A Trendelenberg gait or sign is one in which the patient seems to overbalance or slip to one side when that foot is raised off the ground. Naturally, walking becomes an ungainly exercise. Normally this overbalancing does not occur because the smaller gluteal muscles (gluteus medius and minimus) abduct the trunk to shift its centre of mass over the supporting limb when one foot is off the ground.
A positive Trendelenberg sign may be due to many factors including a congenital dislocation of the hip which has gone unnoticed, an ununited fracture of the upper end of the femur (or conditions such as coxa vara in which the neck of the femur develops at the wrong angle to the shaft) or to paralysis of the smaller gluteal muscles e.g. as a result of polio.