Hamstring Muscles: Anatomy, Function, and Common Injuries

The hamstring muscles are three large muscles that run down the back of your thigh and help control the hips and knees. They're called the:

  • Biceps femoris
  • Semitendinosus
  • Semimembranosus

The hamstring muscles are important for standing, walking, running, and jumping. Common problems that involve the hamstrings are strained muscles, flatback syndrome, spinal disc injury, and sacroiliac (SI) joint dysfunction.

This article covers the anatomy of the hamstring muscles, their functions, common injuries, and how they're treated.


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

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.

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. 

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.

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.

Common Injuries and Treatments

A woman in athletic clothing stretches her hamstring muscles using a park bench.

Lifemoment / Getty Images

Common injuries involving the hamstring muscles include:

Flatback Syndrome

If you look at a spine from the side, you should see two curves—a backward curve (lordosis) where it connects to the pelvic bone, and a forward curve (kyphosis) where it connects to the ribs.

In flatback syndrome, you lose one or both curves. In cases involving lordosis, a possible cause is tight hamstring muscles that keep the pelvis tilted too far forward.

This interferes with a healthy posture, can make it difficult to walk and perform other basic tasks, and cause significant pain.

Treatment for flatback syndrome may involve:

  • Exercises/physical therapy
  • Pain medication
  • Epidural steroid injections
  • Surgery

Spinal Disc Injury

As with flatback syndrome, tight hamstring muscles that alter the tilt of your pelvic bone can cause painful problems with your spinal discs. This happens because the change in angles causes too heavy a load on some discs.

The result can be:

Treatments for a herniated disc and DDD may include:

  • Temporarily decreasing activity
  • Anti-inflammatory medications
  • Epidural steroid injections
  • Physical therapy
  • Surgery, if other treatments fail

Sacroiliac Joint Dysfunction

The SI joints are small, relatively inflexible joints at the base of your spine. Their main purposes are to provide stability and transfer your weight from one leg to the other when you walk.

Tight hamstrings can cause this by putting undue strain on the area. Treatment typically involves:

  • Physical therapy
  • Chiropractic adjustments
  • Anti-inflammatory medications

Strained Hamstring Muscle

Hamstring muscle injuries are often caused by muscle overload, which is when the muscle is stretched too far or suddenly takes on too high a load. This is often the result of an activity such as sprinting.

Treatment for strained hamstring muscles is often:

  • RICE (rest, ice, compression, elevation)
  • Immobilization (splint or brace)
  • Physical therapy
  • Surgery
13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Anne Asher, CPT
Anne Asher, ACE-certified personal trainer, health coach, and orthopedic exercise specialist, is a back and neck pain expert.