Hamstrings
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Three 2-joint muscles — biceps femoris, semitendinosus, semimembranosus — that flex the knee (primary) and extend the hip (secondary); knee-flexion ROM 125°.
Detail
The hamstrings are three muscles: biceps femoris, semitendinosus, and semimembranosus. They function primarily as knee flexors and secondarily as hip extensors, and are named for their cordlike attachments at the knee. All 3 hamstring muscles cross the hip and knee joints (2-joint muscles) and are involved in movement at both joints:
- Knee flexion (leg curl): hip joint stabilized.
- Hip extension (stiff-legged deadlift): knee joint stabilized.
Per the posterior thigh diagram (Yessis, 1992): semimembranosus produces flexion and medial rotation; semitendinosus produces flexion and medial rotation; biceps femoris produces flexion and lateral rotation. Knee-flexion ROM is 125 degrees. Tight hamstrings (and tight Achilles tendon) restrict ROM for eccentric movements in the squat and leg press. Tight hamstrings are also a cause of back problems. To reduce gastrocnemius involvement, keep it in plantar flexion while doing knee flexions. Listed exercises: leg curl and glute-ham developer.
Key facts
- 3 hamstrings: biceps femoris, semitendinosus, semimembranosus.
- Primary: knee flexors; secondary: hip extensors. All are 2-joint muscles.
- Knee flexion ROM: 125°.
- Semimembranosus/semitendinosus: flexion + medial rotation; biceps femoris: flexion + lateral rotation.
- Tight hamstrings restrict squat/leg-press eccentric ROM and contribute to back problems.
Connections
- Hip Extensors — hamstrings extend the hip.
- Knee Joint Range of Motion — knee flexion ROM 125°.
- Two-Joint (2-Joint) Muscles — hamstrings are 2-joint muscles.
- Leg Curl (Hamstring Curl) / Stiff-Legged Deadlift / Glute-Ham Developer — training exercises.
Sources
Exercise guides
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Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.