The possible reasons for uncontrolled shaking of the knee with the leg straightened out include over-exercise, lack of calcium in the body, epilepsy, Parkinson’s disease and so on. 1. Excessive exercise: after strenuous exercise or overwork, because of muscle tension and contraction, it can cause involuntary shaking of the lower limbs, which is a normal physiological phenomenon and can be relieved after rest. 2. Calcium deficiency: because of insufficient calcium intake or absorption disorders, as well as hypothyroidism and other diseases, can lead to a lack of calcium, causing abnormal nerve excitation, there will also be involuntary shaking of the lower limbs. 3. Epilepsy: It is a disease caused by abnormal discharges of neurons in the brain due to abnormal brain development, infection and traumatic brain injury. When the consciousness is clear and focal seizure occurs, it can be manifested as involuntary shaking of the lower limbs. 4. Parkinson’s disease: It is a degenerative disease of the nervous system. Common symptoms include shaking of one hand or one leg, slow movement, stiffness and rigidity of the limbs. The causes of involuntary shaking of the lower limbs after straightening are many, and may also be related to cerebral infarction, cerebral hemorrhage and other diseases, if the symptoms continue to be unrelieved, it is recommended to consult a doctor in a timely manner, to clarify the cause of the disease, and to carry out targeted treatment.