What is Hamstrings?

What is Hamstrings?

The hamstrings are the three long muscles that make up the back of your thigh. Two of them sit toward the inner side, the semitendinosus and the deeper semimembranosus, and one sits toward the outer side, the biceps femoris, which itself has a long head and a short head. With one exception they all start at the ischial tuberosity, the bony sitting bone at the base of the pelvis, and run down to attach below the knee, the biceps femoris onto the head of the fibula on the outside and the two medial muscles onto the inner shin. Because most of the group crosses both the hip and the knee, the hamstrings do two jobs at once: they bend the knee and they extend the hip, driving your thigh backward. That dual role makes them central to almost every powerful lower-body movement, from sprinting and jumping to picking a heavy bar off the floor. The one part that behaves differently is the short head of the biceps femoris, which starts on the femur rather than the pelvis, so it only crosses the knee and only flexes it. The hamstrings are also one of the most frequently strained muscle groups in sport, particularly the biceps femoris during high-speed running, which is why building hamstring strength through a full range and especially in the lengthened position is a cornerstone of injury prevention. For lifters, well-developed hamstrings give the back of the thigh its thickness and separation and balance out the quads at the knee. They are trained through two distinct movement patterns: hip extension, such as the Romanian deadlift, and knee flexion, such as the leg curl.

How it works

The hamstrings are a two-joint muscle group, and understanding that is the key to training them. Three of the four hamstring bellies, the long head of biceps femoris, the semitendinosus, and the semimembranosus, originate on the ischial tuberosity and cross both the hip and the knee. When they contract they can either extend the hip (pull the thigh backward, as when you stand up from a bent-over position) or flex the knee (bend the leg, as in a leg curl), and often both at once. The short head of biceps femoris originates lower, on the linea aspera of the femur, crosses only the knee, and simply flexes it. Because of this arrangement, no single exercise fully trains the group. Hip-extension movements like the Romanian deadlift and good morning load the hamstrings while they are lengthened over the hip, and they preferentially work the two-joint portions near the pelvis. Knee-flexion movements like the seated and lying leg curl load them through knee bend and reach the biceps femoris short head that hip hinges cannot. Research on hamstring architecture and activation supports this functional split, so a complete program uses both patterns. Muscle length also matters a great deal. Training the hamstrings at long muscle lengths, for example the seated leg curl (where the hip is flexed and the muscle is stretched) or the bottom of a Romanian deadlift, produces greater hypertrophy than training them short. A well-known study found greater hamstring growth after training at long versus short muscle lengths, which is why coaches now emphasise the stretched position. The hamstrings also rotate the flexed lower leg: the medial semitendinosus and semimembranosus rotate the shin inward, while the biceps femoris rotates it outward. Their nerve supply comes from the sciatic nerve, with the tibial division driving the semi- muscles and the long head of biceps, and the common fibular division driving the short head, which is one reason the biceps short head behaves independently. Eccentric strength is especially protective: exercises like the Nordic hamstring curl load the muscle as it lengthens and have strong evidence for reducing hamstring strain injuries. Because the group works fast and forcefully during sprinting, it is prone to injury when weak or fatigued, so building both strength and eccentric control matters as much as size.

How to apply it

• Romanian deadlift for hip extension: The RDL is the primary hip-hinge hamstring builder. Push the hips back, keep a soft knee, and feel a deep stretch in the hamstrings before driving the hips forward. It loads the two-joint portions in a lengthened position. Use 6-12 reps with a controlled descent.
• Seated leg curl for the stretch: The seated leg curl flexes the hip, putting the hamstrings on stretch as you curl, and this long-muscle-length position drives more growth than the lying version. Emphasise a full range and a controlled lowering phase, working in the 8-15 rep range.
• Lying and standing leg curls: Prone and standing curls train knee flexion in a shorter, more contracted position and reach the biceps femoris short head that hinges miss. Rotate them in alongside seated curls to train the whole knee-flexion range. Use 10-15 reps with a hard squeeze at the top.
• Nordic curls for eccentric strength: The Nordic hamstring curl loads the muscle as it lengthens and has strong evidence for cutting hamstring strain injuries. Lower your torso as slowly as you can control, then assist yourself up. Start with 3-6 slow reps and build volume gradually since it is very demanding.
• Train the lengthened position: Whichever exercise you pick, emphasise where the hamstring is long: the deep stretch of an RDL or the fully extended knee of a seated curl. Loaded lengthened work is a potent hypertrophy stimulus, so control the eccentric and do not cut the bottom range short.
• Balance both movement patterns: Because the hamstrings both extend the hip and flex the knee, use at least one hip-hinge and one knee-curl exercise each week. Together they cover the whole group, from the proximal two-joint portions to the biceps short head, for complete development.

Types

Worked example

A balanced weekly hamstring plan that pairs a hip-extension movement, a stretch-focused curl, and an eccentric exercise for injury resilience. Loads and reps are examples for an intermediate lifter; scale to your own level and add a little each week.

That is a full hamstring session; many lifters split the hinge and the curls across two lower days. Keep the Nordic curls fresh and slow, and progress the RDL and seated curl loads over the weeks.

Hip hinge vs leg curl for hamstrings

Because the hamstrings act at two joints, neither pattern alone trains the whole group. Using both a hinge and a curl gives the most complete hamstring development.

By goal

• Strength and powerlifting: Prioritise the Romanian deadlift and heavy hip hinges in the 5-8 rep range. Strong hamstrings support the deadlift lockout and squat, and they stabilise the knee, so treat hip-extension strength as a main lift, not an afterthought.
• Hypertrophy and physique: Combine hip-hinge and leg-curl work across 10-16 weekly sets, emphasising the lengthened position on both. Seated leg curls and RDLs with a deep stretch build the back-of-thigh thickness and separation that balance out well-developed quads.
• Athletes and injury prevention: Add eccentric work like the Nordic curl, which has strong evidence for reducing hamstring strains in running sports. Build both hip-extension and knee-flexion strength, and progress high-speed exposure gradually, since most strains happen during fast running.

Common misconceptions

• "Leg curls alone are enough to build the hamstrings." Leg curls train knee flexion but barely load the hamstrings over the hip. Since three of the four bellies also extend the hip, you need hip-hinge work like the Romanian deadlift to grow the proximal, two-joint portions completely.
• "The hamstrings are just one muscle." They are three separate muscles, and the biceps femoris has two heads. The medial semitendinosus and semimembranosus and the lateral biceps femoris behave differently, and the biceps short head only crosses the knee, so no single exercise trains them all equally.
• "Stretching before a workout prevents hamstring strains." Brief static stretching does little to prevent strains. The strongest evidence points to building eccentric strength through exercises like the Nordic curl and training the hamstrings under load in a lengthened position, not to pre-workout stretching.
• "You should always train the hamstrings in the fully contracted, shortened position." Training the hamstrings at long muscle lengths, such as the deep stretch of an RDL or the extended knee of a seated leg curl, produces greater growth than short-length training. Emphasise the stretched range rather than only squeezing at the top.

Related terms

Hamstrings FAQ

What are the three hamstring muscles?

The hamstrings are the biceps femoris on the outer thigh, the semitendinosus, and the deeper semimembranosus on the inner thigh. Most originate on the ischial tuberosity and cross both the hip and the knee. The biceps femoris also has a short head that starts on the femur and only crosses the knee.

What do the hamstrings do?

As a group the hamstrings flex the knee and extend the hip, which powers sprinting, jumping, and lifting a bar off the floor. They also help rotate the flexed lower leg. Because they cross two joints, they often perform hip extension and knee flexion at the same time during movement.

What is the best exercise for the hamstrings?

There is no single best exercise because the hamstrings act at two joints. The Romanian deadlift is the top hip-extension builder and the seated leg curl is the top knee-flexion builder. Using both, plus Nordic curls for eccentric strength, trains the whole group most completely.

Why are seated leg curls better than lying leg curls?

The seated leg curl flexes the hip, which stretches the hamstrings while you work them. Training at this longer muscle length produces greater growth than the shorter position of the lying curl, according to research on muscle length and hypertrophy. Both are useful, but the seated version has an edge for size.

How do I prevent hamstring strains?

Build eccentric strength with exercises like the Nordic hamstring curl, which has strong evidence for cutting strain rates in running sports. Train the hamstrings under load in a lengthened position, keep both hip-extension and knee-flexion strength high, and increase sprinting exposure gradually.

Which hamstring is injured most often?

The biceps femoris, especially its long head, is the most commonly strained hamstring, typically during high-speed running as the leg swings forward and the muscle lengthens under load. This is why lengthened, eccentric training is emphasised for runners and sprint-sport athletes.

Do squats train the hamstrings?

Squats train the hamstrings much less than the quads and glutes. Because the hamstrings shorten at the knee while lengthening at the hip during a squat, their length changes little, so they are not strongly loaded. Add dedicated hip hinges and leg curls to train them properly.

How often should I train my hamstrings?

Twice a week works well for most lifters, letting you split hip-hinge and leg-curl work across two sessions with a day or two of recovery between. Aim for roughly 10-16 hard sets per week, and keep demanding eccentric work like Nordics at a manageable volume.

References

1. Rodgers CD, Raja A. Anatomy, Bony Pelvis and Lower Limb, Hamstring Muscle. StatPearls, NCBI Bookshelf
2. Maeo S, et al. Greater Hamstrings Muscle Hypertrophy but Similar Damage Protection after Training at Long versus Short Muscle Lengths. Med Sci Sports Exerc, 2021. PubMed 33009197
3. Hamstring-to-quadriceps activation ratio during lower-limb strengthening exercises. Res Sports Med, 2024. PubMed 38006325
4. Neuromuscular activation patterns of the hamstrings and trunk during eccentric exercise in athletes and physically active adults. BMC Sports Sci Med Rehabil, 2026. PubMed 42169193