Hamstrings
Two jobs, and most programs only train one of them at length.
2 min read
Function
Four muscles — semimembranosus, semitendinosus, and biceps femoris (long and short heads) — with two separable jobs: knee flexion (bending the knee, as in a leg curl) and hip extension (driving the hip forward, as in a deadlift or hip thrust).
Note the short head of biceps femoris does not cross the hip at all — it originates lower, on the femur, and only flexes the knee. It is the one hamstring head a hip-hinge movement cannot reach.
Origin and insertion
Semitendinosus, semimembranosus and the long head of biceps femoris originate at the ischial tuberosity — the "sit bones." The short head of biceps femoris originates separately, on the femur itself. Semitendinosus and semimembranosus insert on the tibia; both heads of biceps femoris insert at the fibula.
The length nuance that changes exercise selection
Hip-extension exercises (RDL, deadlift, hip thrust) put the hamstrings under load with the hip flexed and lengthened — the muscle is working near its long end.
Knee-flexion exercises (leg curls) can be done two ways, and the difference matters: seated, with the hip flexed, the hamstring is held at a long length throughout the curl. Lying, with the hip extended, it is held short throughout. Same muscle, same joint action, meaningfully different length exposure.
Given that training at long muscle lengths produces measurably more hypertrophy — the mechanism is passive tension through titin plus a bias toward adding sarcomeres in series — the seated curl is the better default, not an interchangeable equivalent to the lying version.
Exercise selection
| Exercise | Job trained | Length | Note |
|---|---|---|---|
| Romanian deadlift | Hip extension | Long | The hip-hinge hamstring stimulus |
| Seated leg curl | Knee flexion | Long throughout | Better default than lying |
| Lying leg curl | Knee flexion | Short throughout | Use only if seated is unavailable |
| Nordic curl | Knee flexion, eccentric-biased | Long | Advanced; very high eccentric demand |
| Good morning | Hip extension | Long | More spinal loading than an RDL |
Neither hip-extension nor knee-flexion work alone is a complete hamstring stimulus. The two are different joint actions and a program needs both — this is why every full hypertrophy program on this site pairs an RDL-family movement with a curl.
Source tier: joint actions and length-tension mechanics are established anatomy. The seated-vs-lying preference follows from the general long-length hypertrophy literature (Wolf et al., 2025 systematic review) applied to this specific joint geometry.
Up next
Glutes
One muscle with four jobs, and the exercise that loads its hardest-to-reach one.
2 min