Hamstrings: Joints Crossed, Anatomy, and Functional Implications

How many joints does the hamstring cross?

The hamstring muscle group primarily crosses two major joints: the hip joint and the knee joint. However, one component, the short head of the biceps femoris, is an exception, crossing only the knee joint.

Understanding the Hamstring Muscle Group

The term "hamstrings" refers to a group of three distinct muscles located on the posterior aspect of the thigh. These muscles are crucial for lower body movement, stability, and athletic performance. Understanding their anatomy, particularly their origins and insertions, is key to comprehending which joints they influence.

The hamstring group consists of:

• Semitendinosus: Located medially, superficial to the semimembranosus.
• Semimembranosus: Located medially, deep to the semitendinosus.
• Biceps Femoris: Has two heads:
  • Long Head: Located laterally.
  • Short Head: Located laterally, deep to the long head.

With the exception of the short head of the biceps femoris, all hamstring muscles originate from the ischial tuberosity (a prominent bony landmark on the pelvis). Their insertions vary, attaching to different parts of the tibia and fibula bones of the lower leg.

The Hip Joint: A Proximal Crossing

All three of the hamstring muscles, except the short head of the biceps femoris, originate from the ischial tuberosity of the pelvis. This anatomical configuration means they cross the hip joint (the articulation between the femur and the acetabulum of the pelvis) proximally.

Because these hamstring components cross the hip joint, they are primary movers for:

• Hip Extension: The action of moving the thigh backward, away from the front of the body. This is a fundamental movement in activities like walking, running, jumping, and squatting.
• Posterior Pelvic Tilt: While primarily known for hip extension, their pull on the ischial tuberosity can also contribute to tilting the pelvis backward.

The Knee Joint: A Distal Crossing

All four components of the hamstring group (semitendinosus, semimembranosus, biceps femoris long head, and biceps femoris short head) cross the knee joint. They insert onto various points on the tibia (shin bone) and fibula (calf bone).

Due to their distal attachments crossing the knee joint, all hamstrings are primary movers for:

• Knee Flexion: The action of bending the knee, bringing the heel towards the glutes. This is evident in movements like hamstring curls, walking, and climbing stairs.
• Knee Rotation (when the knee is flexed):
  • Internal Rotation: The semitendinosus and semimembranosus contribute to internally rotating the tibia relative to the femur.
  • External Rotation: The biceps femoris (both long and short heads) contributes to externally rotating the tibia relative to the femur.

The Unique Case: Biceps Femoris Short Head

The short head of the biceps femoris is a crucial distinction when discussing hamstring anatomy. Unlike its long head counterpart and the other hamstring muscles, the short head of the biceps femoris originates from the linea aspera and lateral supracondylar ridge of the femur itself, not the ischial tuberosity.

This unique origin means that the short head of the biceps femoris crosses only one joint: the knee joint. Consequently, its sole primary action is knee flexion and external rotation of the flexed knee; it does not contribute to hip extension.

Functional Implications for Movement and Training

The fact that most hamstring muscles are biarticular (crossing two joints) has significant functional implications:

• Coordinated Movement: Their ability to act on both the hip and knee allows for powerful and coordinated movements, such as the propulsive phase of running (hip extension and knee flexion).
• Length-Tension Relationship: The effectiveness of the hamstrings can be influenced by the position of both the hip and knee joints. For example, a hamstring curl (knee flexion) is more challenging if the hip is already extended, as the muscle is in a shortened position at the hip. Conversely, a stiff-legged deadlift (hip extension with minimal knee flexion) challenges the hamstrings in a lengthened position.
• Injury Prevention: Understanding their biarticular nature is vital for designing effective injury prevention programs, particularly for common hamstring strains, which often occur during eccentric contractions (muscle lengthening under tension) at high speeds, such as during sprinting.
• Training Considerations: Effective hamstring training programs often incorporate exercises that target both their hip extension function (e.g., Romanian deadlifts, good mornings) and their knee flexion function (e.g., leg curls, glute-ham raises) to ensure comprehensive development and strength across their full range of motion.

Conclusion

In summary, the hamstring muscle group primarily crosses two joints: the hip joint proximally and the knee joint distally. This biarticular nature allows them to perform powerful hip extension and knee flexion, as well as knee rotation. The key exception is the short head of the biceps femoris, which uniquely crosses only the knee joint. This anatomical detail is fundamental to understanding their function in movement and optimizing training strategies.