Sudden pain in the leg can not be bent for several common reasons: First, the meniscus injury of the knee joint, due to the existence of damage to the meniscus, there will be a sudden meniscal injury site stuck in the joint, resulting in sudden pain and joint dysfunction. Second, abnormalities in the muscles of the thigh or lower extremity, such as acute muscle lacerations and even muscle ruptures, and acute muscle spasms can also lead to sudden pain and lower extremity bending disorders. Third, fractures of the bones of the lower extremities, including fractures caused by acute violence, as well as the presence of underlying diseases of the bones, such as tumors or metabolic abnormalities, resulting in pathological fractures of the bones, can produce sudden pain and dysfunction of the bending of the lower extremities.