Hamstring Muscles: Anatomy, Function, and Common Injuries

The hamstring muscles are three large muscles that run down the back of the thigh and help control the hips and knees.

These muscles—the biceps femoris, semitendinosus, and semimembranosus—are used for standing, walking, running, and jumping, among other things.

Hamstring muscle injuries and problems include muscle strains, flatback syndrome, spinal disc injury, and sacroiliac (SI) joint dysfunction.

This article discusses the anatomy of the hamstring muscles. It explains the different muscles, their functions, and common hamstring injuries. It also details treatment for hamstring injuries and how to keep your hamstrings healthy.

Hamstring Muscles Function

Located at the back of the thigh between the pelvis and lower leg, your hamstrings are used to:

• Extend (open) the hip joint, increasing the angle between your pelvis and thigh
• Flex (bend) your knee
• Rotate the leg at the hip and knee joints

You use your hamstrings when standing up from a chair, walking, running, jumping, and climbing stairs.

Anatomy

The hamstring muscle group includes three large muscles:

• Biceps femoris
• Semitendinosus
• Semimembranosus

These muscles attach to the sit bones (ischial tuberosities)—little knobs of bone on the bottom of your pelvis that you can feel when you sit down.

The hamstrings run down the back of the thigh from the sit bones and attach to different points on the lower leg.

Biceps Femoris Hamstring Muscle

The biceps femoris muscle is the lateral (outer) muscle of the hamstring group. As its name implies, the biceps femoris has two heads: one long and one short.

The long head of the biceps femoris attaches to the inner side of your sit bones. The short head attaches to the femur (thigh bone) in three places closer to the knee than the hip.

Both heads attach to the lower leg at the head of the fibula (calf bone), the outer (lateral) condyle of the tibia (shin bone), and the fascia (connective tissue) of the leg.

The biceps femoris works to pull your leg behind you, extending the hip joint. It is also used to bend your knee and rotate your lower leg from side to side.

Semitendinosus Hamstring Muscle

The semitendinosus is a long hamstring muscle in the middle of the back of the thigh. Its main functions are extending and rotating the hip and bending and internally rotating at the knee.

The semitendinosus is attached to the medial (inner) side of the sit bones and divides into two sections by a visible ridge of tendonous tissue called a raphe. It runs down the middle of the back of the thigh, crosses to the inside of the knee, and attaches to the upper part of the tibia (shinbone).

The semitendinosus joins with the sartorius and gracilis muscles to form the pes anserine tendon on the front and side of the tibia. It is also connected to the deep fascia of the leg.

Semimembranosus Hamstring Muscle

The semimembranosus is a broad muscle that is a bit tucked away. It originates at the outer side of the sit bones, higher than the other hamstrings. It attaches to the shin on the back and side of the medial (inner) tibial condyle.

Like the other hamstring muscles, it works to extend the hip joint and bend the knees. It also helps with the internal rotation of the hip and joint.

Hamstring Strains

A hamstring strain is a pull or tear of muscle fibers. Strains are typically caused by muscle overload. A strain can affect the tissue in the belly of the muscle or where the muscle joins with tendons.

Hamstring strains range from mild to severe:

• In a mild (grade 1) strain, the muscle fibers overstretch but do not tear.
• In a moderate (grade 2) strain, one or more hamstring muscles partially tear.
• In a severe (grade 3) strain, a hamstring muscle completely tears away from a tendon or bone.

Causes and Risk Factors

Adolescents and older athletes have the most significant risk of a hamstring strain. Factors that contribute to hamstring strains include:

• Muscle fatigue
• Muscle imbalances
• Muscle tightness
• Not stretching properly or often enough
• Poor muscle conditioning in athletes

Hamstring strains are most common among:

• Basketball players
• Dancers
• Football players
• Runners and sprinters
• Soccer players
  

Symptoms

Symptoms of a hamstring strain include:

• Muscle pain ranging from mildly uncomfortable to severe
• Pain in the back or side of the thigh, hip, or knee
• Swelling in the injured area

Whether or not you can bear weight on the leg depends on the severity. With a grade 1 hamstring strain, you can still usually walk, whereas a grade 3 strain may prevent you from using your leg at all.

Treatment

Strained hamstring muscles are typically treated at home using non-invasive therapies. These include:

• RICE (rest, ice, compression, elevation)
• Immobilization (splint or brace)
• Physical therapy

In some cases, surgery may be needed. A torn muscle (grade 3 strain) may require surgery immediately, while a less severe injury may need surgery if the damage doesn't improve with physical therapy.

Hamstring Muscles and Back Problems

Common issues involving the hamstring muscles that affect the back include:

• Flatback syndrome 
• Spinal disc injury
• Sacroiliac joint dysfunction

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 essential tasks, and causes 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:

• Herniated disc
• Degenerative disc disease (DDD)

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
  

How to Prevent Hamstring Injuries

The most important thing you can do to prevent hamstring injuries and keep your hamstrings healthy is to exercise regularly and consistently year-round. This should include a combination of stretching, strengthening exercises, and cardio. 

In addition, to prevent hamstring injuries:  

• Always stretch before and after exercise. 
• Before stretching, warm up your muscles with 5 to 10 minutes of cardio, such as walking or jogging.
• Do not over-stretch your hamstring muscles.
  
• Slowly increase the duration and intensity of your exercise program.

If you feel any pain in your thigh, stop immediately and give your hamstring time to rest.

Stretches and Strengthening Exercises

Walking, running, and climbing stairs are everyday activities that help to strengthen the hamstring muscles. 

Strengthening exercises that isolate the hamstrings include: 

• Bridges
• Hamstring curls
• Deadlifts

The hamstring muscles are stretched by extending the leg and flexing at the hip. Hamstring stretches can be done in a few different ways:

• Seated: Sit on the floor with one leg straight and one knee bent, so your legs make a figure 4. Bend forward at your hips and reach your hands toward your toes on the straightened leg.
• Standing: Elevate one leg and rest it on a curb or chair. Bend forward at the hips and reach your hand toward your foot.
• Supine: Lie on your back with one leg straight and one knee bent with your foot flat. Use a towel or stretching strap around the foot of your straight leg to pull your leg upward until you feel a stretch at the back of the thigh.

If your hamstrings are tight, you can bend the knee slightly.

Hold the stretch for 15 to 30 seconds, then switch legs. Repeat two or three times per leg.