Knee Muscles Overview

Overview

This lecture covers the main muscles acting on the knee joint, detailing their origins, insertions, and primary functions, with a focus on the quadriceps, hamstrings, and several key accessory muscles.

Quadriceps Muscles

• The quadriceps group consists of four muscles: vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris.
• All quadriceps muscles originate from the anterior hip and femur and insert via the quadriceps tendon into the patella.
• The patella then connects to the tibial tuberosity through the patella tendon.
• Main action: knee extension.
• Rectus femoris also weakly assists in hip flexion due to its origin near the hip joint.

Hamstring Muscles

• The hamstrings include semimembranosus (most medial), semitendinosus, and biceps femoris (with long and short heads).
• Semimembranosus and semitendinosus originate from the ischial tuberosity (“sitting bone”); biceps femoris long head also originates here, while its short head arises from the lateral femur.
• Semimembranosus inserts on the posterior medial tibial condyle; semitendinosus inserts at the pes anserine (superior medial tibia); biceps femoris inserts on the fibular head.
• Main action: knee flexion; also assist in hip extension.

Accessory Muscles of the Knee

• Sartorius and gracilis both cross the medial thigh, weakly flex the knee, and insert at the pes anserine.
• Sartorius also assists in hip flexion; gracilis aids hip adduction.
• Semitendinosus, sartorius, and gracilis share a common insertion, and irritation here can cause pes anserine bursitis.
• Gastrocnemius (calf muscle) has two heads from the posterior femoral condyles and weakly flexes the knee.
• Popliteus originates from the lateral femoral condyle and inserts posteriorly on the tibia, unlocking the knee by rotating femur and tibia.

Key Terms & Definitions

• Quadriceps tendon — Connects quadriceps muscles to the patella.
• Patella tendon — Connects the patella to the tibial tuberosity.
• Pes anserine — Common insertion on the medial tibia for sartorius, gracilis, and semitendinosus.
• Ischial tuberosity — Bony prominence of the pelvis; origin for most hamstrings.
• Popliteus — Small muscle that unlocks the knee from extension.

Action Items / Next Steps

• Review the origin, insertion, and function of the quadriceps, hamstring, and accessory muscles.
• Study the anatomical locations of the pes anserine and ischial tuberosity.
• Prepare to identify these muscles and landmarks in diagrams or models.