes (the most common cause of weakness affecting one side of the body)
- A pinched nerve, as occurs in carpal tunnel syndrome
- A ruptured or herniated disk in the spine
- Pressure on (compression of) the spinal cord, as can result from cancer that has spread to the spinal cord
- Multiple sclerosis