What is leg weakness? Leg weakness can occur as part of a generalized feeling of weakness all over your body, or it may develop in one or both legs. True weakness occurs when the muscles lose their strength, but people often perceive that their muscles are weak when they feel ill or tired. In these cases there is not a true loss of muscle function. The sudden onset of muscle weakness can be a sign of stroke, a potentially life-threatening condition in which part of the brain is deprived of oxygen. This article focuses on conditions that cause a true loss of strength in the leg muscles. Among the most common causes for leg weakness is sciatica, problems with the spinal cord that lead to pinching or compression of the nerves as they exit the spinal cord through the holes between the vertebrae of the backbone. Sciatica may be caused by various conditions. In addition to weakness in one part of the arm or leg, other symptoms such as tingling, burning or pain may occur. Neuromuscular diseases, such as myasthenia gravis (autoimmune neuromuscular disorder that causes muscle weakness) and multiple sclerosis (disease that affects the brain and spinal cord causing weakness, coordination, balance difficulties, and other problems), are other causes for muscle weakness that can occur in the legs. Toxins such as botulinum toxin and certain medications can also cause muscle weakness. Leg weakness of sudden onset can be a sign of stroke, a potentially life-threatening situation. Seek immediate medical care (call 911) for leg weakness that comes on suddenly, particularly if it is felt on one side of the body or is accompanied by any serious symptoms, including blurred vision or double vision, loss of vision, or changes in vision; numbness; paralysis or inability to move a body part; severe headache; high fever (higher than 101 degrees Fahrenheit); loss of bladder or bowel control; fainting or change in level of consciousness; or lethargy. If your leg weakness persists or causes you concern, seek prompt medical care.