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Year : 2021  |  Volume : 21  |  Issue : 2  |  Page : 39-44

Mechanism of hamstring strain injuries in sports: A narrative review

1 Department of Rehabilitation, National Guard Health Affairs, King Abdulaziz Medical City, Riyadh, KSA
2 Department of Physical Therapy, College of Applied Medical Sciences, Imam Abdulrahman Bin Faisal University, Dammam, KSA

Date of Submission08-Aug-2021
Date of Acceptance16-Aug-2021
Date of Web Publication02-Oct-2021

Correspondence Address:
Haifa Saleh Al Mansoof
Department of Rehabilitation, National Guards Health Affairs, King Abdulaziz Medical City, Riyadh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sjsm.sjsm_21_21

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Hamstring strain injury (HSI) is one of the most frequent injuries among athletes in several sports. The high rate of recurrence of this injury advocates that our current understanding of HSI and the risk of its recurrence is incomplete. HSIs are multifaceted and have numerous mechanisms. Having an insight into the injury mechanisms helps determine the severity of the injury, validate clinical practice, manage progress, and prevent occurrence and recurrence of injuries. Hamstring muscles are essential in generating force and controlling the body during running and sprinting activities. It is the force generator in propulsion, swinging leg decelerator, and shock-absorber when impacting the ground. The HIS mechanisms are several, but highlighting the important interaction between different parts (the core and extremities) and systems (the neuromuscular and musculoskeletal) of the body and the effect of having such interaction impaired. The HIS recurrence mechanisms highlight the impact of the residual and persistent risk factors and their ways to help the HIS to reoccur.

Keywords: Hamstring injuries, musclestrain, recurrence, sports population

How to cite this article:
Al Mansoof HS, Almusallam MA, Nuhmani S. Mechanism of hamstring strain injuries in sports: A narrative review. Saudi J Sports Med 2021;21:39-44

How to cite this URL:
Al Mansoof HS, Almusallam MA, Nuhmani S. Mechanism of hamstring strain injuries in sports: A narrative review. Saudi J Sports Med [serial online] 2021 [cited 2022 Nov 26];21:39-44. Available from: https://www.sjosm.org/text.asp?2021/21/2/39/327487

  Introduction Top

The crucial role of hamstring in sports performance is positioned around their function throughout high-speed running.[1],[2] Hamstrings have a significant role in decelerating the extension of the knee during the late swing phase, which allows the foot to adapt ground contact beneath the center of mass of the body, after which they function as an active hip extensor. Hence, two prominent peaks in hamstring muscles excitation occur throughout the late swing and early stance phase of running.[3] Hamstring electromyographic (EMG) activity has been of similar magnitude during the late swing and early stance.[3] Compared with the rest of the running cycle, the amount of excitation in the late swing phase is 2–3 times greater than that of the early swing and late stance phase, and the excitation through the early stance is significantly higher than that of late stance.[3]

Hamstring muscle strains are common sports injuries associated with abrupt acceleration and maximal sprinting or overstretching. The severity of muscle strain injuries is commonly classified as Grade I, mild strain injury with a minimal tear of the musculotendinous unit with trivial loss of strength; Grade II, moderate strain injury (partial tear) of the musculotendinous unit with a marked loss of strength that causes functional impairments; and Grade III, severe injury with a whole rupture of the musculotendinous unit and is accompanied with severe functional disability. Moreover, the severity of a muscle strain injury was measured by the postinjury deficit in the maximal isometric contraction.[4] The postinjury maximum isometric contraction force deficit could be estimated from the muscle strain and the linked elongation speed during the eccentric contraction that induced the injury.[4] The role of muscle elongation speed in estimating the deficit in maximum isometric contraction force after a muscle strain depended on the muscle strain. The role of the muscle elongation speed in the estimation of the severity of strain injury increased as the muscle strain intensified.[4] Therefore, the higher the muscle speed of elongation in an eccentric contraction is, the worse the muscle strain injury will be when the muscle strain is marked.

  Mechanisms of Hamstring Strain Injury Top

Hamstring eccentric contraction and over-stretch related mechanisms

Whether the muscle lengthening belongs to the tendon or muscle (or both), eccentric contractions can cause muscle fiber damage. The greater the activation level of a muscle during an eccentric contraction, the greater mechanical energy the muscle would absorb before a muscle strain injury occurs.[4] Therefore, a suddenly activated eccentric contraction causes severe muscle strain injury.

Hamstring strain injury (HSI) mechanisms include stretching, sliding, turning, twisting, kicking, overuse, jumping, and during escape/saving/take-off exercises in sports such as wrestling.[5],[6]


Within rugby union athletes, kicking was responsible for approximately 10% of HSI and that they were considered the most serious in terms of time lost (36 days lost). In addition, 19.2% of HSI was attributed to kicking the ball at the community level of Australian football.[7] Moreover, up to 55% of HSI was reported in the preferred kicking leg within professional soccer.[5] Although the reason for this was not well established, Rahnama et al.[8] found that the knee flexors of the preferred kick leg were significantly weaker (P< 0.05) than the nonkicking leg when measured at 2.09 rad/s, and that 68% of the athletes tested had differences in strength between legs. Therefore, as the strength deficits have been highlighted as a risk factor for HSI, it is reasonable to assume that the reduction in the strength of the preferred kick leg plays a role in its increased susceptibility to injury, especially when this is coupled with the possibility that the leg is overloaded during the performance.[8]

Stretching and splits

Stretching and performing side and sagittal splits have been reported as HSI mechanisms in various sports, including ballet, dance, rock climbing, tennis, soccer, judo, ice hockey, and gymnastics.[9]

Running and sprinting

Running and sprinting are the primary mechanisms for HSIs.[5],[6] Sprinting and high-speed running was responsible for 70% of HSIs among soccer players[5] and 73% of HSIs among elite Australian footballers.[7] The percentage of HSI attributed to running and sprinting was also reported for other team sports, including American football (48.4%), lacrosse (35.6% male; 48.5% female), basketball (25% male; 35.1% female), and individual sports such as outdoor track and field (58.3% male; 46.9% female) in the National College Athletic Association athlete study.[6]

It has been suggested that injury may occur during a late swing when the hip is flexed and the knee is extended.[10] The peak hamstring active lengthening contraction and overstretch occur during the late swing phase of sprinting before foot contact, creating potential conditions for strain injury to occur. The magnitude of the peak stretch was significantly greater for the biceps femoris long head (BFlh) (9.5% longer than the upright body) than for the semimembranosus (SM) (7.4%) and semitendinosus (ST) (8.1%) muscles.[11] Specifically, because the knee is slightly flexed throughout the late swing, the smaller knee flexion moment arm of the lateral hamstring (biceps femoris [BF]) results in a greater muscle stretching relative to the upright. The lengthening of the fiber/fascicle may be more significant in the BFlh during operation, thus increasing the risk of injury.[12] However, architectural gearing allows the fibers/fascicles to rotate as they shorten/length during contractions, allowing for a more extended length change in muscle relative to the fibers/fascicles, which should provide significant protection against strain injury.[12] The intramuscular architectural structure of the BFlh is nonuniform, with the BFlh pennation being greater in the proximal-medium sections than in the distal and extreme proximal sections. These variations are likely to result in stress and strain across the muscle, increasing the risk of injury. Depending on the task, different hamstring patterns of muscle activation may occur, resulting in shear forces being generated between the BFlh compartments, increasing the risk of injury.[12]

Interactions between fatigue, hamstring muscle activation, and function

Differences between hamstring muscles within the running cycle have been observed in changes in muscle activation with increasing running speed.[3] It appears that BF is activated earlier than ST to prepare for high impact moments when running close to maximum sprinting speed, which could be explained by the vital role of BF in generating forward propulsive force.[3] Since BF and ST muscle bellies share a common proximal tendon origin, differences in activation patterns and timing of peak activation between these muscles may significantly impact HSI risk.[3] These findings suggest that the strong absorption capacity could be remarkably reduced at the most extended length of the hamstring muscles, which could increase the vulnerability to HSI since the production of force during sprinting is highly dependent on the use of elastic tissue energy from the retraction. In addition, athletes who had previous HSI suffered from strength-endurance deficits.[13] Athletes with low hamstring strength-endurance capability may be at higher HIS risk. This low hamstring strength-endurance capability may lead to disruption of individual sarcomeres, leading to large local mechanical strains propagating to the musculotendon junction. Substantial changes in stiffness between the fibers and the tendon can then contribute to the tearing of the fibers at the junction.

Terminal swing in running and sprinting

Indeed, during sprinting, the maximum electromyogram (EMG) activity has consistently been shown to occur during the terminal swing phase.[3] The hamstring muscle-tendon unit only undergoes prolonged activation (eccentric contraction) during the late swing phase.[11] The peak muscle-tendinous stretch was synchronous with peak EMG activation in BFlh during the late swing phase of overground sprinting.[3] As speed increased from 80% to 100%, BF activity increased by an average of 67% during the terminal swing phase, while ST and SM showed an increase of only 37%. The peak hamstring EMG activity during sprinting occurs during the terminal swing. In the terminal swing phase, the BFlh, ST, and SM have peak strain, generate peak force, and execute greater negative energy absorption. The eccentric contraction-induced damage represents the starting point for major muscle strain injuries.

The additional work required upon the hamstrings at this time point is to blame for the high number of HSI.[14] The hamstrings play an important role in horizontal force production during acceleration sprint mechanics.[15] The athletes displaying higher levels of hip extensor torque (eccentric hamstring strength) and the highest hamstring electromyography (EMG) activation during the terminal swing phase were able to generate greater horizontal ground reaction forces.[15]

The hamstrings are vulnerable to injury during the terminal swing for several reasons.[16] The hamstrings appear to be most biomechanically bothered during the terminal swing as it counteracts the inertial force acting about the knee joint by decelerating the swinging shank. The gastrocnemius is the only assistant to the hamstring in this duty.[16]

Initial/early stance

In contrast, several large muscles assist in producing the hip extensor torque in the initial stance.[17] The hamstrings generate <50% of the total hip extensor torque in various functional tasks.[17] The hip, knee, and hamstring mechanics during high-speed running, showing that during initial stance, the net joint torque mainly results from muscle torque (generated by muscle contractions) and external forces (resulting from ground reaction forces).[18] Large loads associated with ground contact may cause HSI.

The swing-stance transition period

A recent study suggested that the risk of maintaining HSI is high in both the late swing and early stance phases, but with different loading mechanisms underpinning it.[19] During maximum sprinting in elite athletes, large passive torques at the knee and hip joints lengthen the hamstring muscles in both phases.[19] The active muscle torques generated mainly by the hamstrings counteracted the passive effects caused by the forward swing of the leg (late swing phase) and the external ground reaction force (early stance phase).[19] As a result, these two phases may be a single-phase (the swing-stance transition period) because the lower extremity joint movements are continuous, and the hamstring muscles function to extend the hip and flex the knee throughout the entire phase.[19]

In addition, the hamstring muscles undergo a stretch-shortening cycle throughout high-speed running action. Traditionally, BFlh has been suggested to shorten during the first part of the swing phase as the knee flexes and the hip moves from extension to flexion. Subsequently, a rapid lengthening of the BFlh occurs as the hip continues to flex while the knee extends throughout the second half of the swing phase. Next, BFlh begins to shorten as the hip extends and the knee flexes in preparation for the foot strike. Throughout the stance phase, the hip continues to extend, and the knee flexes for the first half before starting to extend. Therefore, the length change may depend on the individual athlete's degree of hip and knee extension.

Impaired functional core integrity and pelvic kinematics

Functional core integrity is essential for safe hamstring function during operation.[20],[21] HSI risk in soccer is associated with deviating pelvic kinematics.[20] Lack of active core control appears to increase the risk of a primary HSI. Assessing and correcting running techniques is crucial in preventing HSI.[20] Deficient core stability, allowing excessive pelvis and trunk motion during the swing, is likely to increase primary injury risk. Although sprinting involves a relative risk of hamstring muscle failure in every athlete, running coordination is essential in HSI prevention.[20] The running coordination may be highly associated with the risk of sustained HSI. Lack of control of the lumbopelvic unit (core stability) presented by excessive pelvis and trunk motion during the swing phase is related to the primary risk of injury.[20]

The contralateral hip flexors (i.e., iliopsoas) have as significant an influence on the hamstring stretch as the hamstrings themselves.[10] This occurs because iliopsoas can directly increase anterior pelvic tilting, requiring greater hamstring stretching.[10] Other proximal muscles acting on the pelvis, like the abdominal oblique and erector spinae, also significantly influence the hamstring stretch.[10] The more distal muscles acting around the knee and/or ankle had much less influence on hamstring mechanics.[10],[22] The impaired neuromuscular control of the trunk and pelvis muscles can represent a mechanism of HSI.[10]

There is also a growing body of evidence that lumbopelvic muscle function can play an essential role in HSI risk reduction.[20] Indeed, fatigue has been shown to promote anterior pelvic tilt in soccer players, potentially predisposing them to an increased risk of injury due to increased relative length of BFlh.

Fatigue effect on hamstring muscle coordination and metabolic demands

Hamstring muscles generate large opposing forces during high-speed running (Huygaerts et al., 2021)[12] while also playing a role in developing dynamic knee stability (Huygaerts et al., 2020).[12] There is a possibility that changes in muscle coordination strategies may cause one or more hamstring muscles to be disproportionately activated (Huygaerts et al., 2020), possibly increasing metabolic demand and thus prematurely fatigue the overactive muscles (Huygaerts et al., 2020). With fatigue, lower limb stiffness appears to decrease (Huygaerts et al., 2020),[12] which could lead to the adoption of a “Groucho” running pattern associated with reduced movement efficiency and increased joint moments of strength (Huygaerts et al., 2020). This phenomenon, combined with increased anterior pelvic tilting (due to lumbopelvic instability) during operation, could potentially place BFlh at a relatively longer duration where it is more vulnerable to strain injury (Huygaerts et al., 2020).[12] As a result, the late swing and early stance phases appear to be critical points at which HSI is more likely to occur (Huygaerts et al., 2020). Defining fatigue's influence on tissue behavior in these two phases may be critical to a better understanding of HSI mechanisms (Huygaerts et al., 2020).


A relationship was found between HSI and high-speed running distances over 4 weeks before injury (Duhig, 2017). This relationship suggests that HSI may, at least sometimes, be an overuse injury rather than isolated events associated with a single injurious contraction or sudden stretching (Duhig, 2017). These findings indicate that monitoring of high-speed running volumes may help prevent HSI, as coaches and conditioning staff should be able to prevent athletes from experiencing sudden increases in training loads (Duhig, 2017).

  Hamstring Strain Injury Recurrency-Related Mechanisms Top

Neuromuscular inhibition

The role of neuromuscular inhibition after the injury is a potential mechanism for several maladaptations associated with hamstring reinjury. These maladaptations include eccentric hamstring weakness, selective hamstring atrophy, and knee flexor torque-joint angle shifts. Athletes return to competition after an HSI, having developed maladaptations that predispose them to further injury.

The pain-driven neuromuscular inhibition of voluntary hamstring activation following HIS has a marked effect on hamstring recovery by limiting hamstring exposure to eccentric stimuli over long muscle lengths during rehabilitative exercise. This limited exposure to eccentric stimuli could result in chronic eccentric hamstring weakness, selective hamstring atrophy, and shifts in the torque joint–angle relationship. The previously injured hamstring muscles show functional deficits during the late swing phase of sprinting compared to uninjured contralateral muscles.[3]

Lumbopelvic effect

Recurrent HSIs are common in several elite sporting activities (Saunders et al., 2019). Recurrent HSI is usually a significant warning sign, especially with a history of significant trauma to the lower back or buttocks may occur (Saunders et al., 2019). Recurrence on the same side should raise suspicions of possible injury to the sacroiliac joint, especially in contact sports (Saunders et al., 2019). Diagnosis is suspected if there is a history of trauma to the lower back or buttocks or a history of lateralizing lower back pain without imaging evidence of significant intervertebral disk prolapse (Saunders et al., 2019). Sciatic nerve conductivity impairments may occur in athletes with a history of HSI. Furthermore, significantly reduced hamstring flexibility or increased stiffness has been detected in low back patients.[23]

The extraforaminal L5 nerve root entrapment is one of many explanations for propensity to hamstring and calf symptoms in sport athletes, like piriformis syndrome, sciatic nerve entrapment in the internal obturator muscle, and hamstring syndrome. Perhaps multiple subtle traps may be present simultaneously and additive, leading to the motor dysfunction of the hamstring and calf muscles.[24] Like these syndromes, the anatomical configuration of the lumbosacral ligament trapping of the nerve root L5 has a significant potential to help explain the pathogenesis of the posterior thigh and calf injuries in some athletes.[24]

Most of the HSIs were due to sacroiliac joint dysfunction on the contralateral side (Saunders et al., 2019). Changes on the contralateral side resulting from increased tension in the sacrotuberous ligament will be transmitted to the tendon of the long head of the BF muscle, which is fused with the sacrotuberous ligament in approximately 50% of cases. Increased tension in the sacrotuberous ligament most likely leads to increased tension in the hamstring tendon/muscle, which predisposes it to injury with a rapid increase in dynamic tension, as occurs in many running elites and kicking sports (Saunders et al., 2019).

  Conclusion Top

HSI is one of the most common injuries affecting athletes in many sports. Understanding the injury and injury recurrence mechanisms is crucial for prevention, prediction of injury consequences, and rehabilitation. HSI mechanisms include eccentric-contraction/overstretch, impaired functional core integrity and pelvic kinematics, fatigue, and overuse-related mechanisms. HIS recurrency mechanisms include neuromuscular inhibition and lumbopelvic affection-associated mechanisms. Despite all the multidisciplinary research efforts to deal with the HSI dilemma, there are indeed aspects not discovered yet about the HIS etiology and pathogenesis in the athletic population.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

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