There are many reasons for the sudden weakness of the leg, and the most common ones are meniscal injury, inflammatory disease or anterior thigh injury, as follows: 1, meniscal injury: if the patient has a meniscal injury, when walking, the tibia and femur will stimulate the injured meniscus, which will cause pain and lead to sudden weakness of the leg; 2, inflammatory disease 2, inflammatory diseases: such as synovitis, osteoarthritis, can also lead to sudden weakness of the leg; 3, anterior thigh injury: patients with injuries to the quadriceps muscle, or patellar tendon, patella on the anterior side of the thigh, will feel significant pain when exerting force, resulting in instant weakness and weakness of the leg.