11.6 Appendicular Muscles of the Pelvic Girdle and Lower Limbs

Muscles of the Pelvic Girdle and Thigh

The muscles of the pelvic girdle and thigh are responsible for moving the hip joint and stabilizing the pelvis during standing, walking, and running. These muscles are organized into functional groups based on their location and the movements they produce.

Iliopsoas and Hip Flexors

The iliopsoas is the primary hip flexor, and it's actually two muscles that share a common insertion:

• Psoas major originates from the transverse processes and bodies of the lumbar vertebrae and inserts on the lesser trochanter of the femur.
• Iliacus originates from the iliac fossa and also inserts on the lesser trochanter of the femur.

Together, these two muscles powerfully flex the thigh at the hip joint. They're active every time you take a step or lift your knee.

Gluteal Muscles

The gluteal group sits on the posterior and lateral surfaces of the pelvis:

• Gluteus maximus is the largest and most superficial muscle of the buttocks. It originates from the ilium, sacrum, and coccyx and inserts on the gluteal tuberosity of the femur and the iliotibial (IT) tract. Its primary action is extension and lateral rotation of the thigh. It's most active during powerful movements like climbing stairs or standing up from a seated position.
• Gluteus medius originates from the ilium and inserts on the greater trochanter of the femur. It abducts and medially rotates the thigh. This muscle is critical for stabilizing the pelvis when you stand on one leg.
• Gluteus minimus lies deep to the gluteus medius, shares similar origin and insertion points (ilium to greater trochanter), and performs the same actions: abduction and medial rotation of the thigh.
• Tensor fasciae latae originates from the iliac crest and inserts on the iliotibial tract. It flexes, abducts, and medially rotates the thigh.

Adductor Group

The adductors occupy the medial compartment of the thigh. They all pull the thigh toward the midline (adduction) and most also medially rotate it.

Notice that the gracilis is unique in this group because it crosses both the hip and knee joints, allowing it to flex the knee in addition to adducting the thigh.

Thigh Compartments and Major Muscle Groups

The thigh is divided into three compartments, each with a distinct primary action:

• Anterior compartment contains the quadriceps femoris, a group of four muscles (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) that all extend the knee. The rectus femoris also crosses the hip joint and flexes the thigh.
• Medial compartment contains the adductor muscles and pectineus, which primarily adduct the thigh (described in the table above).
• Posterior compartment contains the hamstrings, a group of three muscles (biceps femoris, semitendinosus, semimembranosus) that flex the knee and extend the hip. The hamstrings are antagonists to the quadriceps at the knee joint.

A helpful way to remember: anterior compartment = extension at the knee, posterior compartment = flexion at the knee and extension at the hip, medial compartment = adduction.

Muscle Function and Interaction

• Origin and insertion points determine a muscle's action and range of motion. The origin is the more stationary attachment, and the insertion is on the bone that moves.
• Muscles are controlled by specific nerves. For example, the femoral nerve innervates the anterior compartment, while the obturator nerve innervates the medial compartment, and the sciatic nerve innervates the posterior compartment.
• Antagonist muscles work in opposition to produce smooth, controlled movements. The quadriceps and hamstrings are a classic antagonist pair at the knee.
• Synergist muscles assist the primary mover. For instance, the gluteus medius and minimus work together as synergists during thigh abduction.

Muscles of the Leg and Foot

The muscles below the knee control movements at the ankle and toes. They're organized into three compartments in the leg, each separated by fascial walls and associated with specific actions and nerve supply.

Anterior Compartment

These muscles sit on the front of the leg and are responsible for dorsiflexion (pulling the foot upward) and toe extension:

• Tibialis anterior originates from the lateral condyle and upper two-thirds of the lateral surface of the tibia. It inserts on the medial cuneiform and base of the 1st metatarsal. It dorsiflexes and inverts the foot. This is the muscle you feel working when you tap your foot.
• Extensor digitorum longus originates from the lateral condyle of the tibia and upper three-quarters of the anterior surface of the fibula. It inserts on the middle and distal phalanges of toes 2–5. It extends the toes and assists with dorsiflexion.
• Extensor hallucis longus originates from the middle third of the anterior surface of the fibula and interosseous membrane. It inserts on the distal phalanx of the great toe. It extends the great toe and assists with dorsiflexion.

Lateral Compartment

The lateral compartment contains two muscles that evert the foot (turn the sole outward) and assist with plantarflexion:

• Fibularis (peroneus) longus originates from the head and upper two-thirds of the lateral surface of the fibula. It inserts on the plantar surface of the medial cuneiform and base of the 1st metatarsal. Its tendon crosses under the foot, which helps support the transverse arch.
• Fibularis (peroneus) brevis originates from the lower two-thirds of the lateral surface of the fibula. It inserts on the tuberosity at the lateral base of the 5th metatarsal.

Posterior Compartment

The posterior compartment is divided into superficial and deep groups. All of these muscles contribute to plantarflexion (pointing the foot downward).

Superficial group:

• Gastrocnemius has two heads: the medial head from the posterior surface of the medial femoral condyle and the lateral head from the posterior surface of the lateral femoral condyle. It inserts on the posterior surface of the calcaneus via the calcaneal (Achilles) tendon. It plantarflexes the foot and also assists with knee flexion because it crosses the knee joint.
• Soleus originates from the posterior surface of the head and upper third of the fibula and the soleal line of the tibia. It also inserts on the calcaneus via the calcaneal tendon. It plantarflexes the foot but does not cross the knee.

Deep group:

• Tibialis posterior originates from the posterior surface of the interosseous membrane and adjacent surfaces of the tibia and fibula. It inserts on the tuberosity of the navicular, cuneiforms, cuboid, and bases of metatarsals 2–4. It plantarflexes and inverts the foot and is a major supporter of the medial longitudinal arch.
• Flexor digitorum longus originates from the posterior surface of the tibia below the soleal line. It inserts on the plantar surface of the distal phalanges of toes 2–5. It flexes toes 2–5.
• Flexor hallucis longus originates from the lower two-thirds of the posterior surface of the fibula. It inserts on the plantar surface of the distal phalanx of the great toe. It flexes the great toe and is important for push-off during walking.

Leg Compartments Summary

Intrinsic Foot Muscles

The foot contains its own set of small intrinsic muscles that enable fine movements of the toes and play a critical role in supporting the arches of the foot during standing and walking. These are analogous to the intrinsic muscles of the hand and are typically covered in less detail at this level, but know that they exist and contribute to balance and gait.