Left leg weakness can be caused by a number of underlying problems. The specific cause can vary depending on a person's other symptoms and whether he or she has any existing health conditions. Some causes of left leg weakness can indicate very serious medical conditions, such as a blood clot, stroke, or neurological problem. Some back and spinal conditions, such as sciatica, can also cause weakness in the left leg. The condition is also commonly caused by injury, as can occur in many types of sports or even during intense exercise.
A condition that occurs most often in the left leg is deep vein thrombosis. This problem occurs when a blood clot forms in one of the leg veins. The condition is dangerous because the blood clot can travel through the vein to the lungs, causing a pulmonary embolism. This is a very serious problem that can be fatal, so immediate medical attention is very important.
Sciatica and other back problems that cause spinal compression are also a common cause of leg weakness on one side. The sciatic nerve runs from the lower back to each leg. When the nerve becomes irritated due to a spinal problem, it can lead to pain and weakness. Other symptoms of sciatica include pain and difficulty standing, sitting, or walking. Depending on the cause of sciatica, it may go away on its own. In other cases, physical therapy, medication, or surgery may be needed.
Another health problem that can cause weakness in the left leg is stroke. As with sciatica, the weakness can occur on the right or left, but is usually only on one side of the body. If the stroke affects the right side of the brain, the weakness will be on the left side. Weakness from a stroke can be felt anywhere in the body, including the legs. A stroke occurs when blood flow to the brain is interrupted. This can cause permanent physical damage. Prompt medical attention is important if a stroke is suspected, as it can help minimize permanent disability.
Left leg weakness can also be caused by injury to the area. Muscle injuries caused by playing sports or exercising can make walking and standing difficult. However, this type of weakness will usually improve over time, unlike the weakness associated with a stroke or other neurological problem. Left leg weakness caused by these conditions usually stays the same for a long period of time or gradually worsens over time.