What Are The Muscles Of The Hamstring Group? | Three Names

The muscles on the back of your thigh do more than bend your knee. They push your leg behind you, slow your shin during a stride, steady your pelvis, and protect the knee when you sprint, climb, or squat.

The hamstring group is usually named as three muscles. One of them, biceps femoris, has two heads, so anatomy charts may show four parts. That split makes sense once you know each part’s start point, end point, and action.

Hamstring Group Muscles And Their Main Jobs

The three named hamstring muscles are biceps femoris, semitendinosus, and semimembranosus. They sit in the posterior thigh, the back compartment between the hip and the knee. Most hamstring tissue starts from the ischial tuberosity, often called the sit bone.

From there, the muscles run down the thigh and attach near the top of the tibia or fibula. Because most of the group crosses both the hip and knee, the hamstrings can extend the hip and flex the knee. That two-joint setup is why they work hard during running, deadlifts, lunges, and downhill walking.

Biceps Femoris

Biceps femoris sits toward the outer back thigh. It has a long head and a short head. The long head starts at the sit bone and crosses the hip and knee. The short head starts from the femur, so it crosses only the knee. Both heads meet in a tendon that attaches near the head of the fibula.

This outer hamstring helps bend the knee and turn the lower leg outward when the knee is bent. The long head also helps extend the hip. In running, it takes heavy load when the leg swings forward and the body prepares for foot strike.

Semitendinosus

Semitendinosus runs down the inner back thigh and has a long, cord-like tendon near the knee. It starts at the sit bone, crosses the hip and knee, then attaches on the upper inner tibia. It sits closer to the surface than semimembranosus, so its tendon can be easier to feel behind the knee.

This muscle helps bend the knee, extend the hip, and turn the lower leg inward when the knee is bent. It also helps control pelvic position when one leg is planted and the other is moving.

Semimembranosus

Semimembranosus is the broad, flatter inner hamstring. It sits deeper than semitendinosus. It starts at the sit bone and attaches on the back and inner side of the upper tibia. Its wide tendon gives it a firm pull near the knee.

This muscle helps with hip extension, knee bending, and inward rotation of the lower leg. It also adds control near the back-inside corner of the knee during cutting, stepping, and decelerating.

Anatomy texts place these muscles in the posterior thigh and describe the biceps femoris, semitendinosus, and semimembranosus as the main hamstring muscles. The NCBI Bookshelf hamstring anatomy chapter also notes the split nerve pattern, with the tibial branch of the sciatic nerve supplying most of the group and the common peroneal branch supplying the short head of biceps femoris.

Why Biceps Femoris Has Two Heads

The word “biceps” means two heads. In the thigh, those two heads share a lower tendon but start in different places. The long head behaves like a classic hamstring because it starts at the pelvis, crosses the hip, crosses the knee, and is supplied by the tibial branch of the sciatic nerve.

The short head is different. It starts on the femur, crosses only the knee, and gets its nerve supply from the common peroneal branch of the sciatic nerve. Many anatomy classes still place it with the hamstrings because it joins biceps femoris and works with the group at the knee.

That detail helps explain why hamstring pain isn’t always in one neat strip down the back of the thigh. Outer-thigh pain near the knee can involve biceps femoris. Inner-thigh tightness or tenderness can point more toward semitendinosus or semimembranosus.

Hamstring Part	Where It Runs	Main Actions
Biceps Femoris Long Head	Sit bone to fibula, outer back thigh	Extends hip, bends knee, rotates shin outward
Biceps Femoris Short Head	Femur to fibula, outer lower back thigh	Bends knee, rotates shin outward
Semitendinosus	Sit bone to upper inner tibia, inner back thigh	Extends hip, bends knee, rotates shin inward
Semimembranosus	Sit bone to back and inner upper tibia	Extends hip, bends knee, rotates shin inward
Common Starting Area	Mostly the ischial tuberosity, or sit bone	Links hip position with thigh pull
Common Nerve Pattern	Mostly tibial branch of sciatic nerve	Sends signals for hip and knee control
Short Head Exception	Starts on femur and has a different nerve branch	Acts at the knee, not the hip

Where The Hamstrings Attach

The upper attachment is mainly the sit bone. You can feel it as the bony point under each side of your pelvis when you sit on a hard chair. This shared upper anchor lets the hamstrings pull the thigh backward during hip extension.

The lower attachments spread out near the knee. Biceps femoris attaches near the fibula on the outer side. Semitendinosus and semimembranosus attach on the inner side of the tibia. This split gives the hamstrings a strong grip on both sides of the knee.

That layout is one reason hamstrings affect both stride power and knee control. The American Academy of Orthopaedic Surgeons notes that the hamstring muscle group helps extend the leg backward and bend the knee, and hamstring strains often happen in sports with sprinting bursts. You can read the AAOS page on hamstring muscle injuries for more on strain patterns and care options.

How The Hamstrings Work During Movement

During walking, the hamstrings help pull the thigh back as your foot moves behind you. During running, they also brake the lower leg as it swings forward. That braking job is demanding because the muscle lengthens while producing force.

In gym lifts, the hamstrings work hardest when the hips hinge. Romanian deadlifts, hip thrusts, back extensions, and glute-ham raises all load hip extension. Leg curls load knee flexion more directly. A balanced plan often includes both patterns because the group works across two joints.

Movement	Hamstring Job	Where You Feel It
Walking	Pulls thigh back and steadies the knee	Mild back-thigh effort
Sprinting	Brakes the shin, then drives hip extension	Strong back-thigh load
Deadlifting	Controls hip hinge and helps stand tall	Stretch and work near back thigh
Leg Curl	Bends the knee against resistance	Work near back of knee and thigh
Cutting Or Pivoting	Helps steer tibia and manage knee position	Outer or inner back-thigh tension

What Tightness Or Strain Can Tell You

A tight feeling in the hamstrings can come from the muscles themselves, but it can also come from nearby joints, nerves, or training load. Sitting all day may make the back thigh feel short, while hard sprinting may leave the tissue sore or strained.

A strain is different from ordinary tightness. It may bring sharp pain, bruising, swelling, weakness, or pain when you try to bend the knee. The Cleveland Clinic’s overview of hamstring muscle anatomy and function names the three muscles and explains common care habits such as warming up, stretching, and not pushing through pain.

Training The Hamstring Group With Less Guesswork

Good hamstring work usually blends knee bending, hip hinging, and slow lowering. The muscle group must shorten, lengthen, and hold tension. Training only one pattern leaves gaps.

Useful choices include:

• Hip hinges, such as Romanian deadlifts, for long-range back-thigh strength.
• Leg curls for direct knee-flexion strength.
• Bridges or hip thrusts for hip extension with glute sharing.
• Slow lowers, such as Nordic-style progressions, when strength and control are ready.
• Warm-ups with easy swings, marching, or light hinges before sprinting.

Start with loads you can control. Smooth reps beat heavy, sloppy reps. The goal is not to crush the hamstrings in one session; it is to build tissue that handles daily movement, sport, and lifting with less drama.

Names Worth Knowing

The hamstring group is simple to name but layered in action. Biceps femoris sits more to the outside, semitendinosus runs along the inner back thigh with a long tendon, and semimembranosus sits deeper and broader on the inner side.

Once you know those three names, anatomy charts make more sense. You can also train with better intent: hip hinges for hip extension, curls for knee flexion, and controlled lowering for the stride-braking job that makes the hamstrings work so hard.