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Hamstring

The hamstrings, also known as the hamstring muscles, are a group of three muscles located in the posterior compartment of the thigh that primarily function to extend the hip joint and flex the knee joint.^[1] These muscles consist of the biceps femoris (long head), semitendinosus, and semimembranosus, all of which originate from the ischial tuberosity of the pelvis and insert into structures on the proximal tibia and fibula.^[1] The biceps femoris short head, while sometimes included in broader discussions of the posterior thigh, originates from the femur and is considered a separate monoarticular muscle that only flexes the knee.^[2] Innervated primarily by branches of the sciatic nerve—with the semitendinosus, semimembranosus, and biceps femoris long head supplied by the tibial division, and the biceps femoris short head by the common fibular division—the hamstrings play a crucial role in locomotion, including walking, running, and squatting, by facilitating pelvic tilting and tibial rotation during knee flexion.^[1] Their biarticular nature, crossing both the hip and knee joints (except the short head), allows coordinated movement but also predisposes them to strain injuries, which are among the most common musculoskeletal issues in sports, particularly sprinting and kicking activities.^[2] Such injuries often affect the biceps femoris long head and can range from mild strains to complete ruptures, with recurrence rates as high as 33% due to factors like muscle architecture and eccentric loading during activity.^[1]

Etymology and Definition

Etymology

The term "hamstring" originates from the combination of two Old English words: "ham," meaning the bend or hollow behind the knee, derived from Proto-Germanic *hamma-, and "string," referring to a tendon due to its fibrous, cord-like appearance.^[3]^[4] The noun form "hamstring" first appeared in English in the mid-16th century, specifically around 1565, as recorded in a translation by Arthur Golding, denoting the tendons at the back of the knee.^[5] This usage reflected the visible, string-like tendons in the posterior thigh, which were prominent in both human and animal anatomy. By the 17th century, the term evolved in English literature and early medical descriptions, with the verb form emerging around 1641 in the writings of John Milton, meaning to disable by cutting these tendons—a practice historically employed to lame livestock or enemies without killing them.^[6]^[3] In modern anatomical contexts, "hamstring" has become the standard term for the group of muscles and tendons in the posterior thigh, retaining its descriptive roots while entering formal medical nomenclature by the 18th and 19th centuries in texts like those of anatomists describing lower limb structures.^[1]

Anatomical Criteria

The hamstring muscles are defined as the three superficial muscles located in the posterior compartment of the thigh: the biceps femoris (long head), semitendinosus, and semimembranosus.^[1]^[7] These muscles are distinguished by their shared anatomical features that enable coordinated movement across the hip and knee joints.^[8] Key criteria for classifying a muscle as part of the hamstring group include a proximal origin from the ischial tuberosity of the pelvis—forming a common tendon in some cases, often described as bipartite due to the medial and lateral attachments—distal insertion on the proximal portions of the tibia or fibula, innervation primarily by the tibial division of the sciatic nerve (L5-S2), and primary actions of knee flexion and hip extension.^[1]^[7] The short head of the biceps femoris represents a partial exception, originating from the linea aspera of the femur and innervated by the common fibular division of the sciatic nerve; due to these differences, it is not classified as a hamstring muscle.^[8] These characteristics ensure the hamstrings act as a biarticular muscle group, spanning both the hip and knee.^[1] Other posterior thigh muscles, such as the adductor magnus, are excluded from the hamstring group despite a partial "hamstring portion" originating from the ischial tuberosity and sharing innervation and hip extension function; this portion is classified within the medial thigh compartment due to its primary adductor role.^[7]^[8] This delineation maintains the hamstrings as a distinct superficial posterior unit.^[1]

Anatomy

Muscle Composition

The hamstring muscle group comprises three primary muscles in the posterior thigh compartment: the semimembranosus, semitendinosus, and biceps femoris, all of which are biarticular structures crossing the hip and knee joints.^[9] The semimembranosus is the deepest and largest of the medial hamstrings, featuring a broad, flat, and membranous shape with a fusiform muscle belly and extensive aponeurotic expansions. Its physiological cross-sectional area (PCSA) measures approximately 15 cm², reflecting its substantial size relative to the other hamstrings, and it exhibits unipennate architecture proximally transitioning to bipennate distally.^[9]^[10] Positioned superficial to the semimembranosus on the medial side, the semitendinosus is a slender, strap-like muscle distinguished by its long, cord-like tendinous portions that dominate its structure, with a fusiform belly spanning about 30 cm. It has a PCSA of around 8 cm² and includes a tendinous inscription separating superior and inferior regions.^[9]^[7] Laterally, the biceps femoris differs from the medial pair by consisting of two heads: the long head, a slender, fusiform, bipennate muscle with a PCSA of about 10 cm² and a length of roughly 42 cm, and the short head, a thinner, broader structure approximately 30 cm long with a PCSA of 3 cm². The long head shares an aponeurosis with the short head, and it is the only hamstring with a fibular insertion.^[9]^[10] In terms of layering, the medial hamstrings form a superficial-to-deep arrangement with the semitendinosus overlying the semimembranosus, while the biceps femoris occupies a distinct lateral position; overall, the semimembranosus contributes the largest volume (up to 324 cm³ in healthy adults), underscoring its dominant role in the group's mass.^[9]^[10]

Origins, Insertions, and Relations

The hamstring muscles, comprising the semitendinosus, semimembranosus, and biceps femoris, exhibit distinct origins that anchor them primarily to the pelvis and femur. The long heads originate from the ischial tuberosity, with the semitendinosus from the inferomedial impression and the long head of biceps femoris sharing a conjoined tendon, while the semimembranosus originates separately from the superolateral aspect; the short head of the biceps femoris arises from the lateral lip of the linea aspera on the posterior femur, positioning it more distally along the thigh.^[1] Regarding insertions, the semitendinosus and semimembranosus converge medially at the knee, with the semitendinosus inserting via a long tendon onto the medial surface of the proximal tibia as part of the pes anserinus, alongside the sartorius and gracilis tendons.^[1] The semimembranosus inserts primarily on the posterior medial tibial condyle, with expansions forming the oblique popliteal ligament and attaching to the medial meniscus and tibia.^[1] The biceps femoris, both heads uniting distally, inserts on the head of the fibula and the lateral tibial condyle, enabling lateral knee stabilization.^[1] Anatomically, the hamstrings occupy the posterior compartment of the thigh, with key relations to surrounding structures that influence their biomechanics. Proximally, their origins at the ischial tuberosity lie adjacent to the sacrotuberous ligament and gluteus maximus, while distally, the semitendinosus and semimembranosus form the superomedial border of the popliteal fossa, and the biceps femoris defines its superolateral boundary, enclosing the fossa alongside the gastrocnemius medially and laterally.^[11] The sciatic nerve courses posteriorly along the thigh, lying deep to the long head of the biceps femoris and entering the popliteal fossa between the hamstring tendons.^[11] In terms of muscular interactions, the hamstrings antagonize the quadriceps femoris across the knee joint for balanced flexion-extension, while proximally, they interface with the adductor magnus, whose vertical fibers blend with the hamstring origins at the ischial tuberosity, contributing to shared hip adduction and extension dynamics.^[11]

Innervation and Vascular Supply

The hamstring muscles are primarily innervated by branches of the sciatic nerve, which divides into the tibial and common peroneal (fibular) nerves within the posterior thigh.^[11] The semimembranosus, semitendinosus, and long head of the biceps femoris receive motor innervation from the tibial division of the sciatic nerve (L5-S2 spinal segments).^[12] In contrast, the short head of the biceps femoris is innervated by the common peroneal division of the sciatic nerve (L5-S1 spinal segments).^[11] The vascular supply to the hamstring muscles arises mainly from the profunda femoris artery (deep femoral artery), with contributions from its perforating branches that supply the mid-thigh portions of the muscle group.^[1] Proximal aspects of the hamstrings receive blood flow from the medial circumflex femoral artery, a branch of the profunda femoris, along with minor input from the inferior gluteal artery.^[13] These vessels form an anastomotic network that ensures robust perfusion during muscle activity.^[12] Due to the close anatomical proximity, the sciatic nerve lies anterior to the hamstring muscles in the proximal and middle thirds of the thigh, increasing the risk of nerve compression or injury from hamstring-related trauma or pathology.^[14]

Function and Biomechanics

Primary Muscle Actions

The hamstring muscles, comprising the biceps femoris (long and short heads), semitendinosus, and semimembranosus, primarily function as biarticular muscles that cross both the hip and knee joints, except for the biceps femoris short head, which acts solely at the knee.^[1] This biarticular configuration allows them to generate unique force vectors by influencing motion at two joints simultaneously, facilitating coordinated lower limb movements.^[15] At the knee joint, the primary action of all hamstring components is flexion, achieved through their insertions on the tibia and fibula, which pull these bones posteriorly relative to the femur.^[1] This flexion is particularly critical during eccentric contractions, where the muscles lengthen under tension to control deceleration of knee extension, such as in the late swing phase of gait to prevent excessive forward momentum of the shank.^[16] The semitendinosus and semimembranosus, as medial hamstrings, also contribute secondary internal (medial) rotation of the tibia during knee flexion, while the biceps femoris induces external (lateral) rotation.^[1] At the hip joint, the long heads of the biceps femoris, semitendinosus, and semimembranosus primarily extend the hip by drawing the femur posteriorly from their common origin on the ischial tuberosity, aiding in propulsion during activities like walking or running.^[1] This extension action complements their knee flexion role, with the biarticular nature enabling efficient energy transfer between joints, though it can impose length-tension constraints based on combined hip and knee angles.^[17]

Role in Locomotion and Daily Activities

The hamstrings are integral to the gait cycle, facilitating smooth transitions between phases through coordinated contractions. In the late swing phase (approximately 50% to 90% of the gait cycle), the hamstrings undergo eccentric lengthening under load to decelerate the forward swing of the leg, absorbing kinetic energy and controlling hip flexion and knee extension to prepare for ground contact.^[18] This eccentric action peaks in the biceps femoris, with negative work increasing with gait speed, such as 0.46 J/kg at maximum speed.^[18] Transitioning to the early stance phase (0% to 50% of the cycle), the hamstrings shift to concentric shortening to extend the hip, contributing to forward propulsion while the foot is fixed on the ground.^[18] Positive work during this phase also escalates with speed, reaching 0.43 J/kg for the biceps femoris.^[18] These dynamics ensure efficient energy transfer and joint stability throughout walking.^[19] In sports activities, the hamstrings support explosive movements by enhancing propulsion and control. During sprint acceleration, they generate substantial hip extensor torques, with eccentric knee flexor peak torque averaging 2.29 Nm/kg and correlating positively with horizontal ground reaction force production (P = 0.04).^[20] This activation, particularly in the biceps femoris during late swing, facilitates the "pawing" action that boosts initial speed.^[20] In jumping, the hamstrings provide stabilization during landing by modulating stiffness and eccentric control, which helps dissipate impact forces and maintain knee alignment through negative mechanical work at the joint (averaging 11.03% body weight × height in high absorbers).^[21]^[22] For daily activities, the hamstrings enable essential lower-body functions through their dual actions of hip extension and knee flexion. In stair climbing, they activate to extend the hip and control descent, working in tandem with other posterior chain muscles to generate the necessary power for ascent.^[19] During squatting, the hamstrings assist in knee flexion and hip stabilization, allowing controlled lowering and rising from a seated position.^[19] In maintaining posture during walking, they contribute to the antagonist relationship with the quadriceps, where the hamstring-to-quadriceps thickness ratio influences knee flexion moments (r = 0.373, P = 0.042), promoting balanced joint kinetics and upright stability.^[23] Additionally, their eccentric role in energy absorption during activities like landing from a step underscores their importance in routine impact moderation.^[22]

Clinical Significance

Common Injuries and Risk Factors

Hamstring injuries primarily manifest as strains, which are classified into three grades based on severity: grade 1 involves mild damage with minimal fiber disruption and no loss of strength; grade 2 represents a moderate partial tear with noticeable weakness and pain; and grade 3 indicates a severe or complete tear leading to significant functional impairment.^[24] Tears can occur at the muscle-tendon junction or within the tendon itself, while tendinopathies, often proximal, involve chronic degeneration and inflammation of the tendon.^[25] Among the hamstring muscles, the biceps femoris is the most frequently affected, accounting for a substantial portion of injuries due to its role in rapid deceleration.^[26] Epidemiologically, hamstring strains constitute 12-24% of all injuries in professional soccer players, with recent data as of the 2021/22 season showing an increase to 24%, and incidence rates ranging from 0.4 to 0.5 injuries per 1,000 training hours and up to 4.99 per 1,000 match hours.^[27]^[28] In sprinters and track athletes, recurrence rates can reach 5-60%, often within months of the initial injury, highlighting a high reinjury burden.^[29] Data from 2020-2025 indicate an overall incidence of approximately 0.5-1.0 injuries per 1,000 exposure hours across various sports, with soccer and sprinting showing elevated rates compared to other disciplines.^[30]^[31] Key risk factors include a history of prior hamstring injury, which increases susceptibility by up to twofold, and older age, with peak incidence in athletes aged 20-30 years due to cumulative wear.^[32]^[33] Muscle imbalances, such as quadriceps dominance where the quadriceps overpower the hamstrings, contribute to vulnerability, alongside fatigue from prolonged activity and poor flexibility in the posterior chain.^[34] These factors are modifiable through targeted training, though neuromuscular deficiencies and excessive training loads exacerbate the risk in high-intensity sports.^[35] Injuries typically arise from eccentric overload, where the hamstrings lengthen under tension during the late swing phase of sprinting or the follow-through of kicking, leading to excessive strain at speeds exceeding 85% of maximum.^[36] This mechanism is compounded by the biarticular nature of the hamstrings, spanning both hip and knee joints, which predisposes them to high mechanical stress during rapid hip flexion and knee extension.^[34] Such overload is prevalent in sports involving explosive movements, accounting for over 70% of cases in soccer.^[37]

Diagnosis and Imaging

Diagnosis of hamstring injuries typically begins with a detailed clinical history, where patients report sudden onset of pain in the posterior thigh, often accompanied by a popping or tearing sensation during activities such as sprinting or kicking.^[38] Physical examination involves palpation along the posterior thigh to identify tenderness, swelling, or ecchymosis, followed by assessment of strength through resisted knee flexion and hip extension, as well as evaluation of range of motion for deficits in knee extension or hip flexion.^[39] These clinical findings help differentiate hamstring strains from other posterior thigh pathologies, such as sciatic nerve irritation or referred lumbar pain.^[38] Imaging modalities are employed to confirm the diagnosis, assess injury severity, and guide management when clinical evaluation suggests significant involvement. X-rays are primarily used to rule out avulsion fractures, particularly at the proximal hamstring origins on the ischial tuberosity in adolescents or during high-force injuries.^[38] Ultrasound serves as an accessible initial imaging tool for acute strains, enabling dynamic assessment of muscle tears and tendon integrity with high sensitivity (approximately 85%) for detecting abnormalities in the early post-injury phase.^[40] Magnetic resonance imaging (MRI) is considered the gold standard for detailed evaluation, utilizing T2-weighted or STIR sequences to visualize edema, hemorrhage, and fiber disruption, while proton density sequences help delineate partial versus full-thickness tears at the musculotendinous junction.^[41] Coronal T2 fat-saturated images provide an overview of injury extent, measuring edema length and tendon retraction to inform prognosis.^[41] Hamstring injuries are commonly graded on a three-tier system based on the degree of muscle fiber disruption observed clinically or via imaging. Grade 1 injuries represent mild strains with less than 10% fiber involvement, manifesting as minimal pain and no significant loss of strength or function.^[42] Grade 2 injuries involve partial tears with 10-50% fiber disruption, leading to moderate pain, swelling, and noticeable deficits in strength and range of motion.^[39] Grade 3 injuries indicate complete ruptures with greater than 50% fiber disruption, often resulting in severe pain, a palpable gap, and substantial functional impairment.^[39] Recent advances since 2020 have incorporated artificial intelligence to enhance MRI interpretation for hamstring injuries, improving diagnostic accuracy and prognostic assessment. Deep learning algorithms now enable automatic 3D segmentation and quantification of hamstring muscle edema on MRI, facilitating precise measurement of injury volume and aiding in the prediction of recovery timelines. Additionally, the Proximal Hamstring Objective Magnetic Resonance Imaging Score (PHOMRIS) provides a reliable, MRI-based grading tool for proximal injuries, correlating imaging features like tendon retraction and edema with clinical outcomes in surgical candidates.^[43]

Treatment and Rehabilitation

The initial treatment of acute hamstring injuries follows the RICE protocol, which involves rest to avoid further strain, ice application to reduce swelling, compression to minimize hemorrhage, and elevation to promote fluid drainage.^[44] Nonsteroidal anti-inflammatory drugs (NSAIDs) are commonly used for pain management and to control inflammation, typically introduced after the first 48 hours to avoid interfering with early healing processes.^[45] Rehabilitation for hamstring injuries is structured in progressive phases aligned with tissue healing timelines. The protection phase, spanning 0-2 weeks post-injury, focuses on minimizing load through relative rest and, if necessary for severe strains, brief immobilization to protect the damaged tissue while initiating gentle isometric exercises at shortened muscle lengths.^[46] The repair phase, from 2-6 weeks, emphasizes controlled mobility with gentle stretching and submaximal strengthening to support collagen alignment and early load tolerance.^[47] In the remodeling phase, beginning around 6 weeks and extending to full recovery, eccentric strengthening exercises—such as Nordic hamstring curls—are introduced to enhance muscle length-tension properties and resilience, allowing gradual return to dynamic activities.^[31] Recent evidence-based guidelines from 2023 highlight the importance of individualized, criteria-based progressive loading in rehabilitation protocols to optimize tissue adaptation and minimize reinjury risk.^[46] These approaches, incorporating sport-specific demands and monitoring symptoms alongside strength metrics, have demonstrated reductions in recurrence rates when including progressive agility, trunk stabilization, and eccentric loading. Typical outcomes for mild to moderate (grade 1-2) hamstring injuries include an average return to sport within 11-25 days, depending on injury location and athlete response, while grade 3 injuries often require surgical intervention for complete tears to restore function.^[48]

Surgical Uses and Prevention Strategies

Hamstring tendons, particularly the semitendinosus and gracilis, are commonly harvested as autografts for anterior cruciate ligament (ACL) reconstruction due to their suitable length, diameter, and biomechanical properties, allowing for quadrupled or five-strand configurations to enhance graft strength.^[49] This approach provides comparable clinical outcomes to other autografts, with low failure rates and good knee stability reported in systematic reviews.^[50] For proximal hamstring avulsions, surgical repair typically involves suture anchors placed at the ischial tuberosity to reattach the tendons, often using 2-5 anchors in configurations like suture bridge or all-suture constructs to achieve secure fixation and minimize displacement under load.^[51] Biomechanical studies confirm that all-suture anchors offer superior load-to-failure resistance compared to traditional titanium anchors, supporting their use in acute repairs to restore function and prevent chronic weakness.^[52] Prevention strategies for hamstring injuries emphasize eccentric strengthening programs, such as the Nordic hamstring exercise (NHE), which has been shown in meta-analyses to reduce injury incidence by up to 51% through improved eccentric strength and muscle fascicle length.^[53] Dynamic warm-ups incorporating progressive sprinting and plyometrics, combined with flexibility routines targeting hamstring extensibility, further mitigate risk by enhancing neuromuscular control and reducing strain during high-speed activities.^[31] Workload monitoring, including tracking acute-to-chronic training ratios and high-speed running volumes, is essential for athletes, as spikes in demands correlate with elevated injury risk, allowing coaches to adjust loads proactively.^[31] Following hamstring surgery, rehabilitation protocols integrate early mobilization—typically starting within days of repair—to promote collagen alignment, prevent adhesions, and avoid stiffness while protecting the repair site through controlled range-of-motion exercises.^[54] Emerging trends in 2025 include biomechanical screening tools, such as field-based protocols using video analysis, to assess sprint mechanics and identify at-risk athletes.^[55] These tools enable personalized prevention by quantifying injury risk factors like sprint mechanics asymmetries, with ongoing validation in elite sports settings.

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Sherry and Best demonstrated a significant reduction in injury recurrence when individuals with an acute hamstring injury were treated using a progressive ...
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A Field-Based Screening Protocol for Hamstring Injury Risk in ... - NIH
Sep 1, 2025 · Abstract. This paper propose a practical field-based screening protocol for evaluating the risk of hamstring injury.Missing: emerging | Show results with:emerging
[56]
Sprint running mechanics are associated with hamstring strain injury
May 24, 2025 · A qualitative screening tool to identify athletes with “high-risk” movement mechanics during cutting: The cutting movement assessment score ...