The most common reasons for legs to be sore and weak are overwork, lumbar intervertebral disc herniation, and venous thrombosis of the lower limbs. 1. Excessive exertion: if the patient has performed strenuous sports or overloaded work, it is easy to cause the accumulation of lactic acid in the leg muscles, resulting in leg weakness, which is usually relieved after sufficient rest. 2. Lumbar disc herniation: the herniated lumbar disc compresses the nerve root, the stability of the lumbar spine is in trouble, which stimulates and presses the muscles and fascia around the lumbar spine, and then the legs become weak and achy. 3. Lower extremity venous thrombosis: due to various reasons, the venous return of the lower extremity is not smooth, so that the blood shows high coagulation state, and eventually form a thrombus, which may cause pain and weakness of the lower extremities, and may also be accompanied by localized swelling. There are many reasons why the legs are always weak and achy, which may be related to hypocalcemia, lumbar spinal stenosis, pyriformis syndrome and long-term low fever, etc. If the symptoms continue without relief, it is recommended to go to the hospital in time to find out the cause of the disease, and be treated by the doctor.