Hamstrings

legs4 muscles

The hamstrings are the three muscles on the back of your thigh. They bend your knee and extend your hip, and most people train only one of those two jobs.

Hamstrings: its 4 muscles highlighted on an anatomical figure, from behind — Biceps Femoris (Long Head), Biceps Femoris (Short Head), Semitendinosus and Semimembranosus.
Where the hamstrings sit, from behind.

Three muscles run down the back of the thigh: biceps femoris on the outside, which has two heads, and semitendinosus and semimembranosus on the inside. All but one of them start on the sitting bone — the bony point you can feel underneath you when you sit on a hard chair — and run down to attach below the knee.

They cross both the hip and the knee, which gives them two jobs. They bend the knee, and they extend the hip. Those happen at opposite ends of the same muscle, and a movement that trains one does not necessarily train the other.

The exception is the short head of biceps femoris, which starts on the thigh bone instead of the pelvis and therefore crosses only the knee. It is the one hamstring that has nothing to do with the hip.

Because they cross two joints, they can be stretched at one end while shortening at the other. During running the hamstrings are decelerating the swinging lower leg while the hip is still extending, which is a demanding combination and the reason hamstring injuries are so strongly associated with sprinting. Nothing here is medical advice — that pattern is a fact about the mechanics, not guidance on avoiding or treating anything.

Their name comes from the tendons behind the knee, which you can feel as two cords on the inside and one on the outside when you bend your knee against resistance.

Training notes

Two jobs, two exercises. Leg curls train knee flexion. Romanian deadlifts, good mornings and back extensions train hip extension. A program with only one of these has trained the hamstrings at one end.

Of the two, hip extension work is the one more often missing, and it loads the muscle at longer lengths, which is where it does most of its useful work. Romanian deadlifts done with a full stretch at the bottom and the knees only slightly bent are the standard answer for good reason.

Nordic curls and slider curls train the muscle eccentrically — resisting the knee straightening — which is a distinct and demanding stimulus that neither leg curls nor deadlifts reproduce well.

The hamstrings work throughout squats and deadlifts, but during a squat they are lengthening at the hip and shortening at the knee at the same time, so their length barely changes. That is real work and it is not the same as training them through a range.

Flexibility and strength are different questions here and often confused. Feeling tight in the back of the thigh is not by itself information about whether the muscle is strong, and this site does not give advice on either stretching or injury.

Muscles in this group

Exercises

Antagonists

Synergists