Heavy stiffness in both legs may be caused by over-exercise, insufficient blood supply to the brain or lumbar disc herniation. 1. Excessive exercise: people who usually lack of exercise, suddenly carry out a lot of exercise, easy to cause muscle and fascia strain after exercise, so the two legs muscle stiffness. 2. Insufficient cerebral blood supply: patients with insufficient cerebral blood supply, due to the intense activities, resulting in insufficient blood supply to the lower limbs, there will be symptoms of heavy legs and stiffness. 3. Herniated lumbar intervertebral disc: Herniated lumbar intervertebral disc leads to soreness or radiating pain in the muscles of the innervated area, which will lead to stiffness and heaviness in the legs. The patient should go to the hospital in time to determine the cause of the condition under the diagnosis of the doctor, and take appropriate treatment according to the different conditions for treatment.