Biceps Femoris: Origin, Insertion, Nerve, Action & Hamstring Strains

The Biceps Femoris is the large, powerful muscle of the lateral hamstring group. Just like the biceps in the arm, it has "two heads" (Long and Short). It is a critical muscle for speed, explosive power, and knee stability, and is notably the most commonly injured muscle in hamstring strains.

[Image of Biceps Femoris muscle anatomy]

Quick Anatomy Snapshot

Head	Long Head	Short Head
Origin (Proximal)	Ischial Tuberosity (Sit bone) - via a conjoint tendon with Semitendinosus.	Linea Aspera (Lateral lip) and Lateral Supracondylar Line of the Femur.
Insertion (Distal)	Head of the Fibula (Lateral side).
Nerve Supply	Tibial Division of Sciatic Nerve (L5, S1, S2).	Common Peroneal Division of Sciatic Nerve (L5, S1, S2).
Primary Actions	Knee Flexion, Hip Extension, External Rotation of tibia.	Knee Flexion, External Rotation of tibia (Does NOT extend the hip).

Deep Dive: The Hybrid Muscle

The Biceps Femoris is anatomically unique because its two heads have different origins and different nerve supplies.

1. Dual Innervation

It is one of the few muscles in the body supplied by two different nerve divisions.
• The Long Head is a "true hamstring" (crosses two joints, supplied by Tibial nerve).
• The Short Head is a "pseudo hamstring" (crosses only the knee, supplied by Common Peroneal nerve).

2. The Lateral Rotator

Unlike the Semitendinosus and Semimembranosus (medial hamstrings) which internally rotate the tibia, the Biceps Femoris passes laterally to insert on the fibula. This allows it to externally rotate the lower leg when the knee is flexed.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Long Head of the Biceps Femoris is the primary decelerator of the leg during the terminal swing phase of running. This is why it is injured so frequently in sprinters—it must contract eccentrically to stop the leg from snapping straight too fast.

Palpation

Have the patient lie prone with the knee slightly flexed. Ask them to hold that position. Locate the two tendons behind the knee (popliteal fossa). The lateral tendon (outer side) is the Biceps Femoris. It is thick and cord-like, attaching to the head of the fibula.

⚠️ Clinical Pathology: Hamstring Strain
The Biceps Femoris Long Head accounts for the vast majority (approx. 80%) of all hamstring strains. This often occurs at the musculotendinous junction near the origin or insertion. "Tight hamstrings" can also posteriorly tilt the pelvis, contributing to flat back posture.

Manual Muscle Testing (MMT)

To isolate the Biceps Femoris from the medial hamstrings, we use tibial rotation.

Testing Tip: Remember: Biceps Femoris = Lateral Hamstring = Lateral Rotation (Toes Out).

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down). Knee flexed to ~45°. Setup: Leg is Externally Rotated (toes pointing out). Action: Patient holds the knee flexion while keeping toes pointed out. Resistance: Applied at the ankle, pushing the leg down into extension (and slightly trying to de-rotate).
Grade 2 (Gravity Eliminated)	Position: Side-lying on the test side (can be awkward) or Prone. Action: Patient flexes the knee with external rotation through range. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the lateral tendon near the fibular head or the ischial tuberosity. Cue: "Try to bend your knee." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Why doesn't the Short Head extend the hip?

Because it originates on the Femur (Linea Aspera), not the Ischium. Therefore, it does not cross the hip joint and has no leverage to move it.

What is the "Conjoint Tendon"?

At the proximal origin (Ischial Tuberosity), the Biceps Femoris Long Head shares a common tendon attachment with the Semitendinosus. A tear here affects both muscles.

Does it form the Popliteal Fossa?

Yes. The Biceps Femoris forms the Superolateral (Upper-Outer) border of the diamond-shaped space behind the knee.

Test Your Knowledge: Biceps Femoris Quiz

1. Which nerve supplies the Short Head of the Biceps Femoris?

2. Where does the Biceps Femoris insert?

3. What is the primary action of the Long Head on the hip?

4. To bias the Biceps Femoris during MMT, the tibia should be:

5. Which head of the Biceps Femoris crosses two joints?

6. The Biceps Femoris forms which border of the Popliteal Fossa?

7. Where does the Short Head originate?

8. Which hamstring muscle acts as an external rotator of the tibia?

9. Which phase of gait is the Biceps Femoris most active (eccentrically)?

10. True or False: The Biceps Femoris Long Head shares a tendon of origin with the Semimembranosus.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Brukner, P., & Khan, K. (2017). Clinical Sports Medicine. 5th ed. McGraw-Hill Education.