The dominant and most superficial muscle of the gluteal region is gluteus
maximus. It is an extremely thick powerful muscle, innervated by the inferior
gluteal nerve (L5 S1,2). 
Gluteus maximus has an extensive origin from:
It has a minor insertion (via its deepest fibres) onto the gluteal tuberosity of the femur. Its main insertion is into the iliotibial tract (a thick band of deep fascia) which runs from the iliac crest to the lateral condyle of the tibia. A second muscle, tensor fasciae latae, also inserts into the iliotibial tract.
Gluteus maximus extends the hip joint.
Why, therefore, is a powerful extensor actually required?
The answer comes in considering movements such as
In each case extension of the hip moves the trunk upwards. The muscle must be extremely powerful to raise the weight of the body against gravity. This is called "forced extension".
Through the iliotibial tract, gluteus maximus gives simultaneous stability to the hip and knee joints.
The patient in the film clip had an aortic aneurysm which necessitated Y-graft surgery. Post-operatively he was hypertensive, in acute renal failure and suffered acute spinal cord ischaemia. This latter resulted in gross muscle weakness of his gluteal muscles.
In this clip the patient demonstrates weak hip extensors. He cannot rise from the chair to a standing position without the assistance of a walking frame. With the frame the movement presents little difficulty for the patient.
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Deep to gluteus maximus lie the two smaller gluteal muscles, gluteus medius and beneath it gluteus minimus (not shown). They are powerful muscles supplied by the superior gluteal nerve (L4,5 S1).
Gluteus medius and gluteus minimus arise from the ilium. Their origins lie between the posterior and anterior gluteal lines (gluteus medius) and the anterior and inferior gluteal lines (gluteus minimus). Both muscles insert onto the greater trochanter of the femur.
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These muscles (together with tensor fasciae latae) are abductors of the hip joint. However it is not immediately clear how useful such a movement might be. abduction of the lower limb (as shown) is an unusual activity.
Why then do we have powerful abductors?
The answer lies in considering how we are able to stand on one leg. As bilaterally symmetrical bipeds our centre of mass passes through the midline of the body (a). When we stand on one leg we should fall over (b). However abduction of the hip pulls the trunk (and moves the centre of mass) over the supporting limb (c) thus preserving balance.
Do we stand on one limb often?
Forty percent of the walking cycle involves standing on one leg (unilateral stance). Patients whose smaller gluteal muscles are not working properly lurch to the unsupported side with each step (Trendelenburg gait).
The film clip depicts the same patient as described before, here he exhibits weak hip abductors. When standing on one leg with the aid of a walking frame the patient is stable because the upper body is supported by the arms. However, when asked to repeat the movement without holding the frame, the patients' pelvis drops on the unsupported side - a positive Trendelenburg sign. He tries to correct this by inclining his upper body over his supporting leg but he is unstable in this position and falls.
In poliomyelitis the virus may attack peripheral nerves such as the superior gluteal nerve which supplies gluteus medius and minimus, leading to a Trendelenburg gait.
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Deep to gluteus maximus are a number of small rotator muscles of the hip, including:
º piriformis
º gemelli muscles
º obturator internus
º obturator externus
º quadratus femoris
The dissectors hand is holding the sciatic nerve which emerges beneath piriformis. It is the largest peripheral nerve in the body.
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1. posterior fracture/dislocations of the hip joint. Unlike the shoulder, traumatic dislocations of the hip are relatively rare. They can occur in a car crash where the knee is pushed backwards, often breaking off the posterior rim of the acetabulum as it goes.
2. Traumatic injury to the back of the thigh where the sciatic nerve is relatively
superficial as it emerges beneath gluteus maximus.
The Russian Nobel Laureate Alexander Solzhenitsyn writes in "The Gulag Archipelago" of how a favourite
KGB torture was to beat the prisoners just below the
gluteal crease thus crushing the sciatic nerve.
3. Intramuscular injections into the buttock must be done in the upper outer quadrant well away from the sciatic nerve.
There is thus a mismatch between the interpretation of "buttock" in ordinary speech and its more technical anatomical use in describing a much larger area.
Everyone giving injections into this region must be aware of the danger of hitting the sciatic nerve.
