Why You Have Small Hamstrings and How to Grow Them

Small hamstrings are one of the most common muscular imbalances in both athletes and desk-bound adults, and they carry real consequences for injury risk, joint health, and athletic performance. The hamstrings sit behind the thigh and cross two joints (hip and knee), yet they receive far less training attention than the quadriceps in front. That imbalance is reflected in the research: hamstring volume correlates strongly with sprint speed, weak hamstrings predict both hamstring tears and knee ligament injuries, and the muscle group shrinks faster with age than many people realize. Understanding why hamstrings stay small and what to do about it is more nuanced than just “do more leg curls.”

How the Hamstrings Are Built

The hamstrings are not one muscle but four: the biceps femoris long head, biceps femoris short head, semitendinosus, and semimembranosus. These muscles share the job of bending the knee and extending the hip, but they are built differently from one another. Detailed cadaver analysis shows that two of the four (the semimembranosus and biceps femoris long head) have shorter fibers and a larger cross-sectional area, making them better suited for producing high force. The other two (semitendinosus and biceps femoris short head) have longer fibers and a smaller cross-sectional area, which favors speed of contraction and range of motion.

1PubMed Central. Unique morphological architecture of the hamstring muscles and its functional relevance revealed by analysis of isolated muscle specimens and quantification of structural parameters

This two-tier architecture matters because “small hamstrings” does not mean all four muscles are equally undersized. Someone may have a relatively well-developed biceps femoris but a visibly thinner semitendinosus, or vice versa. The peak cross-sectional area of each muscle occurs at a different point along the thigh: roughly 38% of thigh length from the knee for the semimembranosus but about 58% for the semitendinosus.

2PubMed Central. Shape factor versus truncated cone-based quantification of quadriceps and hamstring muscle volumes-A choice between accuracy and precision

That unevenness means the visual impression of hamstring size depends on where you look and which individual muscles have grown or atrophied. It also means that a single exercise can leave entire portions of the hamstring group understimulated, a point that becomes important when we talk about training.

Why Hamstrings Stay Small

Several overlapping factors keep hamstrings smaller than the quadriceps. The most straightforward is training bias. Most gym exercises that dominate lower-body routines, like squats, leg presses, and lunges, are quad-dominant. The hamstrings assist during these movements but are not the primary drivers, so they grow less. People who only train through those exercises can develop a noticeable front-to-back imbalance over time.

Sedentary habits compound the problem. Sitting for hours shortens the hamstrings at the hip while keeping them at a slack length at the knee, which reduces the mechanical stimulus they receive during daily life. There is also the matter of fiber-type composition. Despite the common label of a “fast-twitch” muscle group, biopsy data from the biceps femoris long head show a roughly even split between slow-twitch and fast-twitch fibers, with substantial variation between individuals. Importantly, that fiber composition did not predict maximal or explosive strength in the study. Muscle volume alone explained about 40 to 60 percent of the differences in knee-flexor strength across different speeds and contraction types.

3PubMed. The functional significance of hamstrings composition: is it really a “fast” muscle group?

That finding tells you something practical: if you want stronger hamstrings, the single most effective lever is making them bigger. Fiber-type genetics matter less than simply adding muscle tissue.

Functional Inhibition and the “Quiet” Hamstring

One underappreciated reason hamstrings stay small is that some people literally cannot activate them properly, even when they try. After a hamstring strain, the injured limb can show roughly 24% lower voluntary activation compared to healthy controls during maximal eccentric contractions, along with measurably lower strength and electrical activity in the muscle.

4PubMed. Voluntary Activation and Reflex Responses after Hamstring Strain Injury

This inhibition can persist long after the pain has resolved. A case report of a competitive masters sprinter illustrates how severe the problem can get: during a heavy Romanian deadlift, one researcher recorded almost zero electrical activity (0.014 millivolts) from the athlete’s proximal hamstring, even though the athlete was exerting full effort. After a visual biofeedback intervention, activation jumped roughly 40-fold, reaching levels consistent with normal vigorous contraction.

5PubMed Central. Rapid Re-activation of Functionally Inhibited Hamstring Muscles via Real-Time EMG Visual Feedback in an Asymptomatic Masters Sprinter: A Case Report

If the nervous system is not sending a strong signal to the hamstrings, they will not grow no matter how many sets you perform. People who have had even a mild hamstring strain years ago, or who have chronic low back issues, may be unknowingly dealing with this kind of neural shutdown. It is one of the stealthiest reasons for persistently undersized hamstrings.

How Aging Shrinks the Hamstrings

All skeletal muscle declines with age, but the rate varies sharply from one muscle group to another. A comprehensive review pulling together data from nearly 2,000 individuals found that the hamstrings lose about 19% of their size over roughly 50 years of aging, translating to about 0.39% per year. That rate is comparable to the elbow flexors but less severe than the quadriceps (about 27%) or the psoas (about 29%). The most-atrophied individual muscle was the rectus femoris at around 33% loss; the least was the soleus at about 6%.

6PubMed Central. Human skeletal muscle-specific atrophy with aging: a comprehensive review

A five-year longitudinal study of people in their seventies painted an even more striking picture. Over just five years in that age range, the hamstrings shrank by about 5.9%, which was roughly double the rate of the quadriceps (about 3.3%) during the same period.

7PubMed Central. Muscle group-specific skeletal muscle aging: a 5-yr longitudinal study in septuagenarians

That reversal is worth sitting with. Over a lifetime, the quads shrink more in absolute terms. But in the critical eighth decade, the hamstrings atrophy at twice the pace. For older adults who already have relatively small hamstrings from decades of undertraining, this accelerated loss can push the muscle group into a range where daily function and fall risk become real concerns.

Post-Surgical Hamstring Loss

A common and often surprising cause of small hamstrings is ACL reconstruction. The most popular graft technique uses the semitendinosus and gracilis tendons, which are harvested from the inner hamstring area. The body does attempt to regenerate those tendons, but the outcome is usually imperfect: the regenerated tendon typically reattaches at a non-anatomical location, and the muscle belly migrates upward and remains permanently shortened and atrophied.

8PubMed. Regeneration of the semitendinosus tendon and muscle after isolated semitendinosus harvest for primary anterior cruciate ligament reconstruction: A systematic review

At one year after surgery, the semitendinosus and gracilis show significant and persistent atrophy on the operative side, along with measurable hamstring strength deficits.

9PubMed. The effects of semitendinosus and gracilis harvest in anterior cruciate ligament reconstruction

What’s more, these deficits do not fully resolve with time. MRI scans taken nine to eleven years after ACL reconstruction found that the semitendinosus muscle on the surgical side averaged only about 59% of the volume of the non-surgical leg, and the gracilis averaged about 54%. Both muscles also showed fatty infiltration, a sign that the remaining tissue was partly replaced by non-functional fat.

10Journal of Bone and Joint Surgery. Evaluation of Muscle Size and Fatty Infiltration with MRI Nine to Eleven Years Following Hamstring Harvest for ACL Reconstruction

If you have had ACL surgery and notice that one hamstring looks noticeably smaller than the other, this is almost certainly why. The asymmetry is often permanent to some degree, though targeted rehabilitation can recover a meaningful portion of the lost strength.

What Small Hamstrings Mean for Injury Risk

The relationship between hamstring size, strength, and injury has been studied extensively, and the findings are consistent: weaker hamstrings predict more injuries at both the hamstring itself and the knee.

For hamstring strains specifically, a prospective study of 146 professional football players found that eccentric hamstring peak torque below a certain threshold was associated with roughly 5.6 times the odds of hamstring strain injury. A low hamstring-to-quadriceps strength ratio (below about 50%) roughly tripled the odds.

11PubMed. Eccentric hamstring strength deficit and poor hamstring-to-quadriceps ratio are risk factors for hamstring strain injury in football: A prospective study of 146 professional players

The mechanism behind hamstring tears is well understood. During high-speed running, the hamstrings have to produce large eccentric (lengthening) forces during the late swing phase to decelerate the lower leg before foot strike. If the muscle is too small or too weak to handle those forces, it gets overstretched and tears.

12PubMed Central. Hamstring Injury Mechanisms and Eccentric Training-Induced Muscle Adaptations: Current Insights and Future Directions

At the knee, weak hamstrings are one of the commonly identified risk factors for ACL tears, particularly in women. Female athletes who went on to tear their ACL showed about 15% less hamstring strength than male controls, while female athletes who did not tear their ACL were comparable in hamstring strength to the men.

13PubMed Central. The relationship of hamstrings and quadriceps strength to anterior cruciate ligament injury in female athletes

Video analysis of non-contact ACL injuries reinforces the connection: these injuries often happen when the knee is near full extension with the hip flexed and internally rotated. The hamstrings, when strong enough, resist that vulnerable position by pulling the tibia backward and preventing excessive knee extension. Relative hamstring weakness may make athletes more prone to landing in the dangerous posture that tears the ligament.

14Asia-Pacific Journal of Sports Medicine, Arthroscopy, Rehabilitation and Technology. Lower hamstring to quadriceps muscle strength ratio and lower body weight as factors associated with noncontact anterior cruciate ligament injury in male American football players: A prospective cohort study

Hamstring Size and Sprint Performance

If you are trying to get faster, hamstring volume is one of the strongest anatomical predictors of sprint ability. A study comparing national-level sprinters with physically active men found that the sprinters had substantially larger hamstrings and that hamstring volume correlated strongly with 40-meter sprint time and maximum velocity.

15The Journal of Strength & Conditioning Research. Hamstring Muscle Volume as an Indicator of Sprint Performance

Separate research in elite youth football players found that the hamstrings’ ability to produce force rapidly in the first 100 milliseconds of contraction was positively associated with acceleration over the first 5 to 30 meters. Interestingly, hamstring function did not correlate with top-end sprint velocity in that study, suggesting that the hamstrings matter most during the acceleration phase when the body is driving forward at steep angles.

16International Journal of Sports Physiology and Performance. The Influence of Hamstring Muscle Peak Torque and Rate of Torque Development for Sprinting Performance in Football Players: A Cross-Sectional Study

For recreational athletes who have never prioritized hamstring training, this is encouraging: even modest gains in hamstring size can translate into measurably better acceleration.

Pelvic Tilt and the Hamstring Connection

Small, tight hamstrings often get blamed for postural issues, and there is some truth to the connection, but it is not as straightforward as many fitness sources suggest. Biomechanical modeling shows that as the pelvis tilts forward (anterior pelvic tilt), all three hamstring muscles experience increasing tissue elongation, with the greatest stretch occurring in the proximal region closest to the hip. The semimembranosus stretches the most in this scenario.

17PubMed. Anterior pelvic tilt increases hamstring strain and is a key factor to target for injury prevention and rehabilitation

This means that people who stand and move with a pronounced forward pelvic tilt are placing their hamstrings under greater passive strain even before they start running or lifting. If the hamstrings are already small and weak, that extra stretch during dynamic activities pushes them closer to their strain threshold.

However, the static story is more complicated. One study comparing individuals with and without low back pain found that hamstring muscle length was significantly reduced in people with low back pain, but hamstring length did not correlate with pelvic tilt range during forward bending in either group.

18PubMed. Hamstring muscle length and pelvic tilt range among individuals with and without low back pain

The takeaway is that tight hamstrings and weak hamstrings are different problems that sometimes overlap. A short, stiff hamstring does not necessarily control pelvic position during movement, and a forward-tilted pelvis can increase injury risk for the hamstrings regardless of their flexibility. Both length and strength need attention.

Growing Bigger Hamstrings Through Exercise Selection

Not all hamstring exercises produce the same growth in the same parts of the muscle. Research using electromyography confirms that the hamstrings can be regionally targeted by choosing different exercises. For example, the lying leg curl produced significantly greater activation in the lower lateral and lower medial hamstrings compared with the single-leg deadlift.

19The Journal of Strength & Conditioning Research. Regional Differences in Muscle Activation During Hamstrings Exercise

A modeling study that combined EMG data with chronic training outcomes found that the Nordic hamstring exercise primarily grew the semitendinosus (about 24% hypertrophy), while the single-leg deadlift primarily grew the semimembranosus (about 11% hypertrophy). The two exercises were not interchangeable for building a complete hamstring group.

20PubMed. Neuromechanical Determinants of Muscle-Specific Hamstring Damage and Hypertrophy across Exercises: An EMG-Informed Modeling Approach

Eccentric training, where the muscle lengthens under load, stands out as particularly effective for hamstring development. Nine weeks of Nordic hamstring exercises increased the biceps femoris long head muscle volume by about 8%, while knee-flexion strength jumped by about 40%. The training also increased fascicle length, meaning the muscle fibers literally grew longer, by roughly 19% in the central region and 33% toward the lower end of the muscle.

21PubMed Central. Multiscale hamstring muscle adaptations following 9 weeks of eccentric training

Eccentric training at long muscle lengths, such as during Romanian deadlifts performed through a full range of motion, shows similarly favorable adaptations. This kind of training has been shown to increase fascicle length and improve both concentric and eccentric strength, with large effect sizes for fascicle length changes and voluntary activation improvements.

22PubMed. Effects of eccentric training at long-muscle length on architectural and functional characteristics of the hamstrings

The practical message is that someone with small hamstrings should not rely on a single exercise. A combination of hip-dominant movements (like Romanian deadlifts or hip extensions) and knee-dominant movements (like Nordics or lying leg curls) covers different portions of the muscle group. Eccentric emphasis in both categories appears to deliver the fastest architectural and strength gains.

Hamstrings, Knee Osteoarthritis, and Daily Function

Outside of sport, small hamstrings create problems for joint health that many people do not connect to the muscles behind their thigh. In patients with knee osteoarthritis, adding hamstring strengthening exercises on top of the standard quadriceps program produced significant improvements in pain, walking speed, and scores on functional-performance questionnaires compared to baseline.

23PubMed Central. Treatment of knee osteoarthritis in relation to hamstring and quadriceps strength

The hamstrings stabilize the knee from behind and share the job of shock absorption during walking, stair descent, and transitions from sitting to standing. When they are weak, the joint surfaces bear more compressive load and the quadriceps must compensate, which can worsen the anterior-directed forces that accelerate cartilage wear. For older adults or anyone with early knee pain, hamstring size is not a cosmetic matter but a joint-protection issue.

How Hamstring Size Is Measured

If you are curious whether your hamstrings are genuinely undersized or just undertrained, there are clinical ways to find out. MRI remains the gold standard for measuring individual muscle volumes, but it is expensive and impractical for most people. Ultrasound has emerged as a valid and reliable alternative. Panoramic ultrasound, performed by a trained operator, shows good agreement with MRI for hamstring cross-sectional area and volume.

24Scientific Reports. Panoramic ultrasound vs. MRI for the assessment of hamstrings cross-sectional area and volume in a large athletic cohort

Three-dimensional ultrasound has pushed accuracy even further. Compared with MRI, biases for muscle volume ranged from less than 1% to about 2%, with excellent test-retest reliability across sessions.

25PubMed. Validity and Reliability of 3-D Ultrasound Imaging to Measure Hamstring Muscle and Tendon Volumes

These tools are used primarily in research and elite sports settings, but sports-medicine clinics increasingly offer ultrasound-based muscle assessments. For most people, a simpler functional test is more practical: if your hamstring-to-quadriceps strength ratio is well below 50% on an isokinetic machine, or if you cannot control a slow eccentric Nordic curl through at least half the range of motion, your hamstrings are almost certainly too small or too weak relative to your quads.

An Evolutionary Footnote

It is worth noting that relatively small hamstrings are, in a sense, part of the human design. When our ancestors transitioned from arboreal life to upright walking and long-distance running, the musculature around the hip and thigh reorganized. The gluteus maximus expanded enormously, the quadriceps grew to handle the demands of an extended-knee gait, and the hamstrings’ moment arms shifted to favor energy-efficient walking over raw power. The result was improved endurance but reduced capacity for explosive movements like tree climbing and, by extension, all-out sprinting.

26PubMed Central. Evolution of the human hip. Part 2: muscling the double extension

Modern life has stacked further disadvantages on top of that evolutionary trade-off. We sit more than any ancestor did, train our quads more readily because they are visible in the mirror, and often ignore the hamstrings until they get hurt. The result is a muscle group that is biologically designed to be modest in size and culturally set up to be even smaller.