The hamstrings (biceps femoris long head, semimembranosus, and semitendinosus) are the primary knee flexors and important hip extensors. They work eccentrically to decelerate the swinging lower leg and control knee extension during landing.
| Origin | Ischial tuberosity — semimembranosus from the superolateral facet, biceps femoris long head and semitendinosus from the inferomedial facet |
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| Insertion | Tibia and fibula — see individual muscle entries |
| Nerve Supply | Tibial nerve for all three long heads (L5, S1, S2); Common peroneal nerve for the biceps femoris short head (L5, S1, S2) |
| Blood Supply | Perforating arteries from the profunda femoris |
| Actions | Flex the knee — primary function; Extend the hip — the most powerful hip extensors alongside gluteus maximus; Decelerate the swing phase of gait |
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Hamstring tears are the most common acute muscle injury in sport. Proximal hamstring avulsion from the ischial tuberosity produces the "waterskiing injury" and requires surgical reattachment if completely avulsed. The hamstring-to-quadriceps ratio (H:Q ratio) below 0.6 is a risk factor for hamstring strain. Hamstrings are the graft donors for ACL reconstruction.
Palpated posterior in the thigh during resisted knee flexion — semimembranosus medially, semitendinosus as a cord posteromedially, and biceps femoris laterally.
Complete hamstring detachment from the ischial tuberosity producing ecchymosis, weakness of hip extension and knee flexion, requiring surgical reattachment for athletes.
