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.