Hamstring Muscles: Anatomy, Injuries, and Exercises

The hamstrings are a group of muscles that cross the hip and knee joints that are responsible for walking, running, jumping, and many other physical activities. The hamstrings flex the knee joint and extend the thigh backward to propel movement. 

Located at the backs of the legs, the hamstrings are the opposing muscles to the quadriceps. Because the hamstrings originate at the sitting bones, they are naturally stretched while sitting. However, long periods of sitting may affect how they function and lead to tightness.

Hamstring injuries are quite common. In fact, they are the most prevalent injury in sports. Learn how the hamstrings function and why strengthening and stretching exercises can help prevent injury from occurring (and reoccurring).

Anatomy of the Hamstrings

The tendons of the hamstring muscles attach to bone at the ischial tuberosity of the hip (more commonly known as the sitting bones). This region is located at one end of the linea aspera, a ridge along the femur (thigh) bone. The hamstring tendons also flank the space behind the knee. 

There are three main muscles that comprise the hamstring muscle group. The biceps femoris is a big, long muscle located at the back of the thigh that includes both a long head and a short head. The semitendinosus is located at the posterior and medial (inner) part of the thigh and the semimembranosus is the most medial of the hamstring muscles. Here's a closer look at the hamstring muscle group.

Biceps Femoris

The long head and short head of the biceps femoris are two muscles that work together to externally rotate the thigh at the hip and extend the leg backward. They are the main contributors to hip extension but they also flex and laterally rotate the knee at the joint. The biceps femoris attaches to the main part of the lateral tibia (shin bone) and is innervated by the sciatic nerve.

• Long head biceps femoris: The most lateral hamstring, the long head or "superficial" biceps femoris inserts on the lateral side of the fibula (lower leg bone) and originates from the inner sides of the sitting bones.
• Short head biceps femoris: The short head of the biceps femoris originates in three places on the femur bone that are situated closer to your knee than your hip.

Semitendinosus

Like the biceps femoris, the semitendinosus is a big, long hamstring muscle. It originates from the inside of the back of the thigh near the biceps femoris at the inner sides of the sitting bones. It also crosses the knee and connects to the inner side of the upper part of the shin bone. It also attaches to the fascia of the leg. The semitendinosus extends the thigh backward and allows for the medial ((i.e., turning the lower extremity inward) rotation of the thigh and knee joint.

Semimembranosus

The most medial muscle, the semimembranosus, inserts on the inner portion of the shin bone. Like the semitendinosus, the semimembranosus is a broad muscle, though it is more tucked away. It originates at the sitting bones, similar to the other muscles in the hamstring group, however, it attaches higher up. It also connects to the inner portion of the upper tibia (knee joint). The semimembranosus extends the hip joint and flexes and also medially rotates the knee toward the midline.

Hamstring Injuries

Overuse injuries to the hamstrings are common, particularly in sports like soccer, football, basketball, and tennis, where running is combined with rapid starts and stops. The long head of the biceps femoris is particularly prone to injury in sports such as these, likely because it exerts the most force compared to the other muscles in the hamstring group.

Hamstring sprains and tears are also relatively common. And they can become more serious when there is significant bruising behind the thigh. Repetitive stress injuries from running or walking are also a common cause of hamstring pain and injury.

Strains and Contusions

The onset of injury to the hamstring muscle group is often sudden and is usually identified as a strain (sprain or tear) or contusion (bruising). Strains range from mild to severe and include the following traits.

• Mild strains involve minimal damage to the muscle and heal quickly and can be treated with rest and over-the-counter pain medication.
• Moderate strains cause a partial rupture of the muscle and result in a loss of function.
• Severe strains result in a total rupture of tissue and lead to short- or long-term functional disability.

Contusions are caused by an external force making contact with the hamstring muscles, such as with many contact sports. Symptoms of contusions include:

• Muscular pain
• Swelling
• Bruising and discoloration
• Limited range of motion
• Stiffness

In addition, studies show that 12–33% of hamstring injuries are reoccurring. If you play a sport and sustain a hamstring injury, you will likely need to fully rehabilitate before returning to normal activity. This will allow the muscle group ample time to recover and repair, which can help prevent a relapse.

Strengthening Exercises

Walking, running, and climbing and descending stairs all help to build the functional fitness of the hamstring muscle group. Hamstring exercises can be beneficial for anyone, but they can be especially helpful for people who run or cycle, both of which target the quadriceps. It's important to balance the development of the quadriceps with cross-training that includes adequate strength and conditioning of the hamstrings.

Various isolation and compound exercises for the hamstrings can also be employed in rehabilitation settings or in bodybuilding. Exercises that involve knee flexion and hip extension are commonly used to build the hamstring muscles. Here are a few fundamental moves to try.

• Basic bridges: This simple exercise isolates and strengthens the hamstrings and gluteal muscles. Press your feet into the floor and engage your glutes to fire your hamstrings and lift your hips.
• Single-leg bridges: Similar to basic bridges, single-leg bridges target the hamstrings and glutes with the added challenge of leg lifts to promote core stability. Maintain the lift of your hips and pelvis by using the strength of your glutes and hamstrings rather than your back muscles.
• Leg curls: Also known as hamstring curls, these exercises are commonly done with gym equipment to strengthen the hamstring muscles and calves. They can also be performed with an exercise ball by lying on your back and placing your heels on the ball, and then rolling the ball in toward you as you bend your knees and lift your hips.
• Squats: This classic move can be performed with or without weights to work the hamstrings, glutes, and quadriceps. Keep your back straight and head upright as you reach your seat back to lower into a squat.
• Walking lunges: This stability exercise strengthens the hamstrings, quadriceps, glutes, calves, and core muscles while testing your balance. Keep your torso tall and upright as you step forward and back.

Basic Stretches

Hamstring flexibility is important for runners and may help prevent injury and delayed onset muscle soreness (DOMS) after exercise. Tight hamstrings may limit your range of motion when you straighten your knee. You might also feel a cramp at the back of the knee.

Hamstring stretches can be incorporated into just about any regular stretching and flexibility routine. The following hamstring stretches can be performed daily to improve flexibility, promote recovery, and prevent injury.

Seated Stretch

1. Sit on an exercise mat on the floor with both legs stretched out in front of you and pull the flesh away from your sitting bones so you can feel them making contact with the floor.
2. Bend one knee to slide the foot in toward the opposite knee, placing your foot flat on the floor.
3. Hinge at your hips and reach your hands toward the toes of the straightened leg. If there is a lot of tightness, you can micro-bend that knee.
4. Hold the stretch for 15–30 seconds.
5. Switch sides.

Supine Stretch

1. Lie on your back on an exercise mat with your knees bent and place your feet on the floor about hips-distance apart.
2. Lift one leg toward the ceiling, maintaining a neutral spine.
3. Reach behind the back of the thigh and gently tug the leg in closer. Optional: Allow the knee to bend slightly to increase the range of motion.
4. Hold for 15–30 seconds.
5. Lower the leg and then switch sides.

Standing Stretch

1. Start standing tall and upright with your feet about hips-distance apart.
2. Take a natural step forward with your heel and keep the toes lifted.
3. Place your hands on your hips as you sit back slightly and hinge forward.
4. Allow your spine to naturally round forward as you reach for the lifted toes.
5. Let your knees soften as you reach your seat back a little more and drop your chin toward your chest.
6. Hold for 15–30 seconds.
7. Bring your hands back to your hips to stand back up and step the feet back together. Switch sides.