Lying down and not being able to lift the leg straight may be caused by insufficient leg strength, low flexibility of the leg and external injuries. 1. Insufficient leg strength: when the patient lies down and the leg is not straight, it may be due to insufficient leg strength, resulting in the inability to support the activities of the leg. Thus, the situation of lying down and not being able to straighten the leg. 2. Low flexibility of the legs: the leg cannot be straightened when lying down may be due to the low flexibility of the patient’s body, and the elasticity and stretching ability of the muscles and ligaments at the joints of the legs are insufficient. Therefore, the range of motion of the legs is low, and the situation that the legs cannot be straightened when lying down. 3. External injury: lying down leg lifting to not straight may be the patient’s leg by external injury, resulting in leg muscle or ligament damage. Thus, it is easy to appear the situation that the leg can not be straightened. When the patient lies down the leg lifting to extend the situation, if the combination of pain and other discomforts, it is recommended that the patient go to the hospital in time to accept the doctor’s professional diagnosis and treatment.