Biceps femoris

Proximal attachment: 

The two heads of the biceps femoris have different  attachment sites, 

• The long head attaches to the ischial tuberosity along with other muscles of the hamstring group.  
• The short head attaches to the linea aspera and the lateral supracondylar line of the femur. 

Distal attachment: 

Both heads combine to form a single tendon that attaches to the head of the fibula and the lateral tibial condyle distally, forming the lateral hamstrings. 

Nerve innervation: 

• The Biceps femoris gets innervated by the Sciatic nerve(L4- S3).
• While the long head of the biceps femoris is innervated by the tibial component of the sciatic nerve, the short head gets innervation from the common fibular component of the sciatic nerve.  
• It is the only muscle in the hamstring muscle group to have dual nerve innervation.

Blood supply: 

Arterial supply: 

• For the proximal one-third: inferior gluteal  artery
• For the middle one-third: Deep femoral artery (profunda femoris artery) 
• For the distal one-third: popliteal artery

Venous drainage: 

Inferior gluteal vein, deep femoral vein, and popliteal veins 

Function

The biceps femoris has functions in both the hip and knee joints, but its two heads differ in their contributions.

• The long head attaches to the ischial tuberosity proximally, allowing it to cross both the hip and knee joints. This makes it a two-joint muscle, responsible for movements at the hip as well as at the knee.
• In contrast, the short head attaches to the linea aspera and lateral supracondylar line of the femur proximally. Since it does not cross the hip joint, it solely contributes to functions at the knee joint. 

Concentric (shortening) action: 

Standard mover action:(distal attachment moving towards proximal) 

At the Knee joint: 

• The biceps femoris muscle produces knee flexion, along with other hamstring muscles. 
• When the knee is flexed, the biceps femoris (being the lateral hamstrings) can produce lateral rotation of the tibia relative to the femur. 

At the Hip joint: 

• The long head of the biceps femoris contributes to the hip extension. 
• Additionally, it medially rotates the thigh at the hip joint. 

Reverse mover action:(proximal attachment moving towards distal)

At the Knee joint: 

• The biceps femoris can flex the thigh at the knee joint along with other hamstring muscles (as in Nordic curl– while bringing the body up from the curl). 
• Although the reverse mover action is not typically observed in everyday functional activities, it is essential in special circumstances, such as the Nordic curl. 

At the Hip joint: 

• The long head of the biceps femoris helps produce posterior pelvic tilt at the hip joint.   

Eccentric (lengthening) action: 

• The biceps femoris contributes to the control and deceleration of the knee joint extension.  
• The long head controls and slows down the flexion at the hip joint. 
• Additionally, it also contributes to the controlled anterior pelvic tilt.  

Isometric function: 

• When the biceps femoris contracts isometrically, it helps stabilize the knee joint and hip joint during various lower limb activities. 
• The distal tendon contributes to the posterolateral stability of the knee.  

What is the purpose of your Rectus Femoris? 

• The main purpose of the rectus femoris (along with other hamstrings) is to help with walking, and mainly during sprinting, jumping, and climbing. 
• The biceps femoris, especially its long head, works with the gluteus maximus to generate a powerful hip extension for an efficient push-off. As the leg swings forward, it contracts eccentrically to slow the thigh and prevent the knee from overextending before foot strike. This action deloads the ACL ligament and prevents its injury. 
• The long head helps stabilize the Pelvis, especially during trunk flexion, and both heads provide posterolateral stability to the knee joint. 
• Although it is a crucial muscle for sprinting, it is the most commonly injured muscle because of the following reasons: 
  1. It has a higher eccentric demand and bears more load in decelerating the leg at high speed, compared to the semitendinosus and semimembranosus. 
  2. The long head of the biceps femoris has a complex architecture that provides strength and speed but also makes it vulnerable to injuries. 
  3. The two heads of the biceps femoris have different innervation (long head- tibial nerve and short head- common fibular nerve). Sometimes this leads to an asynchronous activation and uneven force distribution, resulting in injury.

The False Hamstrings- Short head: 

The short head of the biceps femoris is sometimes referred to as the false hamstrings, because it varies from other hamstring muscles in several criteria, which include: