Your hamstrings bend your knees, help straighten your hips, and act as brakes for your legs during walking and running. This group of three muscles runs along the back of each thigh, connecting your pelvis to bones just below the knee. They’re involved in nearly every lower-body movement you do throughout the day, from climbing stairs to sitting down to sprinting.
The Three Muscles Behind Your Thigh
The hamstring group consists of three muscles: the biceps femoris (which has two separate portions, a long head and a short head), the semitendinosus, and the semimembranosus. All three run vertically along the back of the thigh. The biceps femoris sits on the outer side, while the semitendinosus and semimembranosus sit on the inner side.
What makes the hamstrings unusual is that most of them cross two joints. They attach to the sit bones of your pelvis at the top and to the bones of your lower leg at the bottom, spanning both the hip and the knee. This two-joint design is what allows them to perform two very different jobs: bending the knee and extending the hip.
Bending the Knee
The most straightforward job of the hamstrings is knee flexion, the motion of bringing your heel toward your backside. Every time you take a step, pedal a bike, or lower yourself into a chair, your hamstrings are pulling your lower leg backward at the knee joint.
The inner and outer hamstrings contribute differently depending on knee position. The outer muscle (biceps femoris) activates most strongly at shallower angles of knee bend, around 30 degrees, while the inner muscles peak closer to 60 degrees. Rotation of the lower leg also shifts the workload: turning your foot outward recruits the outer hamstring more, and turning it inward targets the inner two. This is why physical therapists sometimes use foot position cues during hamstring rehab exercises.
Extending the Hip
When you stand up from a bent-over position, push off during a stride, or thrust your hips forward at the top of a deadlift, your hamstrings contribute to hip extension. In this role, they work alongside your glutes to straighten the hip joint.
That said, the hamstrings aren’t the primary drivers of hip extension. Research using muscle imaging after exercise shows that the hamstrings are more heavily recruited during knee flexion than during hip extension. The gluteus maximus, which is a much larger muscle, handles most of the heavy lifting at the hip. The hamstrings play more of a supporting role, especially during explosive movements like jumping and sprinting where their contribution to hip extension is relatively small. Still, the hamstrings produce more force during hip extension when the knee is kept straight, because the stretched position puts them at a better mechanical advantage.
Braking Your Legs While Running
Perhaps the most demanding job hamstrings perform is decelerating your leg during the swing phase of running. When your leg swings forward through the air, the hamstrings have to lengthen under tension to slow that forward momentum before your foot hits the ground. This controlled lengthening is called eccentric contraction, and it places enormous stress on the muscle fibers.
During high-speed running, the hamstrings lengthen under load from roughly the midpoint of each stride cycle through 90% of it, then switch to shortening as your foot contacts the ground. Two distinct loading peaks occur: one in late swing (just before your foot lands) and one in early stance (just after). The amount of braking work the hamstrings do increases significantly as running speed goes up, which is why hamstring strains so often happen during sprinting rather than jogging. Research on running biomechanics confirms that the negative, braking work performed by the hamstrings happens exclusively during the swing phase, making that the window when strain injuries are most likely.
Protecting the Knee From Injury
One of the hamstrings’ most important but least obvious roles is protecting the anterior cruciate ligament, the ACL. The ACL prevents your shinbone from sliding forward relative to your thighbone. The hamstrings, attaching to the back of the lower leg on both sides, pull the shinbone backward and directly resist that same forward sliding motion.
Stiffer, stronger hamstrings reduce the amount of forward tibial translation during dynamic movements like landing from a jump. Cadaveric studies confirm that hamstring force can directly limit ACL loading caused by forward shear on the shin. People with ACL-deficient knees who have greater hamstring stiffness actually function better than those with weaker hamstrings, because the muscles partially compensate for the missing ligament. The hamstrings also influence side-to-side knee loading, meaning their protective effect extends beyond just the front-to-back plane.
This is why so many ACL injury prevention programs emphasize hamstring strengthening. The conventional recommendation is that your hamstrings should produce at least 60% of the force your quadriceps can generate. When the hamstrings are strong enough to fully brake the quads during rapid movements, a ratio known as the “point of equality,” knee joint protection is optimized.
Controlling Posture and Pelvic Position
Because the hamstrings attach to the pelvis, they directly influence how your pelvis tilts and, by extension, the curve of your lower back. Tight hamstrings pull the bottom of the pelvis downward, restricting the pelvis from tilting forward. This matters more than you might expect.
When you bend forward at the waist, the movement normally comes from a coordinated combination of lumbar spine flexion, forward pelvic tilt, and hip flexion. This is called the lumbar-pelvic rhythm. If tight hamstrings restrict the pelvic tilt component, your body compensates by increasing flexion through the lumbar spine. Over time, this compensation creates excessive mechanical stress on the lower back. Studies on soccer and basketball players have found that hamstring tightness restricts anterior pelvic tilt and can flatten the normal lumbar curve, a condition sometimes called lumbar hyperkyphosis. These altered movement patterns are linked to increased rates of low back pain and spinal injuries in athletes.
Reducing Injury Risk
Hamstring strains are one of the most common injuries in sports that involve sprinting, and they have notoriously high recurrence rates. The Nordic hamstring curl, an exercise where you kneel and slowly lower your body forward while resisting with your hamstrings, has become the gold standard for prevention. A meta-analysis of soccer players found that injury prevention programs including Nordic hamstring curls reduced hamstring injury rates by up to 51% compared to teams using no prevention measures.
The exercise works by strengthening the hamstrings eccentrically, training them in exactly the way they function during the high-risk swing phase of running. This builds the muscle’s capacity to absorb force while lengthening, which is the specific demand that causes most hamstring tears. For anyone who runs, plays field sports, or does activities involving rapid acceleration and deceleration, eccentric hamstring training is one of the most effective injury prevention strategies available.