What constitutes the hamstring muscles?

Anatomy of the Hamstring Muscles

The hamstring muscle complex consists of three major muscles in the posterior thigh: the biceps femoris, semimembranosus, and semitendinosus. 1, 2

Detailed Anatomy

Origin and Insertion

• Origin: All three muscles (except the short head of biceps femoris) originate from the ischial tuberosity 2, 3
• Insertion:
  • Biceps femoris: Inserts into the head of the fibula
  • Semimembranosus and semitendinosus: Insert into the medial aspect of the tibia 2

Specific Anatomical Features

Biceps Femoris

• Consists of two heads:
  • Long head: Originates from the ischial tuberosity
  • Short head: Originates from the linea aspera of the femur
• Forms a conjoint tendon with the semitendinosus at its origin 3

Semitendinosus

• Has a unique tendinous inscription (raphe) present in 96% of individuals
• A group of semitendinosus fibers consistently arise from the proximal aspect of the long head of biceps femoris, located approximately 67 ± 12 mm from the ischial tuberosity 4, 5
• This secondary origin has a width of about 10.9 ± 5.3 mm and thickness of 3.2 ± 1.4 mm 4

Semimembranosus

• Has a twist in its proximal tendon
• Origin angle at the ischial tuberosity is approximately 6 degrees (compared to 12 degrees for the conjoint tendon) 3

Architectural Relationships

• The conjoint tendon (biceps femoris long head and semitendinosus) has a steeper angle of origin (median 12°) compared to the semimembranosus (median 6°) 3
• Overlapping proximal and distal tendons exist in both the biceps femoris long head and semimembranosus 5

Functional Aspects

• Primary actions: Hip extension and knee flexion 2
• The hamstrings cross both the hip and knee joints (except the short head of biceps femoris)
• During contraction, the horizontal component of the semitendinosus can pull the long head of biceps femoris medially 4

Clinical Significance

• The hamstring complex is among the most commonly injured muscle groups in athletes, particularly during sprinting activities 2, 4
• The different angles of origin between the conjoint tendon and semimembranosus may contribute to injury patterns, with approximately 9% more force applied to the conjoint tendon origin 3
• Injury patterns differ by activity:
  • Sprint injuries typically affect the biceps femoris
  • Stretching injuries typically affect the semimembranosus 2
• The presence of the raphe in the semitendinosus may play a protective role against gross muscle injury 5

Imaging Considerations

• MRI and ultrasound are complementary modalities for diagnosing hamstring injuries
• Key factors in assessment include injury length, connective tissue involvement, and tear location 2
• The most frequent injury location is where the long head of biceps femoris joins with the semitendinosus 4

Understanding the complex anatomy of the hamstring muscles is essential for proper diagnosis, treatment, and rehabilitation of hamstring injuries, which are particularly common in athletes who engage in sprinting and activities requiring forceful stretching.