Hamstring Muscles, Pelvic Position, and Your Back Pain

Hamstrings are the big muscles located at the back of your thigh. At the hip, the hamstrings play a role in a posture condition known as flat low back. This is because the result of their contraction at this location is a pulling down of the pelvis in back, a move also known as a posterior pelvic tilt. The posterior pelvic tilt, in turn, tends to elongate the natural low back curve, overstretching and/or weakening the muscles in that area and possibly predisposing you to disc injury. Tight hamstrings may also play a role in sacroiliac dysfunction. 

Become acquainted with each of the hamstring muscles, including where they attach and what they do

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Hamstring Muscles

A image of the back of a skeleton covered with muscles walking.
Hamstring muscles power walking and other movements because they are located at the back of your thigh. newartgraphics

The hamstrings are a group of four muscles: long head of the biceps femoris, short head of the biceps femoris, semitendinosus, and semimembranosus. Each hamstring crosses two joints—the hip and the knee. This means when they contract they can either take your lower extremity back behind you (called hip extension) or bend the knee (called knee flexion.) While the hamstrings can produce both movements at the same time, they can’t do so to their full capacity. It’s one movement or the other if you’re looking to go all the way with a hamstring muscle contraction.

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Biceps Femoris Hamstring Muscle

An image of the lower half of a skeleton plus hamstring and calf muscles.
The hamstring muscles are located at the back of your thighs. MedicalRF.com/MedicalRF.com/Getty Images

The biceps femoris muscle is one of the big, long muscles at the back of your thigh. (The other one is the semitendinosis; the biceps femoris is lateral, or toward the outside relative to the semitendinosis.) 

The biceps femoris has two heads—a long and a short. At the hip, both the long and short heads rotate the thigh outwardly and extend it backward. At the knee, they flex and laterally rotate the joint. Of all the hamstring muscles, the biceps femoris is the biggest contributor to hip extension.

The long head of the biceps femoris originates on the inner side of your sitting bones, which are little knobs of bone that are technically called the ischial tuberosities. The ischial tuberosities are located on the underside of your pelvis. (You likely can feel them when you sit down.)

The short head of the biceps femoris originates on three places on the femur (i.e., your thigh bone) that are located more towards your knee than your hip. Note that the short head of the biceps femoris is the only part of the hamstring muscle group that does not cross two joints. For this reason, some experts don’t consider it to be a hamstring muscle at all. It's actually missing in some people.

Both the long and short head of the biceps attach on the lower leg, also in three places: The head of the fibula bone, the outer (called lateral) condyle of the tibia bone, and the fascia of the leg. The multitude of attachment sites on the lower leg may make for more tears here than in the other hamstring muscles. 

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Semitendinosus Hamstring Muscle

Muscle diagram highlighting the semitendinosus hamstring muscle.
The semitendinosus hamstring muscle. MedicalRF.com/MedicalRF.com/Getty Images

The semitendinosus is another long, big hamstring muscle. At first glance, it is situated medially, or toward the inside of the back of the thigh, relative to the biceps femoris. The semitendinosus originates at the inner side of your sitting bone.

As with the other hamstring muscles, the semitendinosus muscle crosses the knee. It attaches at the upper part of your tibia bone (the shin bone) on the inner, or medial side. It also attaches on the deep fascia of the leg. The semitendinosus, along with the satorius and gracilis muscles, is one of three muscles that come together to form the pes anserine tendon that terminates on the anteromedial (the area to the front and side of the) tibia.

Although many anatomy books don’t show this, the semitendinosus is divided into two sections by a visible ridge of tendonous tissue called a raphe.

At the hip, the semidendinosus extends the thigh back and also helps or assists with medial rotation of the thigh. At the knee, this muscle bends and medially rotates the joint.

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Semimembranosus Hamstring Muscle

Muscle diagram of lower extremity shows hamstrings.
Hamstring muscle group consists of semimembranosus, biceps femoris and semitendinosus. MedicalRF.com/MedicalRF.com/Getty Images

The semimembranosus, the third of the hamstring muscles, is a broad muscle (like the semitendinosus.) Unlike the semitendinosus muscle, at first glance, it seems a bit tucked away. Like the other two hamstrings, it originates at the ischial tuberosity (your sitting bone), but the attachment site is located higher up and more to the outside (called lateral) than either one. It attaches on the posteromedial (i.e., back and side) area of the medial (inner) tibial condyle.

At the hip, the semimembranosus extends the joint, and helps with medial rotation (i.e., turning the lower extremity inward.) It also flexes the and medially rotates the knee.

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  1. NHS. Common posture mistakes and fixes.