According to Physiopedia, the Trendelenburg gait is a condition caused by weakness or instability in the abductor muscles of the hip joint. This weakness in the hip abductors makes it difficult for the pelvis to support the body's weight.
Physiopedia states that when an individual without a weakness in this area walks, the leg on the ground takes the weight of the entire body while the other leg is in the air and moving forward. During this movement, the pelvis on the side of the raised leg is also raised.
When there is a weakness in the hip musculature on one side, the pelvis on the affected side drops instead of rising during the walking movement. As a result, affected individuals tend to lean to the affected side, changing their center of gravity as a coping mechanism that reduces the pelvic drop.