Do hamstrings consist of muscles and tendons?

Anatomy of the Hamstring Muscles and Tendons

Yes, hamstrings consist of both muscles and tendons, forming a complex musculotendinous unit in the posterior thigh that plays a critical role in knee flexion, hip extension, and overall lower extremity function.

Hamstring Muscle Components

The hamstring muscle complex consists of three distinct muscles:

1. Biceps Femoris:

   • Has two heads:
     • Long head (originates from ischial tuberosity)
     • Short head (originates from linea aspera of femur)
   • Forms the lateral portion of the hamstring group
   • Inserts on the fibular head and lateral tibia

2. Semitendinosus:

   • Originates from ischial tuberosity via a conjoint tendon shared with biceps femoris long head
   • Contains a unique tendinous inscription (raphe) present in 96% of individuals 1
   • Forms part of the medial hamstring group
   • Has longer muscle fascicles compared to other hamstring muscles

3. Semimembranosus:

   • Originates independently from the ischial tuberosity (separate from the conjoint tendon)
   • Forms part of the medial hamstring group
   • Has a characteristic twist in its proximal tendon 1

Tendon Anatomy and Architecture

The hamstring tendons have several notable characteristics:

• Proximal Tendons:

  • Biceps femoris long head and semitendinosus share a conjoint tendon at the ischial tuberosity 2, 1
  • Semimembranosus has an independent tendon attachment at the ischial tuberosity
  • The angle of origin differs between tendons (conjoint tendon median angle: 12°, semimembranosus: 6°) 3

• Distal Tendons:

  • Overlapping proximal and distal tendons in both biceps femoris long head and semimembranosus 1
  • Semitendinosus has a long distal tendon that forms part of the pes anserinus

Architectural Variations and Clinical Implications

The hamstring muscles show significant intra- and inter-muscular variations:

• Biceps Femoris Long Head:

  • Has shorter and more pennated fibers compared to other hamstring muscles 4
  • Proximally has shorter and more pennated fibers with a narrower aponeurosis than distally 4
  • Most commonly injured hamstring muscle 5

• Semitendinosus:

  • Displays less relative strain than other hamstrings during joint motion
  • Has greater length, longer fascicles, and possibly a longer tendon 4
  • Contains a tendinous inscription (raphe) that may protect against gross injury 1

• Semimembranosus:

  • Similar architecture to biceps femoris long head but with different proximal attachment and moment arm 4

Clinical Relevance

The unique anatomy of the hamstring complex has important implications:

1. Injury Patterns:

   • The biceps femoris long head is most frequently injured, possibly due to its architectural properties and angle of origin 3, 5
   • The conjoint tendon is subject to concentrated traction stress from both medial and lateral hamstrings 2

2. Rehabilitation Considerations:

   • Hamstring exercises should target all components due to architectural differences 6
   • Strengthening programs should include both eccentric and concentric components 6
   • Flexibility training of hamstrings is recommended as part of comprehensive exercise programs 7

3. Surgical Applications:

   • Hamstring tendons (particularly semitendinosus and gracilis) are commonly used as autografts for ACL reconstruction 6
   • When using hamstring autografts for ACL reconstruction, surgeons may favor them to reduce the risk of infection, though they may have higher graft failure rates compared to bone-patellar tendon-bone grafts 6

Understanding the complex anatomy and biomechanics of the hamstring muscles and tendons is essential for proper diagnosis, treatment, and rehabilitation of hamstring injuries, as well as for surgical planning when using hamstring tendons as grafts.