📝 STANDARD DESCRIPTION:
Type: Big muscle group on the front of thigh.
4 Heads:
1. Rectus Femoris (only one from ilium — biarticular, crosses both hip & stifle).
2. Vastus Lateralis (lateral side of femur).
3. Vastus Medialis (medial side of femur).
4. Vastus Intermedius (front of femur, deep).
Common Insertion: Tibial tuberosity, via PATELLA + 3 patellar ligaments (in horse/ox) or single patellar ligament (dog).
Function: The MAIN extensor of the STIFLE joint. Rectus femoris also flexes the hip.
Nerve: Femoral nerve.
📚 DETAILED DESCRIPTION:
QUADRICEPS — 'FOUR HEADS' OF THE FRONT THIGH
The quadriceps femoris is a big muscle group on the front of the thigh. It is the MAIN extensor of the stifle (knee).
FOUR HEADS:
• Rectus Femoris: Only head originating from the ilium (above the hip). Therefore it crosses TWO joints (hip + stifle) → it can also flex the hip in addition to extending the stifle. Forms the topmost belly of the front thigh.
• Vastus Lateralis: From the lateral surface of femur.
• Vastus Medialis: From the medial surface of femur.
• Vastus Intermedius: From the front (cranial) surface of femur — deepest head.
All four heads converge to form a strong common tendon, which contains the patella (largest sesamoid). From the patella, the patellar ligaments transmit the force to the tibial tuberosity.
FUNCTION: Strong extension of the stifle — keeps the leg straight under body weight. Without quadriceps, the animal cannot stand because the stifle would buckle.
NERVE: Femoral nerve (from lumbar plexus). Damage to femoral nerve = quadriceps paralysis = animal cannot bear weight on that limb (the limb buckles forward at every step).
// COMPARATIVE ANALYSIS
| HORSE |
Quadriceps + medial patellar ligament + medial parapatellar fibrocartilage = key components of stay apparatus (stifle lock). |
| DOG |
Same 4 heads. Single patellar ligament inserts on tibial tuberosity. |
CLINICAL RELEVANCE
Femoral Nerve Paralysis: Quadriceps cannot contract → cannot extend the stifle → the limb buckles when bearing weight → 'dropped hind quarter'.
IM injection sites: Vastus lateralis is a safe IM site in dogs and cats — large mass, away from sciatic nerve.
