Sudden weakness in the legs when walking may be meniscus injury, synovial crease syndrome, lumbar disc herniation, etc.. You should consult a doctor in time to find out the cause of the disease. 1. Meniscus injury: It is caused by indirect violent trauma such as knee twisting, or it may be caused by long-term weight-bearing cumulative injury. Due to the structural instability of the knee joint and knee pain, it can lead to the symptom of a sudden weakness of the leg when walking. 2. Synovial fold syndrome: It is caused by abnormal synovial folds around the knee joint due to trauma, chronic irritation and scarring. When the proliferated synovial bundles slide over the femoral condyles, it will lead to a sudden weakness of the leg when walking. 3. Lumbar disc herniation: disc degeneration is the root cause of lumbar disc herniation, which is related to pregnancy, injury accumulation, psychological factors and genetic factors. Lumbar disc herniation compresses the sciatic nerve for a long period of time, which can cause nerve edema, leading to a decrease in the muscle strength of the muscles of the lower limbs, and the symptom of a sudden softness of the legs when walking will appear. There may be other reasons for the sudden softness of the legs when walking, so you should consult a doctor in time to clarify the specific cause and carry out appropriate treatment if necessary.