Sitting down left leg pain may be due to sciatica, trauma, lumbar disc herniation and other causes. 1. Sciatica: a clinical syndrome of burning, cutting pain and numbness along the sciatic nerve pathway caused by nerve root compression, radiating from the lumbosacral region through the buttocks to the lower limbs and even to the calves and ankles. Therefore, when the left leg has sciatica, the patient can appear to sit down the left leg pain phenomenon. 2. Trauma: It can also be caused by violence, sharp objects and other traumatic injuries. Sitting down can involve the injured part, thus appearing the phenomenon of sitting down left leg pain. 3. Lumbar disc herniation: it is mainly caused by the degeneration of lumbar discs, abnormal bearing and other reasons. When the herniated disc compresses the nerve root on the left side, it will cause radiating pain in the left lower limb, and thus the patient may experience pain in the left leg. It is recommended that patients with left leg pain go to the hospital in time and follow the doctor’s instructions for treatment to avoid delaying their condition.