The hamstrings are a group of three muscles running along the back of your thigh, from your sit bones down to the bones of your lower leg. They bend your knee, straighten your hip, and help rotate your leg. Together, they’re essential for walking, running, jumping, and even just standing upright.
The Three Hamstring Muscles
Each of the three hamstrings has a slightly different position and role, though they work together as a unit.
Biceps femoris sits on the outer side of the back of your thigh. It’s the only hamstring with two parts: a long head that runs from your sit bone all the way down, and a short head that starts partway down your thighbone. The long head handles knee bending, hip extension, and outward rotation of the lower leg. The short head only bends the knee and rotates the lower leg, since it doesn’t cross the hip joint.
Semimembranosus is the innermost hamstring, positioned deepest in the back of your thigh. It bends your knee, extends your thigh at the hip, and rotates both your hip and lower leg inward.
Semitendinosus sits between the other two muscles and performs the same actions as the semimembranosus: knee flexion, hip extension, and inward rotation of the hip and lower leg.
How They Work Together
The hamstrings are “two-joint muscles,” meaning they cross both your hip and your knee (with the exception of the short head of the biceps femoris). This dual role makes them uniquely important for coordinating movement between your upper and lower leg.
Their three primary jobs are bending your knee, straightening your hip, and rotating your hip joint. But these simple descriptions understate how much work the hamstrings actually do. During walking or running, they’re most active in the late swing phase (when your leg is swinging forward and about to hit the ground) and into early stance. In that moment, they act like brakes, arresting the forward swing of your leg by slowing hip flexion and knee extension simultaneously. Without that braking action, your knee would hyperextend with every step.
During the stance phase of walking, their role gets more complex. They help drive your hip into extension (pushing your body forward over your planted foot), restrain your knee from snapping backward, and initiate the slight knee bend that absorbs impact when your foot hits the ground.
Nerve Supply
The hamstrings get their nerve signals from the sciatic nerve, which is the largest nerve in the body. But the three muscles don’t all connect to the same branch. The tibial division of the sciatic nerve controls the semimembranosus, the semitendinosus, and the long head of the biceps femoris. The short head of the biceps femoris is the odd one out: it’s controlled by the common peroneal (fibular) division. This split in nerve supply is one reason the short head is sometimes considered functionally distinct from the rest of the hamstring group.
Why Hamstring Injuries Are So Common
Hamstring strains are one of the most frequent soft tissue injuries in sports. A 21-season study of men’s professional soccer published in the British Journal of Sports Medicine found that hamstring injuries made up 19% of all injuries over the study period, and the proportion climbed from 12% in the first season to 24% in the most recent season.
The reason comes down to how the muscles work. During sprinting, the hamstrings have to lengthen while they’re actively contracting (an “eccentric” contraction) to slow your swinging leg. That combination of stretch and force puts enormous strain on the muscle fibers, especially near the tendons. Fast, explosive movements like sprinting, kicking, and sudden acceleration or deceleration are the most common triggers.
Strains are graded on a three-point scale:
- Grade 1: A mild strain with very little torn muscle. You’ll feel tightness and mild pain but can usually still walk.
- Grade 2: A partial tear with noticeable loss of strength, more significant pain, and often some bruising or swelling.
- Grade 3: A complete tear. You may hear or feel a pop, and the muscle loses most or all of its function.
Common symptoms across all grades include pain on the back of your thigh (especially when moving), stiffness, swelling, tenderness, and sometimes a visible bump or knot. With grade 2 and 3 strains, sitting can be particularly uncomfortable, especially on the affected side, because the injury often involves the area near the sit bone where the muscle attaches. Higher-grade tears also cause bruising that can spread down the back of the leg over several days.
Strengthening Your Hamstrings
Because the hamstrings are most vulnerable during eccentric contractions, eccentric exercises are the most effective way to build resilience. The Nordic curl is the best-studied example. You kneel with your ankles anchored under something stable, then slowly lower your torso toward the ground by extending your knees while keeping your hips and spine straight. The hamstrings work hard to control the descent, building strength at the exact muscle lengths where injuries tend to happen. You can use your hands to push back up to the starting position, since the lowering phase is the key part of the exercise.
Beyond Nordic curls, other effective movements include Romanian deadlifts, single-leg deadlifts, and hip bridges. What these share is that they load the hamstrings through their full range, particularly in lengthened positions. Because the hamstrings cross both the hip and knee, the best training programs include exercises that emphasize hip extension (like deadlifts) and exercises that emphasize knee flexion (like Nordic curls or leg curls) rather than relying on just one pattern.
People who sit for long periods often develop relatively tight, weak hamstrings because the muscles spend hours in a shortened position. Regular strengthening and stretching helps counteract this, improving both injury resilience and everyday movement quality.