The Trendelenberg Sign results from unilateral disrupted function of the primary abductor muscles of the hip –gluteus minimus and gluteus medius. Both muscles are innervated by the superior gluteal nerve, and their function can be compromised by damage to the nerve resulting from hip dislocation, hip surgery, or motor neuron disease such as poliomyelitis, direct damage to the muscle bellies, or avulsion of their distal attachment from the femur can lead weakness or loss of hip abduction. The Trendelenberg Sign is most apparent during the walking gait cycle. When the weight of the body is supported by the leg on the lesioned side, the pelvis “rises” ipsilaterally. In fact, this more accurately described as a dipping of the pelvis toward the contralateral side. Because the pelvis cannot be maintained in a level plane by the lesioned abductors, the patient “falls” toward the good side and simultaneously leans the torso toward the lesioned side in an attempt to maintain balance. The mechanics behind the Trendelenberg Sign are difficult but can be made easier by first reviewing basic principles of joint movements. Joint movements can be described in two different ways, depending upon which skeletal elements are “fixed” or immobile. For instance, performing a dumbbell curl is an example of elbow flexion. More specifically, this motion can be described as “flexion of the forearm at the elbow joint.” The bones of the forearm are in motion, while the humerus is relatively immobile --held in place by the muscles of the shoulder and chest. Likewise, performing a pull-up is also an example of elbow flexion. Now, however, we can describe the movement as “flexion of the humerus at the elbow joint.” The humerus is in motion while the bones of the forearm are immobilized by the fixation of the hands to the bar. By applying this principle to the movement of hip abduction, we can better appreciate how the Trendelenberg Sign comes about. Hip abduction is the responsibility of gluteus medius and gluteus minimus, both of which are attached proximally to the pelvis and distally to the femur. Typically, hip abduction is visualized as raising the lower limb away from the body in a lateral direction, thereby making the angle between the thigh and torso more acute. During this type of hip movement –described as “abduction of the femur at the hip joint”- the pelvis remains fixed, while the femur is pulled laterally by the contraction of gluteus medius and minimus on that side. However, we know that we can also achieve hip abduction in the opposite direction: that is, by tilting the pelvis laterally. This motion also decreases the angle between the thigh and the torso, and can be described as, “abduction of the pelvis at the hip joint”. Both of these examples of hip abduction depend on proper function of the gluteus medius and minimus muscles. In fact, raising the leg laterally while standing requires the simultaneous action of the abductors on BOTH sides of the body. Obviously, the abductors on the right are contracting in order to raise the right lower extremity. But at the same time, the abductors on the left side –the supporting side- are contracting in order to immobilize the pelvis and maintain it in a neutral plane. They are, in fact, pulling the pelvis laterally in order to offset the weight of the unsupported right leg. This action, abduction of the hip on the side that is supported by contact with the ground, is KEY to understanding the Trendelenberg Gait. Let’s examine the normal walking gait. When the person’s weight is supported by the left leg, the right leg is unsupported as it swings forward. The weight of the unsupported right leg should tilt the pelvis downward towards the right side. However, the abductors on the left side offset this action by pulling the pelvis towards the fixed and stable left leg, thereby maintaining the pelvis in a neutral or non-tilted plane. Now, Let’s compare that with the walking gait of someone with paralyzed hip abductors on the left side. When this person’s weight is supported by the left leg, and the right leg is unsupported as it swings forward, the paralyzed hip abductors on the left side are unable to offset the weight of the right leg. This causes the pelvis to tilt downward, throwing the body’s center of gravity off. In order to counteract this imbalance, these persons will often maintain their center of gravity by leaning their torso back to the left each time the right leg is unsupported. This seems like a relatively simple concept, but confusion can set in unless you remember that the dysfunctional abductor muscles are on the same side as the supporting leg. The simplest way to assess the integrity of the hip abductors in the clinic is to ask your patient to stand motionless on one leg. If the hip abductors connected to the supporting leg are intact, the pelvis will remain level and the torso will not lean toward the supported side of the body. If, on the other hand, the abductors of the supporting leg are injured, the patient exhibit the Trendelenberg sign. The pelvis will tilt away from the supported side and the patients will either lose their balance or try to maintain their center of gravity with a compensatory leaning of the torso toward the side of injury.