Hamstrings

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.

• 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.

• hip-extensors — hamstrings extend the hip.
• knee-joint-rom — knee flexion ROM 125°.
• two-joint-muscle — hamstrings are 2-joint muscles.
• leg-curl / stiff-legged-deadlift / glute-ham-developer — training exercises.