Weakness in the knees from walking is most commonly associated with meniscal injuries of the knee, often accompanied by pain. Meniscal injuries can be followed by weakness in the legs, as well as atrophy of the quadriceps muscle, which can lead to weakness in both knees. Another common cause is chondromalacia patellae, in which the patient walks with weakness in the knee, especially when walking up and down stairs. It is recommended that when walking knee tenderness occurs, promptly visit a hospital to identify the specific cause with the help of a doctor to avoid delaying treatment.