What are the three hamstring muscles?

Biceps femoris, semitendinosus and semimembranosus. All three originate on the sitting bone and cross both the hip and the knee, so they extend the hip and flex the knee. The short head of biceps femoris is a partial exception — it starts on the femur and only crosses the knee.

Are leg curls or Romanian deadlifts better for hamstrings?

Neither replaces the other. Romanian deadlifts train hip extension with the hamstrings lengthened; leg curls train knee flexion, including the short head of biceps femoris that hinges cannot reach. A complete hamstring programme includes both.

Why do hamstrings get injured so often?

Because they work eccentrically at high speed during sprinting, decelerating the lower leg just before the foot lands, while being lengthened at the hip at the same time. That combination of high force and long muscle length is where most strains occur. Eccentric training such as Nordic curls has good evidence for reducing that risk.

How do I stretch tight hamstrings?

Standing or supine hamstring stretches held for around 30 seconds are the usual approach. Worth knowing though: a feeling of hamstring tightness is often a stiffness or neural-tension issue rather than genuinely short muscle, and strengthening through a full range frequently helps more than stretching does.

Lower body

Hamstrings

Biceps femoris, semitendinosus and semimembranosus

Also called: hams, hamstring muscles, back of thigh

The hamstrings are three muscles on the back of the thigh: biceps femoris, semitendinosus and semimembranosus. They cross both the hip and the knee, so they do two jobs — bending the knee and extending the hip. Training only one of those leaves half the muscle undertrained.

back

The hamstrings highlighted on the back of the body.

Hamstrings anatomy

All three hamstrings originate on the ischial tuberosity — the sitting bone — and run down the back of the thigh. Biceps femoris inserts on the outside at the head of the fibula; semitendinosus and semimembranosus insert on the inside of the upper tibia.

Because they cross two joints, the hamstrings are in a constant trade-off: they shorten at the knee while lengthening at the hip during a hinge, and vice versa. This is why a leg curl and a Romanian deadlift feel like completely different exercises despite training the same muscles.

The short head of biceps femoris is the exception — it originates on the femur rather than the pelvis, so it only crosses the knee and only contributes to knee flexion.

The hamstrings are among the most commonly strained muscles in sport, usually during high-speed running where they work eccentrically to decelerate the shin.

Part	What it does
Biceps femoris (long head)	Lateral hamstring. Crosses hip and knee; also externally rotates the flexed knee.
Biceps femoris (short head)	Crosses only the knee, so it is trained by leg curls but not by hinges.
Semitendinosus	Medial, superficial. Knee flexion, hip extension, internal rotation of the flexed knee.
Semimembranosus	Medial, deep and broad. Same actions as semitendinosus.

Origin: The ischial tuberosity of the pelvis, for all three. The short head of biceps femoris originates instead from the linea aspera of the femur.
Insertion: Biceps femoris: the head of the fibula. Semitendinosus: the medial surface of the tibia (pes anserinus). Semimembranosus: the medial condyle of the tibia.

What the hamstrings does

Knee flexion — bending the knee, bringing the heel toward the glute.
Hip extension — driving the thigh backward, alongside the glutes.
Controlling hip flexion eccentrically — decelerating the torso as you hinge forward.
Decelerating the lower leg during sprinting, which is where most hamstring strains happen.
Rotation of the flexed knee — internal (semi- muscles) and external (biceps femoris).

Best hamstrings exercises

Exercise	Equipment	Role	Level	Why
Romanian deadlift	Barbell or dumbbells	primary	Intermediate	The best hip-extension hamstring builder. Trains the muscle in a stretched position under heavy load.
Lying leg curl	Machine	primary	Beginner	Direct knee flexion, and the only way to train the short head of biceps femoris.
Seated leg curl	Machine	primary	Beginner	Hip flexed means the hamstrings start lengthened — evidence suggests this position produces more growth than lying curls.
Nordic hamstring curl	Bodyweight + anchor	primary	Advanced	Heavily eccentric. Well supported by research for reducing hamstring strain risk in athletes.
Good morning	Barbell	primary	Advanced	Loaded hip hinge with a long lever. Effective, and demanding on the lower back.
Single-leg RDL	Dumbbell or kettlebell	primary	Intermediate	Adds a balance and pelvic-stability demand, and exposes side-to-side differences.
Glute-ham raise	GHD machine	secondary	Advanced	Trains both hamstring functions at once — knee flexion and hip extension.

How to train the hamstrings

Sets per week

10–16 hard sets

Rep range

6–12 for hinges, 8–15 for curls

Frequency

2 sessions per week

Do both a hinge and a curl every week. Hinges train hip extension; curls train knee flexion. Neither covers the other, and the short head of biceps femoris only responds to curls.
Seated leg curls place the hamstrings in a more lengthened position than lying curls, and current evidence favours training at longer muscle lengths for growth.
Eccentric-focused work like Nordic curls has good evidence behind it for reducing hamstring strain rates in sprinting athletes. Introduce it gradually — the soreness from the first session is memorable.

Stretches and mobility

Standing hamstring stretch

Heel on a low step, leg straight, hinge forward from the hips with a flat back. 30 seconds each side.

Supine hamstring stretch with strap

Lie on your back, loop a strap around the foot and raise the straight leg. Easier to control than a standing version.

Common mistakes

Only doing leg curls

Curls train knee flexion but skip hip extension entirely, which is where the hamstrings do most of their work in sport and in heavy lifting. Add a Romanian deadlift.

Turning RDLs into squats

If the knees travel forward and the hips drop, it stops being a hinge and the hamstrings stop being stretched. Push the hips backward, keep the shins near-vertical, and let the knees bend only slightly.

Stretching a strained hamstring aggressively

Pulling hard on a recently strained muscle tends to irritate it. Hamstring strains are common and are worth a proper rehabilitation approach rather than improvised stretching.

Hamstrings FAQ

What are the three hamstring muscles?: Biceps femoris, semitendinosus and semimembranosus. All three originate on the sitting bone and cross both the hip and the knee, so they extend the hip and flex the knee. The short head of biceps femoris is a partial exception — it starts on the femur and only crosses the knee.
Are leg curls or Romanian deadlifts better for hamstrings?: Neither replaces the other. Romanian deadlifts train hip extension with the hamstrings lengthened; leg curls train knee flexion, including the short head of biceps femoris that hinges cannot reach. A complete hamstring programme includes both.
Why do hamstrings get injured so often?: Because they work eccentrically at high speed during sprinting, decelerating the lower leg just before the foot lands, while being lengthened at the hip at the same time. That combination of high force and long muscle length is where most strains occur. Eccentric training such as Nordic curls has good evidence for reducing that risk.
How do I stretch tight hamstrings?: Standing or supine hamstring stretches held for around 30 seconds are the usual approach. Worth knowing though: a feeling of hamstring tightness is often a stiffness or neural-tension issue rather than genuinely short muscle, and strengthening through a full range frequently helps more than stretching does.

References

This page is general educational information about exercise and anatomy. It is not medical advice and not a substitute for assessment by a qualified clinician. If you have pain, an injury or a health condition, speak to a professional before changing how you train.