Muscles of the Thigh

Overview of Thigh Muscle Compartments

• The muscles of the thigh are organized into $3$ distinct functional and anatomical compartments:

  • Anterior Compartment: Primarily responsible for hip flexion and knee extension.

  • Medial Compartment: Primarily consists of adductors; these muscles act to pull the limb toward the midline.

  • Posterior Compartment: Primarily consists of the hamstrings; these muscles are responsible for knee flexion and hip extension.

• These muscles are bi-articular in many cases, meaning they act at both the hip (acetabulofemoral joint) and the knee (tibiofemoral joint).

Hip and Knee Joint Kinematics

• Hip Movements (Acetabulofemoral Joint):

  • Flexion: Involves muscles of the anterior compartment lifting the leg anteriorly.

  • Extension: Involves muscles of the posterior aspect bringing the leg posteriorly.

  • Abduction: Pulling the leg away from the midline on the coronal plane using the anteroposterior axis of rotation. Key muscles include gluteus medius, gluteus minimus, tensor fascia lata, and the iliotibial (IT) tract.

  • Adduction: Pulling the limb toward the midline, primarily involving the medial thigh muscles.

  • External Rotation: Turning the limb outward. Muscles involved include the deep external rotators: piriformis, obturator internus, obturator externus, gemellus superior, gemellus inferior, and quadratus femoris.

  • Internal Rotation: Rotating the limb inward toward the midline.

• Knee Movements (Tibiofemoral Joint):

  • Flexion: Pulling the lower leg toward the posterior midline of the thigh, driven by the posterior compartment.

  • Extension: Straightening the leg (e.g., kicking a soccer ball), driven by the anterior compartment (quadriceps).

The Anterior Compartment of the Thigh

• The anterior compartment contains $8$ muscles that share a common fascial space (with minor exceptions regarding innervation for specific muscles).

• Psoas Major and Iliacus (Iliopsoas):

  • Psoas Major Origin: Lower thoracic and lumbar vertebrae ($T12$ to $L5$).

  • Iliacus Origin: Iliac fossa (internal aspect of the iliac wing).

  • Insertion: Both muscles run underneath the inguinal ligament to insert into the lesser trochanter of the femur.

  • Primary Action: Hip flexion.

• Sartorius:

  • Characteristics: The longest muscle in the human body.

  • Origin: Anterior superior iliac spine (ASIS).

  • Path: Runs down the anterior surface, traverses to the medial side, wraps around the knee, and inserts distal to the knee.

  • Insertion: Medial aspect of the tibia, just medial to the tibial tuberosity (below the medial condyle).

  • Primary Action: Flexes the hip and enhances knee flexion (unique as the only anterior muscle to do both). It also assists in external rotation of the hip.

• Quadriceps Femoris: A group of $4$ powerful muscles that extend the knee via a common insertion in the patella and the tibial tuberosity via the patellar ligament.

  • Rectus Femoris: The most superficial muscle of the group. It originates at the anterior inferior iliac spine (AIIS) and the notch on the acetabulum. It is bi-articular, meaning it flexes the hip and extends the knee.

  • Vastus Lateralis: Originates on the greater trochanter and the lateral side of the linea aspera on the posterior femur. It wraps around to insert into the lateral aspect of the patella.

  • Vastus Medialis: Originates on the intertrochanteric line and the medial side of the linea aspera. It wraps around to insert into the medial aspect of the patella.

  • Vastus Intermedius: Located deep to the rectus femoris. It originates from the anterior and lateral aspects of the femoral shaft.

• Pectineus:

  • Origin: Pectineal line of the pubic bone (anterior aspect of the pubis around the obturator foramen).

  • Insertion: Pectineal line of the femur.

  • Action: Hip flexion, adduction, and external rotation.

  • Classification Debate: While it acts as an adductor and can be innervated by the obturator nerve, it is grouped with the anterior compartment because its primary innervation is usually the femoral nerve ($\text{the nerve of the anterior compartment}$) and its primary role is hip flexion.

The Medial Compartment (The Adductors)

• These muscles primarily originate around the obturator foramen and insert along the posterior femur.

• Adductor Brevis:

  • Depth: The deepest of the adductor muscles.

  • Origin: Inferior pubic ramus/ischiopubic ramus.

  • Insertion: Linea aspera on the posterior femur.

  • Action: Strong adduction and assists with external rotation due to its posterior insertion.

• Adductor Longus:

  • Position: Superficial to the adductor brevis.

  • Origin: Pubic bone.

  • Insertion: Linea aspera, distal to the adductor brevis insertion.

  • Action: Adduction of the hip.

• Adductor Magnus:

  • Size: The largest muscle in the medial compartment.

  • Origin: Inferior ischiopubic ramus and the ischial tuberosity.

  • Insertion: Expands the entire length of the femoral shaft, from the gluteal tuberosity and pectineal line down to the adductor tubercle on the medial supracondylar ridge.

  • Adductor Hiatus: A gap in the muscle that allows the femoral artery and vein to transition from the anterior/medial compartment to the posterior popliteal fossa. At this landmark, the femoral artery becomes the popliteal artery.

  • Functional Parts: The fibers from the ischial tuberosity to the adductor tubercle can assist in hip extension, effectively serving a hamstring-like role.

• Gracilis:

  • Characteristics: The most medial, most superficial, and longest (slender) muscle of the medial thigh.

  • Origin: Shaft of the descending pubic rami (near the pubic symphysis).

  • Insertion: Medial aspect of the tibia, near the sartorius insertion.

  • Action: Adducts the thigh and supports knee flexion when the knee is already flexed.

• Obturator Externus: Located on the external surface of the obturator membrane; it wraps underneath the acetabulum to the posterior femur to aid in external rotation.

The Posterior Compartment (The Hamstrings)

• The hamstrings are hip extensors and knee flexors. They are commonly injured during activities such as running.

• Biceps Femoris:

  • Long Head: Originates at the ischial tuberosity. It is bi-articular (extends hip, flexes knee).

  • Short Head: Originates from the medial aspect of the lower linea aspera on the femoral shaft. It only acts at the knee (flexion).

  • Insertion: Combined tendon inserts into the lateral aspect of the tibia and the head of the fibula.

  • Action: Knee flexion, hip extension (long head only), and external rotation of both the hip and knee.

• Semimembranosus:

  • Depth: Deep to the semitendinosus.

  • Origin: Ischial tuberosity.

  • Insertion: Medial aspect of the tibia (below the tibial plateau).

  • Action: Hip extension, knee flexion, and internal rotation.

• Semitendinosus:

  • Characteristics: Has a small muscle belly and a very long, prominent tendon.

  • Origin: Ischial tuberosity (most medial origin point).

  • Insertion: Medial aspect of the tibia, near the gracilis and sartorius.

  • Action: Hip extension, knee flexion, and internal rotation.

Knee Terminal Extension Mechanics and the Popliteus

• Closed Packed Position (The Screw-Home Mechanism):

  • During full knee extension, the tibia passively rotates laterally ($\text{extrinsic rotation}$) underneath the femur.

  • Causation: The medial condyle of the femur and the medial meniscus are larger than their lateral counterparts. When the lateral condyle completes its roll during extension, the medial side continues moving, causing a "twisting" or "locking" effect.

  • Benefit: This mechanism allows for a stable, passive standing position that requires minimal muscular contraction.

• Popliteus Muscle:

  • Role: Known as the "key" to unlocking the knee.

  • Function: It is a small, oblique muscle (similar to the pronator teres in the arm) that medially rotates the tibia to unscrew the lock.

  • Outcome: This realignment of the medial and lateral condyles enables the rolling mechanism required for the knee to initiate flexion properly.