Big Picture

The bony component of the gluteal (buttocks) region consists of two pelvic bones (os coxae) joined anteriorly by the symphy-sis pubis and posteriorly by the sacrum. Each os coxa is composed of three fused bones: ilium, ischium, and pubis. The bones of the gluteal region contain foramina (notches), which serve as conduits for nerves and blood vessels that travel between the pelvis, gluteal region, perineum, and lower limb. Muscles of the gluteal region primarily act on the hip joint.

Actions of the Hip Joint

The hip joint is a synovial, ball-and-socket joint. The “ball” is the head of the femur, and the “socket” is the acetabulum of the pelvic bone. The motions of the hip joint are as follows (Figure 35-1A):

• Flexion. Movement anterior in the sagittal plane.
  
• Extension. Movement posterior in the sagittal plane.
  
• Abduction. Movement away from the midline in the frontal plane.
  
• Adduction. Movement toward the midline in the frontal plane.
  
• Medial rotation. Movement toward the midline in the transverse or axial plane.
  
• Lateral rotation. Movement away from the midline in the transverse or axial plane.
  
• Circumduction. A combination of hip joint motions that produces a circular motion.

Big Picture

The muscles of the gluteal region primarily act on the hip joint, producing extension, medial rotation, lateral rotation, and abduction. In addition to producing motion, the muscles of the gluteal region are important for stability of the hip joint as well as for locomotion.

Gluteal Muscles (Figure 35-1B)

• Gluteus maximus muscle. Attaches proximally on the ilium behind the posterior gluteal line, the sacrum, the coccyx, and the sacrotuberous ligament; distally, the muscle attaches at the iliotibial tract and the gluteal tuberosity of the femur. The gluteus maximus muscle is a powerful extensor of a flexed femur at the hip joint and a lateral stabilizer of the hip joint. The inferior gluteal nerve (L5, S1, S2) innervates this muscle.
  
• Gluteus medius muscle. Attaches proximally on the ilium between the anterior and posterior gluteal lines; distally, the muscle attaches on the greater trochanter of the femur. The gluteus medius muscle abducts and medially rotates the femur at the hip joint. In addition, the gluteus medius holds the pelvis secure over the stance leg, preventing pelvic drop on the opposite swing side during gait. The superior gluteal nerve (L4, L5, S1) innervates this muscle.
  
• Gluteus minimus muscle. Attaches proximally on the ilium between the anterior and posterior gluteal lines; distally, the muscle attaches on the greater trochanter of the femur. The action of the gluteus minimus muscle is the same as that of the gluteus medius—it abducts the femur at the hip joint, holding the pelvis secure over the stance leg and preventing pelvic drop on the opposite swing side during gait and hip medial rotation. The inferior gluteal nerve (L5, S1, S2) innervates this muscle.
  
• Tensor fascia lata muscle. Attaches more anteriorly than the other muscles of the gluteal region. The tensor fascia lata muscle attaches proximally at the lateral aspect of the iliac crest between the anterior superior iliac spine and the tubercle of the crest. Distally, the muscle attaches to the iliotibial tract of the fascia lata, which extends to the tibia. Because of its distal insertion, the tensor fascia lata muscle is unique when compared to the other gluteal muscles. The main action of this muscle is to stabilize the knee in extension and in hip flexion. The superior gluteal nerve (L4, L5, S1) innervates this muscle.

Deep Hip Rotator Muscles

The deep hip rotator muscles all have several common characteristics—they are deep to the gluteal muscles, they arise from the pelvis, they share common attachments around the greater trochanter of the femur, and they laterally rotate the hip. The deep hip rotator muscles are as follows (Figure 35-1C):

• Piriformis muscle. Attaches proximally on the anterior surface of the sacrum; distally, the muscle attaches at the greater trochanter of the femur. The piriformis muscle laterally rotates the femur at the hip joint. The nerve to the piriformis muscle (S1, S2) innervates this muscle.
  
• Superior gemellus muscle. Attaches proximally at the ischial spine; distally, the muscle attaches on the greater trochanter of the femur. The superior gemellus muscle laterally rotates the femur at the hip joint. The nerve to the obturator internus and superior gemellus muscles (L5, S1, S2) innervates this muscle.
  
• Obturator internus muscle. Attaches proximally on the deep surface of the obturator membrane and surrounding bone; distally, the muscle attaches at the greater trochanter of the femur. The obturator internus muscle laterally rotates the femur at the hip joint. The nerve to the obturator internus and superior gemellus muscles (L5, S1, S2) innervates this muscle.
  
• Inferior gemellus muscle. Attaches proximally on the ischial spine; distally, the muscle attaches at the greater trochanter of the femur. The inferior gemellus muscle laterally rotates the femur at the hip joint. The nerve to the inferior gemellus and quadratus femoris muscles (L4, L5, S1) innervates this muscle.
  
• Quadratus femoris muscle. Attaches proximally at the lateral aspect of the ischium just anterior to the ischial tuberosity; distally, the muscle attaches on the intertrochanteric crest. The quadratus femoris muscle laterally rotates the femur at the hip joint. The nerve to the inferior gemellus and quadratus femoris muscles (L4, L5, S1) innervates this muscle.

Big Picture

The lower limb is innervated by the ventral rami from nerve roots L1–S4, which form two separate networks of nerves and are referred to as the lumbar plexus (L1–L4) and the sacral plexus (L4–S4). The lumbar plexus communicates with the sacral plexus via the lumbosacral trunk (L4, L5), which descends into the pelvic cavity to contribute to the sacral plexus. Most of the sacral plexus is divided into anterior and posterior divisions. The anterior division provides the primary motor innervation to the posterior compartment of the thigh and leg. The posterior division provides the primary motor innervation to the anterior and lateral compartments of the leg.

Anterior Division of the Sacral Plexus

The anterior division of the sacral plexus consists of ventral rami from L4 to S4, which form a network and give rise to the following five nerves (Figure 35-2A):

• Pudendal nerve (S2–S4). Exits the pelvis via the greater sciatic foramen, enters the gluteal region, and courses to the perineum through the lesser sciatic foramen. The pudendal nerve provides motor innervation to the muscles of the pelvic floor and sensory innervation to the skin of the perineum, penis, and clitoris.
  
• Posterior femoral cutaneous nerve (S1–S3). Exits the pelvis via the greater sciatic foramen, inferior to the piriformis muscle. The posterior femoral cutaneous nerve receives half of its innervation levels (S1 and S2) from the posterior division of the sacral plexus and the other half (S2 and S3) from the anterior division. The nerve remains deep to the gluteal maximus muscle and emerges at the inferior border, providing sensory innervation to the posterior region of the thigh.
  
• Nerve to the superior gemellus and obturator internus muscles (L5–S2). Exits the pelvis via the greater sciatic foramen, inferior to the piriformis muscle, and provides motor innervation to the superior gemellus muscle. The nerve then reenters the pelvis, via the lesser sciatic foramen, providing motor innervation to the obturator internus muscle.
  
• Nerve to the inferior gemellus and quadratus femoris muscles (L4–S1). Exits the pelvis via the greater sciatic foramen, inferior to the piriformis, and travels along the deep surface of the superior gemellus muscle and the obturator internus tendon, providing innervation to the inferior gemellus and quadratus femoris muscles on their deep surface.
  
• Tibial nerve (L4–S3). The tibial nerve (a division of the sciatic nerve) exits the pelvis via the greater sciatic foramen to enter the gluteal region inferior to the piriformis muscle. The nerve descends along the posterior aspect of the thigh, providing motor innervation to the hamstring muscles (excluding the short head of the biceps femoris muscle) and a hamstring head of the adductor magnus muscle in the medial compartment of the thigh. The tibial nerve descends through the popliteal fossa and enters the posterior compartment of the leg, deep to the gastrocnemius and soleus muscles. It provides motor innervation to the posterior compartment of the leg as well as to the plantar muscles of the foot. Sensory branches provide cutaneous innervation to the posterolateral region of the leg and the lateral region of the foot.