Hamstring Muscles: Names, Functions, and Importance in Movement

What are hamstring muscles called?

The hamstring muscle group comprises three distinct muscles located at the posterior aspect of the thigh: the biceps femoris, semitendinosus, and semimembranosus. These muscles are crucial for lower body movement, primarily responsible for hip extension and knee flexion.

The Hamstring Muscle Group: An Overview

Often referred to collectively as "the hamstrings," this powerful muscle group is located on the back of the thigh, extending from the pelvis down to just below the knee. While commonly treated as a single unit in casual conversation, understanding their individual names and functions is fundamental for proper training, injury prevention, and rehabilitation. These muscles are predominantly biarticular, meaning they cross and act upon two joints: the hip joint and the knee joint, making them vital for a wide range of movements, from walking and running to jumping and squatting.

The Three Primary Hamstring Muscles

The hamstring group is composed of three distinct muscles, each with unique attachments and subtle differences in their actions:

• Biceps Femoris The biceps femoris is the most lateral (outermost) of the hamstring muscles. It is unique in having two heads:

  • Long head: Originates from the ischial tuberosity (a prominent bony projection on the bottom of the pelvis, often called the "sit bone").
  • Short head: Originates from the linea aspera (a ridge on the posterior surface) of the femur (thigh bone).
  • Insertion: Both heads merge to insert primarily onto the head of the fibula (the smaller bone of the lower leg) and the lateral condyle of the tibia (the larger shin bone).
  • Actions: The long head performs hip extension and knee flexion, as well as external rotation of the flexed knee. The short head contributes only to knee flexion and external rotation of the flexed knee.

• Semitendinosus Located on the medial (inner) side of the posterior thigh, the semitendinosus is superficial to the semimembranosus. It gets its name from its long, cord-like tendon.

  • Origin: Ischial tuberosity.
  • Insertion: It inserts, along with the sartorius and gracilis muscles, onto the medial aspect of the proximal tibia, forming part of a common tendonous attachment known as the pes anserinus.
  • Actions: Hip extension, knee flexion, and internal rotation of the flexed knee.

• Semimembranosus This muscle lies deep to the semitendinosus on the medial side of the posterior thigh. Its name reflects its broad, flat, and membranous tendon of origin.

  • Origin: Ischial tuberosity.
  • Insertion: Primarily onto the medial condyle of the tibia.
  • Actions: Hip extension, knee flexion, and internal rotation of the flexed knee.

Functional Significance and Biomechanics

While each hamstring muscle has specific roles, their collective action is paramount for efficient movement:

• Hip Extension: All three hamstrings (specifically the long head of the biceps femoris, semitendinosus, and semimembranosus) are powerful hip extensors, working in conjunction with the gluteus maximus. This action is crucial during the propulsive phase of walking, running, and jumping, as well as in movements like deadlifts and good mornings.
• Knee Flexion: All three muscles are primary knee flexors, responsible for bending the knee. This function is vital for the swing phase of gait, decelerating the leg, and performing exercises like leg curls.
• Rotational Control: The semitendinosus and semimembranosus contribute to internal (medial) rotation of the flexed knee, while the biceps femoris facilitates external (lateral) rotation of the flexed knee. This rotational control is essential for agility, cutting movements, and stabilizing the knee joint.
• Adductor Magnus (Hamstring Portion): It's also worth noting that the "hamstring portion" of the adductor magnus muscle, which originates from the ischial tuberosity and inserts on the adductor tubercle of the femur, also acts as a powerful hip extensor and is sometimes functionally grouped with the hamstrings due to this shared action and origin.

Importance in Performance and Injury Prevention

A thorough understanding of the individual hamstring muscles is not merely academic; it has significant practical implications for fitness and health:

• Athletic Performance: Strong and well-balanced hamstrings are critical for athletic endeavors requiring speed, power, and agility. They are key accelerators in sprinting and powerful decelerators, protecting the knee.
• Injury Prevention: The hamstrings are one of the most commonly injured muscle groups, particularly in sports. Hamstring strains and tears often occur during high-speed running or movements involving rapid changes in direction, especially during the eccentric (lengthening) phase of muscle contraction. Imbalances between the quadriceps and hamstrings, or between the individual hamstring muscles, can increase injury risk.
• Rehabilitation: Targeted exercises for each hamstring muscle or specific portions can be vital during rehabilitation from injury, ensuring comprehensive recovery and strength restoration.
• Postural Stability: These muscles also contribute to pelvic stability and overall lower body alignment, influencing posture and reducing the risk of lower back pain.

Conclusion

The "hamstrings" are not a single muscle but a powerful trio: the biceps femoris, semitendinosus, and semimembranosus. Each contributes uniquely to the complex movements of the hip and knee, making them indispensable for both daily activities and peak athletic performance. Recognizing their individual roles allows for more informed training strategies, better injury prevention, and a deeper appreciation for the intricate design of the human body.