Hamstrings
The hamstrings are a group of three muscles on the posterior thigh: biceps femoris, semitendinosus, and semimembranosus. They are powerful knee flexors and hip extensors, crucial for walking, running, and jumping.
Origin, Insertion, Action, Innervation
OOrigin
All three originate from the ischial tuberosity. Biceps femoris long head: Ischial tuberosity. Biceps femoris short head: Linea aspera of femur. Semitendinosus: Ischial tuberosity. Semimembranosus: Ischial tuberosity.
IInsertion
Biceps femoris: Head of fibula. Semitendinosus: Upper medial tibia (pes anserinus). Semimembranosus: Posterior medial condyle of tibia.
AAction
- • Flexion of the knee (all three muscles)
- • Extension of the hip (all three muscles except biceps short head)
- • Lateral rotation of the flexed knee (biceps femoris)
- • Medial rotation of the flexed knee (semitendinosus and semimembranosus)
NInnervation
Sciatic nerve - tibial division (L5, S1, S2) for all except biceps femoris short head, which is innervated by the common fibular (peroneal) division.
Blood Supply
Perforating branches of profunda femoris artery
Clinical Relevance
Hamstring strains are common in sports, especially during sprinting. The muscles are prone to injury because they cross two joints and have a high proportion of fast-twitch fibers. Tight hamstrings can contribute to lower back pain and postural problems.
Palpation
Palpate on the posterior thigh. The biceps femoris tendon is lateral (to the fibula) and the semitendinosus/semimembranosus tendons are medial. Have the patient flex the knee against resistance.
Study Tips
- ✓THREE muscles - Biceps femoris (2 heads), Semitendinosus, Semimembranosus
- ✓All from ischial tuberosity except biceps short head (linea aspera)
- ✓Biceps = lateral (fibula), Semi's = medial (tibia)
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Common questions about the hamstrings
The three hamstring muscles are: biceps femoris (with long and short heads), semitendinosus, and semimembranosus. They occupy the posterior compartment of the thigh.
All hamstring muscles originate from the ischial tuberosity ('sit bone') except the short head of biceps femoris, which originates from the linea aspera of the femur.
Hamstring strains are common because these muscles cross two joints (hip and knee), have a high proportion of fast-twitch fibers, and are eccentrically loaded during activities like sprinting. They also have a complex nerve supply.
Biceps femoris is lateral and inserts on the fibula head; it laterally rotates the flexed knee. Semimembranosus is medial and inserts on the posterior medial tibia; it medially rotates the flexed knee.