Hamstring Strain Injuries: Incidence, Mechanisms, Risk Factors, and Training Recommendations

Abstract

Hamstring strain injury (HSI) is one of the most commonly reported sports injuries. This has led to a substantial amount of research aimed at identifying factors that increase the risk of an athlete suffering a HSI. The identification of risk factors allows practitioners to plan intervention programs with the aim of reducing the rate and severity of HSI. As a multitude of factors contribute to the risk of HSI, interventions should be multifaceted in nature. This review outlines the incidence, mechanisms, and risk factors for HSI and provides evidence-based training recommendations to reduce the rate and severity of HSI.

INTRODUCTION

Hamstring strain injuries (HSIs) are one of the most commonly reported lower limb injuries, with high incidence and reinjury rates across a number of sports (^{12,16,26,29,31,76,77,79,102,114}). These injuries can be viewed as acute (i.e., as a direct result of an impact or traumatic event with sudden feelings of pain), overuse (i.e., exposure to inappropriately high training load/volume over an extended period) and chronic or recurrent (i.e., the repeat injury of the same muscle site due to a reduction in function and/or lack of appropriate healing and rehabilitation, which may also take the form of an acute injury) (¹⁸). In some cases, HSI can be severe in nature, which has been previously defined as an injury that takes greater than 28 days to recover (²⁹). Often, HSI leads to a significant loss of athlete playing time, which may have a detrimental effect upon team performance and subsequent financial losses for sporting organizations (^41,44). A report in Australian football from the 2012 season estimated that HSI could cost clubs up to $245,842 per season (⁴⁴). This was seen as an increase of 71% in comparison with the figures reported for the 2003 season (⁴⁴).

This has led to a substantial amount of research aimed at identifying risk factors that predispose athletes to suffering a HSI. These risk factors have been classified into 2 groups: modifiable and nonmodifiable (⁵⁶). The modifiable risk factors are those that can be altered through a training intervention and include reduced eccentric strength, fatigue, flexibility, high-speed running loads, and insufficient or inadequate warm-up. However, despite identifying several risk factors that contribute to HSI risk, a substantial amount of research evaluating HSI prevention programs have centered solely around the development of eccentric hamstring strength. These have often included the use of the Nordic hamstring curl (^2,90,107). In some cases, interventions of this nature have reduced HSI by 65% (²), as well as significantly reducing the time lost to HSI (⁹⁰).

Despite this ongoing research, and subsequent training recommendations, HSI have been reported to have increased annually within professional soccer (³¹), athletics (⁷²), and in cricket (⁷⁷) since the introduction of the 20 over format (a faster paced game played across 20 overs per team). Although challenging to fully explain, this may be due to the lack of emphasis placed upon the other modifiable risk factors within HSI prevention programs. Furthermore, after injury, the hamstrings not only seem to suffer from a loss of strength (^{25,50,54,64,73,74}), but also flexibility as well (^50,64), which is believed to contribute to the risk of reinjury. Therefore, it seems that additional factors, and not just eccentric hamstring strength alone, warrant particular attention within HSI prevention programs. In order for these programs to be successful, practitioners should have a thorough understanding of the different types of HSI, the injury mechanisms, and the potential risk factors associated with HSI. Thus, the purpose of this review is to summarize the injury mechanisms, injury rate, and risk factors on HSI, with a focus on providing evidence-based guidelines for multifaceted injury prevention programs.

Throughout this review, it is important to have an appreciation for different injury definitions used within the literature when comparing any research of this nature. For example, Orchard et al. (⁷⁶) define an injury as one that causes an athlete to miss only match-playing time. By contrast, Ekstrand et al. (²⁹) includes any injury that prevents a player from taking part in training and competition. These differences in methodologies may have an influence over the prevalence and severity of reported HSI.

HAMSTRING ANATOMY

Having an understanding of the basic hamstring anatomy and function can aid to improve the understanding of HSI risk. The hamstring muscle group consists of 3 major muscles of the posterior thigh: semitendinosus, semimembranosus, and biceps femoris (long and short head) (^18,108,113). The biceps femoris long head, semitendinosus, and semimembranosus have a biarticular formation where they cross both the knee and hip joint. This biarticular formation causes the hamstring to stretch at 2 points, a factor often hypothesized to contribute to the high rate of HSI (¹¹⁴). The biceps femoris long head originates from the medial facet of the ischial tuberosity through its proximal tendon and distally inserts to the lateral surface of the fibula head (^18,108,113). The semitendinosus also originates at the ischial tuberosity before extending and inserting distally at the medial surface of the tibia (^18,108,113). The semimembranosus proximal tendon arises from the lateral aspect of the ischial tuberosity and extends distally to attach at the posterior aspect of the medial tibial condyle (^18,108,113). The biceps femoris short head arises from the femur and inserts at the fibula head, making it a uniarticular muscle that crosses only the knee joint (^18,108,113). The isolated function of the hamstring muscle group is to shorten concentrically to produce knee flexion and hip extension. During more integrated or dynamic muscle actions (e.g., jumping, sprinting, and changing direction), the hamstrings aid in the stabilization of the lumbopelvic hip complex and knee joints (^51,86).

Of particular interest regarding HSI is the intramuscular or central tendon, which descends down the length of the muscle belly (^17,55). The intramuscular (central) tendon acts as a supporting structure to which the muscle fibers attach (¹⁷). When this tendon is injured or damaged, the injury is considered to be more severe with increased return to training and competition (^17,22,55,82). This is highlighted in the study by Comin et al. (²²) who identified 45 biceps femoris injuries, of which 12 also involved the central tendon. It was reported that the recovery times for those injuries involving the central tendon that did not require surgical intervention (71 days) were significantly (p < 0.01) longer than those not involving the central tendon (21 days) (²²). Therefore, the intramuscular tendon has important implications for injury prevention and rehabilitation.

FUNCTIONAL ROLE OF THE HAMSTRINGS IN ATHLETIC PERFORMANCE

The predominant role of the hamstrings within sports performance is often centered around their function during high-speed running. Their primary role during this is to decelerate knee extension during the terminal swing phase (a point in the running cycle where neither limb is in contact with the ground), so that the foot can make ground contact under the body’s center of mass, after which they act as an active hip extensor (^86,87). During the terminal swing phase, the biceps femoris long head, semitendinosus, and semimembranosus exhibit peak strain, produce peak force, and perform greater negative energy absorption (⁸⁶). It is a common theory that the additional work placed upon the hamstrings at this time point is responsible for the high number of HSI (^21,86,87).

Furthermore, the hamstrings seem to play an important role in horizontal force production during acceleration sprint mechanics (⁶⁸). It has been proposed that those 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 (⁶⁸). The important role of the hamstrings during running performance is further supported by Kyrolainen et al. (⁵³) who suggested that as running speed increases, so too does force production, which can be partly attributed to the action of the hamstrings (⁵³). Therefore, as the hamstrings seem to play a prominent role in speed development, it is essential for practitioners to have an understanding of appropriate training methods that optimize both their health and performance.

HAMSTRING STRAIN INJURY

Hamstring strain injuries are one of the most commonly reported sports injuries (^{12,16,18,26,29,31,76,77,79,102,114}). A HSI is commonly classified as a grade I–III strain depending on its level of severity (¹⁸). A grade I strain typically affects a small number of muscle fibers; grade II, a significant amount of muscle fibers; and grade III, a complete tear of the muscle (¹⁸). Using similar grade classifications, Ekstrand et al. (³⁰) reported return to play times to be 17 ± 10 days (grade I), 22 ± 11 days (grade II), and 73 ± 60 days (grade III) within professional soccer. In more recent times, additional injury grading systems have been proposed to increase their specificity and provide clearer information on return to play times (^20,69,81). Pollock et al. (⁸¹) suggest that alongside grading the injury severity on a scale of 1–4 (small, moderate, extensive, or complete tear), an additional suffix of (myofascial, musculotendinous, or intratendinous) should also be included to indicate the location of the injury. Similarly, Chan et al. (²⁰) proposed a new classification system, which included lesion site (proximal musculotendinous junction, muscle, or distal musculotendinous junction), with muscle injuries having 2 additional suffixes including location (proximal, middle, or distal) and anatomical site (intramuscular, myofascial, myofascial/perifascial, myotendinous, or combined). Including such information within injury classifications has been proposed to aid practitioners with both injury prevention and rehabilitation practices (^20,69,81).

HAMSTRING STRAIN INJURY TYPE

A type I strain is commonly referred to as a sprinting-related strain and is typically reported in sports such as rugby, athletics, and the various football codes (^{5,16,26,29,76,79,114}). These often occur when the hamstring muscle group are required to work eccentrically (produce force while lengthening) to decelerate the limb and control knee extension during the terminal swing phase of high-speed running (^21,42,57,88). This mechanism of injury has been supported by the work of Heiderscheit et al. (⁴²) and Schache et al. (⁸⁸) who studied the time frame of hamstring injury during running and concluded that injury occurred during the late swing phase. Schache et al. (⁸⁸) further reported that during the injury phase, the biceps femoris reached a peak length estimated to be 12% greater than that seen during upright posture and exceed the normalized peak length of the medial hamstrings. Furthermore, Higashihara et al. (⁴⁵) reported significant increases in hamstring activation when running speeds were increased from 85 to 95% of an individual's maximum velocity. Oftentimes, the biceps femoris is the main site of damage in type I strains, with Askling et al. (⁵) stating that the biceps femoris (long head) was the primary injury location in all 18 hamstring injuries suffered by elite-level sprinters within their study. A further 8 sprinters (44%) suffered additional injury, with 7 at the semitendinosus and one at the biceps femoris short head (⁵).

Type II hamstring strains are commonly seen as stretch-related injuries (¹⁸). These injuries most commonly occur during combined excessive stretching into hip flexion and knee extension (⁶). Askling et al. (⁶) report that these types of injury can occur in several sports (soccer, dance, judo, gymnastics, and sprint running) and during different athletic actions (high kicking, stretching, and sagittal and side splits). However, this is most commonly seen among dancers, with Askling et al. (³) reporting that 66% of acute HSI occurred during a sagittal plane split and 12% during a side split. These injuries commonly affect the semimembranosus, with Askling et al. (⁶) reporting this occurrence in 83% of type II strains, with all semimembranosus strains also involving its proximal free tendon. It is important for practitioners to understand which type of HSI is most likely to occur in their athletic population, enabling more specific rehabilitation protocols to be applied.

Although athletes suffering a type I strain often initially present with greater functional deficits compared with type II strains, their recovery time has been reported to be quicker (⁴). The study by Askling et al. (⁴) demonstrated that athletes who suffered from both type I and II strains could perform strength and flexibility assessments at >90% of the uninjured leg 6 weeks after injury. However, their self-reported time to return to preinjury levels of performance were markedly different (type I: average of 16 weeks [range = 6–50 weeks] and type II: average of 50 weeks [range = 30–76 weeks]), identifying the need for both subjective and objective information during the rehabilitation period (⁴). It should be noted that in the work of Askling et al. (⁴), these 2 different types of HSI were present in 2 different sports populations (type I: sprinters and type II: dancers), which may have influenced the difference in recovery times.

HAMSTRING STRAIN INJURY INCIDENCE, TIME LOSS, TIME OF INJURY INCIDENCE, AND TYPICAL SEVERITY

The incidence and time loss of HSI across several sports is summarized in Table 1. Within professional soccer, HSI incidence has been widely reported. Petersen et al. (⁷⁹) reported an average of 3.4 (range = 1–5) HSI per club per season, Woods et al. (¹¹⁴) reported a higher average of 5.0 (range = 0–16) per club, and Ekstrand et al. (²⁹) claimed that clubs could expect around 7 HSIs per season. This is similar to those reported for Australian football, where Orchard et al. (⁷⁶) reported 6 injuries during the 1995 season. Hamstring strain injury incidence also highlighted in the more recent 2018 Australian Football League (AFL) injury report, with 6.35 new HSIs per club per season (¹). The similar number of HSI per club per season reported within these 2 studies provides some evidence that HSI occurrence within AFL has remained consistent across 23 seasons (^1,76). Furthermore, the AFL injury report also demonstrated a HSI reinjury rate of 20%, defined as the same injury type, on the same side in the same season (¹). Injury incidence rates have also been reported for rugby union (5.6 per 1,000 player hours) (¹⁶), cricket (22.5 per 1,000 team days) (⁷⁷), and a range of National Collegiate Athletic Association (NCAA) sports (3.05 per 10,000 athlete exposures) (²⁶). Finally, within a cohort of student dancers, a retrospective analysis found that 51% of athletes reported suffering posterior thigh pain at some point in their careers (³). Thus, the prevalence of HSI seems common across a multitude of sporting populations.

Table 1: Hamstring injury incidence and time lost across a number of sports

Table 1-A: Hamstring injury incidence and time lost across a number of sports

Time loss due to hamstring injury can be seen as a more important factor than injury incidence because ultimately, the amount of time missed by an athlete may have a direct effect on team performance and results (⁴¹). This is highlighted in the 2018 AFL injury report, which noted that during a 22 game season, clubs could expect to lose 25.19 matches to HSI, which may ultimately have a detrimental effect upon team selection (¹). The reporting of time loss in professional soccer seems to be fairly consistent across the literature, with Woods et al. (¹¹⁴) (18 days), Ekstrand et al. (³¹) (17 days), and Petersen et al. (⁷⁹) (21.5 days) all reporting similar average time-loss values per injury. Woods et al. (¹¹⁴) further report that during this 18-day time-loss period, athletes are likely to miss 3 competitive soccer matches. Within NCAA athletes, 37.7% of HSIs incurred a time loss of <24 hours, with 6.3% reported to miss >3 weeks (²⁶). The severity of hamstring injuries was further displayed by Ekstrand et al. (²⁹) who stated that 12% of injuries classed as severe (time loss >28 days) were seen to be hamstring injuries.

Hamstring strain injury incidence (^31,79,114) and rate (²⁶) is reported to be more prevalent during competition than in training. This may indicate the increased intensity of match-play but also suggests that training may not sufficiently prepare athletes for the demands of competition (²⁹). This notion is further supported by Ekstrand et al. (²⁹) who states that hamstring strains are more prevalent in-season compared with preseason, highlighting the importance of continually training the hamstring group all year round within athlete development programs. Furthermore, it has been reported that 47% of hamstring strains in professional soccer occur in the final third of the first and second halves, suggesting that fatigue may be a contributing factor (¹¹⁴).

A magnetic resonance imaging (MRI) study of hamstring injuries within professional soccer highlighted 207 injuries, of which 13% were classified as grade 0 (negative MRI with no visible pathology), 57% grade 1, 27% grade 2, and 3% grade 3 (³⁰). Similar findings could be seen within a second study in professional soccer that reported 1,614 hamstring injuries, with 10% reported as minimal, 21% mild, 54% moderate, and 15% severe in nature (³¹). The findings within these studies would suggest that most hamstring injuries within soccer athletes are minimal to moderate in nature and/or classified as grades 0 to 2.

MECHANISMS OF INJURY

Among the literature presented in Table 1, running and sprinting was shown to be the primary mechanism for hamstring injury (^{16,26,30,36,114}). Ekstrand et al. (³⁰) highlight that sprinting and high-speed running were responsible for 70% of hamstring injuries among soccer players. Similarly, Gabbe et al. (³⁶) found that 73% of hamstring injuries among elite Australian footballers (AF) could be attributed to running or sprinting. These figures are much higher than those reported by Woods et al. (¹¹⁴) who claimed running was responsible for 57% of hamstring injuries. The percentage of HSI attributed to running and sprinting has also been reported for other team sports including American football (48.4%), lacrosse (men 35.6%; women 48.5%), basketball (men 25%; women 35.1%), and individual sports such as outdoor track and field (men 58.3%; women 46.9%) in a study of NCAA athletes (²⁶). Furthermore, within rugby union athletes, the “back” playing positions have been shown to suffer a greater incidence of hamstring injury, possibly because of the greater demand of high-speed running upon this playing group (¹⁶).

Other hamstring injury mechanisms reported within the literature include stretching (^6,30,114), sliding (³⁰), turning (^30,114), twisting (^30,114), kicking (^6,30,114), overuse (^26,30), jumping (^30,114), and during escape/sparring/take-down maneuvers in sports such as wrestling (²⁶). Collectively, although these actions are not as common as sprint-related injuries for team sport athletes, however, their importance should not be understated. The action of kicking, either a ball or an opponent, has also been highlighted within the literature as a HSI mechanism (^6,15,37). Previously, Askling et al. (⁶) has identified HSI during high kicking actions in ballet, taekwondo, and soccer. Within rugby union athletes, Brooks et al. (¹⁵) explained that kicking was responsible for approximately 10% of HSI, and that these were seen as the most severe in terms of time lost (36 days lost). Furthermore, Gabbe et al. (³⁷) reported that in community level Australian football, 19.2% of HSI were attributed to kicking the ball. In addition, within professional soccer, up to 55% of HSI have been reported in the preferred kicking leg (³⁰). Furthermore, Lord et al. (⁵⁹) reported that 100% (n = 20) of the injured subjects within their study suffered the HSI within the preferred kicking leg. Although the reason for this has not been well established, Rahnama et al. (⁸³) found the knee flexors of the preferred kicking leg to be significantly weaker (p < 0.05) than the nonkicking leg when measured at 2.09 rad/s, and that 68% of athletes tested had between-limb differences in strength >10%. Therefore, as strength deficits have been highlighted as a risk factor for HSI, it is reasonable to assume that the reduction in strength of the preferred kicking leg plays a role in its increased susceptibility to injury, especially when this is coupled with the possibility that this limb is overloaded during performance (⁸³).

Stretching and performing side and sagittal splits have been reported as a mechanism of injury in a variety of sports including ballet, dance, rock climbing, tennis, soccer, judo, ice hockey, and gymnastics (⁶). This type of injury is commonly reported within dancers. In fact, Askling et al. (³) stated that within a cohort of student dancers, 88% of acute HSIs were suffered during slow activities such as performing splits. In other sports, such as professional soccer, stretching- and sliding-related HSI have been reported with less regularity, with Ekstrand et al. (³⁰) stating that they account for 5% of all HSI. Finally, 13 HSI were reported in a population of NCAA wrestling athletes, with actions such as sparring, takedown maneuvers, and performing escapes all reported to be responsible for 15.4% (each) of all HSI (²⁶). Although running and sprinting are reported as the most common causes for HSI, identifying other possible mechanisms is important information that can enable practitioners to understand the risk of HSI and develop appropriate injury prevention plans for their given sport.

INJURY RISK FACTORS

Several risk factors relating to hamstring injury and reinjury have been reported within the literature. These can be categorized into 2 distinct groups: modifiable and nonmodifiable. Risk factors classified as modifiable are often seen as factors where the risk can be reduced through a targeted training intervention (e.g., increasing an athlete's strength). Nonmodifiable risk factors are those which are out of the control of the athlete and practitioner (e.g., age of the athlete). Oftentimes, these risk factors can be specific for each sport. For example, high-speed running loads are likely a risk factor in soccer as opposed to wrestling, whereas some may be global to all athletes, such as poor levels of strength. These risk factors and their implications for training are discussed in the following section.

MODIFIABLE RISK FACTORS

FATIGUE

As previously mentioned, hamstring injuries often occur toward the end of match-play, presumably with fatigue being a contributing factor. Several authors have investigated the role fatigue may have upon other established risk factors, such as eccentric knee flexor strength and, therefore, the ability of the hamstrings to generate and tolerate force. Small et al. (⁹⁵) and Greig (³⁹) both used a soccer-specific fatiguing protocol to measure the impact of fatigue upon measures of torque obtained through isokinetic dynamometry at contraction speeds of 60, 120, 180, and 300°/s, respectively. It was found in the work by Small et al. (⁹⁵) that eccentric peak hamstring torque and the functional hamstring:quadricep (H:Q) ratio (eccentric hamstrings versus concentric quadriceps) were significantly reduced during the fatigue inducing protocol (⁹⁵). Furthermore, the authors found no significant changes in concentric peak torque of both the hamstrings and quadriceps (⁹⁵). Greig (³⁹) also found no significant changes in concentric knee flexor and extensor peak torque at all contraction speeds tested but were able to demonstrate significant reductions in peak eccentric hamstring torque, which were more evident at the faster contraction speeds. This indicates that the hamstrings are more greatly affected when having to produce force quickly when fatigued, which may be particularly relevant when considering the relationship between high-speed running and HSI (³⁹).

The reduced ability of the hamstrings to produce force when under fatigue is also supported by Lord et al. (^58,59). Their first study highlighted significantly reduced mean horizontal force production in limbs previously suffering a HSI during a 10 × 6-s repeated sprint test on a nonmotorized treadmill (⁵⁸). The second study measured peak concentric knee flexor and extensor torque during isokinetic testing measured at 180°/s, after the completion of the same 10 × 6-s repeated sprint protocol (⁵⁹). They found significant reductions in isokinetic knee flexor torque and the concentric H:Q ratio only in limbs that had previously suffered a hamstring injury (⁵⁹). Furthermore, the decline in knee flexor torque was also able to correctly identify the previously injured limb with 100% accuracy (⁵⁹). Therefore, it seems that fatigue may also play a prominent role in hamstring reinjury rates as the previously injured limb seems to suffer greater loss of function when in a fatigued state (^58,59). Coratella et al. (²³) also found significant increases in peak joint torque angle, both during concentric and eccentric contractions, after a fatigue-inducing protocol, which consisted of the Loughborough Intermittent Shuttle Test (a 20-m shuttle run that involves sprinting, walking, and running at 55 and 95% of an individual's maximal aerobic speed [MAS]). The authors hypothesized that these fatigue-induced changes (where the hamstring exerts greater force at shorter muscle lengths) may highlight their impaired ability to act against the quadriceps during near maximal knee extension when the hamstring is in a lengthened position (²³). However, it should be noted that these measurements were made in a seated position and therefore are not indicative of sprint running gait (²³).

The reduced ability of the hamstrings to produce force at longer muscle lengths, and maybe more importantly absorb opposing force, may help to enhance the understanding of fatigue as a risk factor for injury. During sprint running, the hamstrings work both eccentrically to decelerate knee extension to counteract inertia of the leg swing during the terminal swing phase and concentrically as an active hip extensor (^57,78). It is at this time (terminal swing) that the biceps femoris, semitendinosus, and semimembranosus are subjected to peak strain, force, and energy absorption (⁸⁶). The reduced ability of the hamstrings to both absorb energy and produce force once fatigued is likely to impair their ability to perform subsequent tasks, and when accompanied by increased quadriceps dominance (as indicated by the reduced H:Q ratio), this may predispose the hamstrings to heightened injury risk (^{21,23,57,86,95}). Furthermore, altered hamstring muscle activation patterns (once fatigued), have been proposed as a possible cause of injury (^80,110). Pinniger et al. (⁸⁰) explain that under fatigue, there is a significant increase in the duration of hamstring EMG activity due to the earlier onset of muscle activation. It has been suggested that this may be a mechanism to overcome the reduced force generation capabilities of the hamstring muscle group (^80,110).

BICEPS FEMORIS FASCICLE LENGTH

The contribution of the biceps femoris fascicle length in HSI occurrence and reoccurrence has been discussed within the literature (^9,34,101,102). The prospective research by Timmins et al. (¹⁰²) in elite soccer players reported that short biceps femoris fascicle lengths of <10.56 cm increased the risk of HSI by 4.1-fold. Furthermore, a retrospective study by Timmins et al. (¹⁰¹) found that after injury, both fascicle length and fascicle length relative to muscle thickness was significantly (p < 0.001) reduced compared with the uninjured contralateral limb.

It has been hypothesized that the contributing mechanism to the increased risk of injury to shorter fascicles may be owed to a reduced number of in-series sarcomeres, which may be excessively lengthened during eccentric contractions (^9,102). This may be exacerbated further after injury with the presence and formation of scar tissue, which may increase the burden placed upon the fascicles during excessive lengthening (^34,52,93). Therefore, it can be seen that short biceps femoris fascicle lengths may play a role in both first-time HSI and injury reoccurrence and should be a factor that is considered in both injury prevention and rehabilitation programs.

HIGH-SPEED RUNNING LOADS

Running at high speed or sprinting has already been identified as a mechanism for hamstring injury. It has been previously well reported that spikes in athlete load increase the risk for soft-tissue injury, and that appropriately planned vigorous training may decrease the risk of injury (^38,48). Malone et al. (⁶³) studied exposure to high-velocity running events in 37 elite Gaelic football athletes and found that both underexposure and overexposure to these events increased the risk of injury. Specifically, those performing 6–10 maximal velocity efforts per week were at reduced risk of injury compared with those completing <5 efforts, and those completing >10 bouts at a significantly higher risk of injury (⁶³). They further explained that those athletes who were exposed to events over 95% of their maximal velocity benefitted from a protective effect of training (⁶³). A secondary study by Malone et al. (⁶¹) reported that large weekly changes of 351–455 m in high-speed running (>14.4 km/h) and 75–105 m of sprint speed (>19.8 km/h) increased the risk of injury. Furthermore, athletes who completed a moderate distance (high speed: 701–750 m; sprint speed: 201–350 m) were at reduced risk compared with those who completed relatively low amounts (high speed: <674 m; sprint speed: <165 m) (⁶¹).

This is somewhat supported by Duhig et al. (²⁸) who reported that athletes completing higher than typical mean (calculated from each athlete's 2 yearly session average) high-speed running (>24 km/h) distances in the 4 weeks before injury were at greater likelihood of suffering a hamstring injury. Furthermore, the study by Ruddy et al. (⁸⁵) involving 220 elite Australian footballers supports the monitoring of running distances completed above 24 km/h in relation to HSI. They reported that absolute weekly distance covered above 24 km/h (>653 m, relative risk [RR] = 3.4), absolute week to week change in distance covered above 24 km/h (>218 m, RR = 3.3), relative week to week change in distance covered > 24 km/h (>2.00, RR = 3.6), and distance covered above 24 km/h expressed as a percentage of that covered above 10 km/h (>2.5%, RR = 6.3) provided the largest significant risk factors of suffering a HSI in the subsequent week (⁸⁵). However, despite the significant RR values, the authors report a substantial overlap in running distances between those subsequently injured and those uninjured (⁸⁵). Therefore, although providing an association between distances covered above 24 km/h and HSI, it was not possible to predict HSI at the individual athlete level, which is further highlighted by none of the absolute running variables reporting both sensitivity and specificity values above 0.6 (⁸⁵). Although not all the aforementioned studies are specific to hamstring injury, it seems that inappropriate high-speed and sprint running loads may lead to an increase in soft-tissue injury. Therefore, as high-speed running and sprinting have been reported mechanisms for hamstring injury, exposure to these types of events warrants particular attention as a risk factor for HSI.

STRENGTH AND INTRALIMB AND INTERLIMB ASYMMETRY

Hamstring strength and asymmetry have been widely proposed as modifiable risk factors (^{12,19,25,75,76,100,102,116}). Asymmetry may present in 2 forms: interlimb (the difference between 2 limbs) (^12,25) and intralimb (the difference between the quadriceps and the hamstrings within the same limb) (¹¹⁶). Intralimb differences are often reported as a ratio (¹¹⁶), whereas interlimb differences are typically displayed as a percentage (^12,25). Oftentimes, the concentric H:Q has been investigated to highlight strength discrepancies between the hamstrings and quadriceps. The literature highlights that a significant reduction in the H:Q ratio was evident in subsequently injured limbs in comparison with uninjured athletes and/or the uninjured limb (^19,76,116). In-fact, Yeung et al. (¹¹⁶) explain that when measuring concentric strength at an angular velocity of 180°/s, a ratio lower than 0.6 led to a 17 times increased risk of injury. However, in a study of 614 elite soccer players across 4 competitive seasons, the H:Q was not supported as a potential risk factor for future HSI, with the authors reporting no relationship between H:Q measurements and subsequent HSI (¹⁰⁹). No significant differences were noted between the injured and uninjured limbs (n = 167) in the concentric H:Q measured at 60 and 300°/s (¹⁰⁹). Furthermore, after multiple logistic regression analysis, odds ratios were also reported to be nonsignificant (n = 563) at both 60 and 300°/s (¹⁰⁹). Thus, given the sample size and time course of the study, the value of the H:Q in relation to HSI prediction could be questioned.

Because of the previously mentioned primary role of the hamstring muscles (to function eccentrically to decelerate knee extension during the late swing phase), it may be argued that a more functional assessment of the H:Q ratio would be to assess the eccentric action of the hamstrings versus the concentric action of the quadriceps (^23,24). This method was retrospectively used by Croisier et al. (²⁴) who discovered significant imbalances in the functional ratio between the injured (0.73 ± 0.24) and uninjured limb (0.90 ± 0.16; p < 0.01) within subjects with previous hamstring injury. However, a prospective study by Bennell et al. (¹¹) found no predictive benefit of isokinetic testing, including the comparison of functional H:Q ratio. Similarly, Van Dyk et al. (¹⁰⁹) found no significant differences between injured and uninjured limbs when studying the functional H:Q ratio. However, it should be noted that eccentric hamstrings torque was measured at 60°/s and concentric quadriceps torque at 300°/s. Furthermore, the hamstrings were not tested eccentrically at faster contraction speeds (like the quadriceps), which may be more indicative of high-speed running (¹⁰⁹). As previously mentioned, Small et al. (⁹⁵) found significant reductions in the functional H:Q ratio during a multidirectional soccer-specific fatigue-inducing protocol, which may suggest that performing these ratios within a fatigued state may be more sensitive to injury prediction. However, it should be noted that Small et al. (⁹⁵) did not report upon any relationships with injury, and therefore, further prospective research within this area is warranted.

It has been reported across several studies that hamstring injury often occurs within the weaker limb, indicating that between-limb strength differences may be a factor for consideration with HSI (^25,76,100). The work of Sugiura et al. (¹⁰⁰) explains that significant interlimb asymmetries existed between injured and noninjured limbs in isokinetic testing of both the eccentric hamstrings and concentric hip extensors (which include but are not limited to the hamstrings). Furthermore, Orchard et al. (⁷⁶) found that a significantly increased risk of injury was present if an individual displayed a hamstring to opposite hamstring ratio of <0.92. Despite this being a useful finding, this value reported as a percentage difference between limbs may be of more practical use and better understood by practitioners in the field. This is further supported by the study of Croisier et al. (²⁵) involving 462 soccer players. They found that those with significant imbalances (>15% bilateral difference in concentric or eccentric hamstring strength) had a 4–5 times increased risk of injury (²⁵). The authors reported that reducing these imbalances to <5% significantly reduced the risk of injury from a RR ratio of 4.66–1.43 (²⁵).

Strength imbalances were further highlighted as a risk factor when tested during the Nordic hamstring exercise (¹²). In a prospective study, it was found that the subsequently injured limb was significantly weaker than the uninjured contralateral limb, and that differences of ≥15% and ≥20% increased the risk by 2.4- and 3.4-fold, respectively (¹²). Further measurements made during the performance of the Nordic hamstring exercise provide additional support for strength as a risk factor. Both Opar et al. (⁷⁵) and Timmins et al. (¹⁰²) report that weaker limbs and athletes were at an increased risk of injury. In a population of 210 elite Australian footballers eccentric strength below 256 Newtons (N) at the start of preseason and 279 N at the end of preseason were said to increase risk by 2.7- and 4.3-fold, respectively (⁷⁵). This is further corroborated by Timmins et al. (¹⁰²) who found that for every 10 N increase in eccentric knee flexor strength, the risk of injury fell by 8.9%. Finally, a reduction in hamstring strength, in comparison with uninjured limbs/subjects, after a hamstring injury has been widely reported among the literature (^50,54,73,74). Although this does not add any prospective predictive value per se, as it is unknown whether the reduction in strength can be attributed to previous injury or if the weakness is the result of previous injury; thus, testing previously injured athletes may provide some value. As strength deficits and previous injury have been identified as risk factors, coupled with the role that the normalization of strength imbalances can play on reducing risk (²⁵), identifying those individuals still at risk after previous injury may help in the planning of targeted training interventions.

INSUFFICIENT/INADEQUATE WARM-UP

Oftentimes, an appropriately planned warm-up that adequately prepares an athlete for training and match-play has been recommended to reduce injuries (^32,70,97,99), although there is a lack of empirical evidence to support this theory for HSI. A systematic review by Fradkin et al. (³²) found insufficient evidence to both promote or discourage pre-exercise warm-up for the reduction of injury occurrence. Of the 5 studies included within the review, 3 found that the inclusion of the warm-up significantly reduced injury, whereas 2 found no significant effect upon injury occurrence (³²). The authors conclude that although there is insufficient evidence to support or discourage the implementation of a warm-up to prevent injuries, the weight of evidence is in favor of implementing a warm-up strategy (³²).

Recently, structured warm-up protocols, such as the Fédération Internationale de Football Association (FIFA) 11+ (also referred to as the FIFA Medical Assessment and Research Centre 11+), have been implemented with the aim of reducing lower limb injury occurrence (^40,94,97). Soligard et al. (⁹⁷) reported that although statistical significance had not been reached, a reduction in overall lower limb injury could be seen due to the implementation of the structured warm-up intervention. When looking at the hamstring specifically, the intervention group (n = 1,055) suffered 5 injuries, with the control group (n = 837) suffering 8 injuries (⁹⁷). However, the incidence per 1,000 playing hours was 0.1 in the intervention group and 0.2 in the control group, which was not significantly different (⁹⁷). It should be noted that hamstring injury rates before intervention were not reported, which would have allowed for better comparisons to be made as to the effectiveness of the intervention.

The reports by Silvers-Granelli et al. (⁹⁴) and Grooms et al. (⁴⁰) both support the value of the FIFA 11+ program within athletic training after discovering significant reductions in hamstring injuries compared with a control group and a reference group, respectively, across one entire season. It was reported that 55 and 16 HSIs were experienced by the control and intervention group, respectively, resulting in the intervention reducing the likelihood of injury 2.74-fold (p < 0.001) (⁹⁴). However, a better understanding of the intervention's success could have been gained if these HSI occurrences had been compared with those experienced during the preintervention period. In the study by Grooms et al. (⁴⁰) the intervention group, who performed the FIFA 11+ program 5–6 times per week, reported only one hamstring injury compared with the 5 reported by the control group. However, it should be noted that the FIFA 11+ program includes the Nordic curl exercise, which has been widely reported to reduce hamstring injuries (^2,90,107). Therefore, it could be suggested that increases in strength, derived through the inclusion of the Nordic curl, are the largest factor in reducing HSIs within the FIFA 11+ program, and not the overall process of performing a warm-up. Although the potential benefit of the warm-up is not fully supported by the research provided here, there is some evidence to suggest that an appropriately planned warm-up may aid the reduction of injuries.

Oftentimes, flexibility/dynamic stretching exercises are included as part of a warm-up routine (⁴⁹). However, the evidence to suggest that altered levels of flexibility are a risk factor for hamstring injury is inconsistent across the literature. Bennell et al. (¹⁰) studied 67 Australian football players and concluded that there were no significant differences in hamstring flexibility between those who subsequently sustained an injury and those who remained uninjured. Similarly, Orchard et al. (⁷⁶) found no correlation between injury and hamstring flexibility as measured through the sit and reach test in a population of Australian footballers. However, it should be noted that the sit and reach test is not specific to hamstring flexibility, and often results can be impaired by an athlete’s hip mobility and their ability to flex the spine (⁷⁶). It is also worth noting that the test is unable to differentiate between limbs, potentially masking any imbalances that may be present (⁷⁶). These findings are further supported by both Hennessy and Watson (⁴³) and Yeung et al. (¹¹⁶) who also found no correlation between hamstring flexibility and injury.

By contrast, Witvrouw et al. (¹¹²), prospectively studied the relationship between hamstring flexibility and hamstring injury among 146 professional soccer players. They reported that, in comparison with their uninjured counterparts, those injured displayed significantly reduced levels of flexibility (<90° during passive straight leg raise; p = 0.02) (¹¹²). The differences in the results presented here may be partly attributed to the different methods used to ascertain hamstring flexibility. However, both Witvrouw et al. (¹¹²) and Yeung et al. (¹¹⁶) measured flexibility through a passive straight leg raise and reported opposing results. With such discrepancies existing within the literature, the role of flexibility in hamstring injury should be viewed with caution, especially when a multitude of factors may contribute to hamstring injury.

It should be noted that in a retrospective study performed by Jonhagen et al. (⁵⁰), previously injured sprinters showed significantly reduced hamstring flexibility during a passive hamstring raise compared with a group of uninjured sprinters (average RoM = 67.2° versus 74.1°; p < 0.05). The reduction in flexibility after hamstring injury is further supported by Maniar et al. (⁶⁴) whose meta-analysis showed reduced hamstring flexibility up to 40 days after injury. Therefore, it may be more important to consider flexibility as a risk factor in those previously suffering from a HSI to reduce the risk of a subsequent injury.

LUMBO-PELVIC HIP CONTROL

Despite only a relatively small amount of current evidence, lumbo-pelvic hip control should be considered as a potential risk factor for HSI. An increased anterior pelvic tilt during sprint running is believed to place the hamstrings into an elongated position, thus increasing the strain placed upon them (^47,96). This may be particularly critical during the terminal swing phase, when the biceps femoris long head is already placed under increased stretch, which may be further exacerbated by the presence of an anteriorly tilted pelvis (^21,47,96). This may result in an increased chance of suffering a HSI; however, further research is required in this regard to fully corroborate such a theory.

Furthermore, restricted sagittal plane motion at the hip, as measured using the modified Thomas test, has been shown to reduce gluteal activation (⁶⁷). This may be important to HSI risk, as the work by Schuermans et al. (⁸⁹) highlights proximal neuromuscular control as another risk factor for HSI. They studied muscle activation (through surface EMG) during sprint running in a population of 60 amateur soccer players (⁸⁹). During the 1.5 season follow-up period, they reported that those athletes not suffering a HSI had significantly (p = 0.027) greater gluteal muscle activity during the front swing phase and greater trunk muscle activity (p = 0.042) during the back swing (⁸⁹). Therefore, it may be hypothesized that restricted motion at the hip has the potential to inhibit gluteal activation, and subsequently proximal neuromuscular control, which could lead to an increased risk of suffering a HSI (^67,89).

NONMODIFIABLE RISK FACTORS

PREVIOUS INJURY

A previous HSI has often been identified as a risk factor for future HSI (^{12,26,31,36,77,79,102,114}). Reinjury rates have typically been reported at 12–13% (^26,31,114), with Petersen et al. (⁷⁹) reporting greater values of 25%. However, it is important to note the significantly different methodological approaches in the work of Petersen et al. (⁷⁹), who define a HSI as any self-reported posterior thigh pain, irrespective of time loss, which may account for the reported increased reinjury rate. Gabbe et al. (³⁶) found that among Australian footballers, a HSI sustained within the previous 12 months, to be the strongest independent predictor of future injury (odds ratio = 4.3; p = 0.003). It has also been reported among international cricketers that after a HSI, an athlete is at 3.7 times higher risk of suffering a further injury within the same season and at 2.7 times higher risk in subsequent seasons (⁷⁷). This risk factor is slightly lower than those reported for both rugby union athletes (4.1 times higher) (¹²) and Australian footballers (4.9 times higher) (¹¹⁰). Previous knee (p = 0.039) and groin (p = 0.015) injuries were also reported as significant risk factors for future HSI (¹¹⁰).

Although previous injury is seen as a nonmodifiable risk factor, it has been highlighted that those with previous injury had reduced eccentric hamstring strength (¹⁰²) and interlimb asymmetries (¹²) when performing the Nordic hamstring exercise. Furthermore, short biceps femoris fascicle length was also reported as a contributor to multiple hamstring injuries (¹⁰²). Therefore, it could be speculated that improving these physical attributes may aid in the prevention of repeat HSI (^12,102).

AGE

A study by Gabbe et al. (³⁵) identified that athletes ≥25 years of age had a higher hamstring injury incidence (19.2%) than those ≤20 years of age (6.9%). A separate study by Gabbe et al. (³⁷) found that athletes ≥23 years old were at a greater risk of hamstring injury. It has also been reported that for every 1 year increase in age, the risk of hamstring injury increases by 1.3-fold when assessed independently of previous injury (¹¹⁰). It was also reported among a large cohort of track and field athletes that masters athletes (>40 years of age) were significantly more likely to suffer a HSI than high school and collegiate athletes (⁷²). It has been hypothesized that the role of age in increased injury risk may be attributed to increased body weight, decreased hip flexor flexibility (³⁵), reduced eccentric hamstring strength, and short biceps femoris fascicle length (¹⁰²). Therefore, maintaining optimal body composition, flexibility of the hamstring and hip musculature, and eccentric hamstring strength may be beneficial to hamstring injury prevention among older athletes.

PRACTICAL APPLICATIONS: INJURY PREVENTION PROGRAM

As the literature highlights several contributing factors to HSI and reinjury rates, injury prevention programs should be multifaceted in nature and address all the potential modifiable risk factors. The program outlined (Figure 1) is aimed at team sport athletes, who are at greatest risk of suffering a type I strain (sprint-related). The program is divided into 4 stages, with stages 1–3 representing a preseason period and stage 4, an in-season phase, which may be implemented for maintaining performance levels. The program outlined in Figure 2 is aimed at athletes who are at greater risk of suffering a type II strain (such as dancers and combat athletes) and is divided into 3 progressive stages, which can be implemented in the lead up to a competition. It is intended that both injury prevention programs should not stand alone and instead should be integrated into the wider athlete performance plan.

Figure 1.: Potential evidence-based HSI prevention program focusing on type I strains. HSI = hamstring strain injury.

Figure 2.: Potential evidence-based HSI prevention program focusing on type II strains. HSI = hamstring strain injury.

WARM-UP

Although there is not overwhelming evidence to suggest that a structured warm-up is beneficial to the reduction of HSI, warm-ups are common place in sports performance to prepare the athlete both mentally and physically for activity (⁴⁹). During this preparation phase, team sport athletes should be gradually exposed to maximal velocity efforts (i.e., 40-m sprints at 65, 75, 85, 95, and 100% of perceived maximal velocity), as well as gradually increasing kicking (for appropriate sports) distances (i.e., 10-, 20-, 30-, 40-m kicks), particularly as these events have been highlighted as injury mechanisms within these populations (^{6,15,16,26,30,36,37,114}). Similarly, in sports where type II HSI is more likely, performing sport-specific movements (i.e., high kicks, sagittal, and side splits) at gradually increasing intensity and range can be incorporated into the preparation phase of the warm-up.

The warm-up also affords practitioners with a period in which to deliver training protocols, which cannot only aid athletic movement competencies (⁴⁹), but also injury prevention (^40,94,97). The FIFA 11+ recommends the integration of the Nordic curl as part of a structured warm-up to prevent HSI (^40,94,97). Furthermore, other exercises, including those that may play a role in increases in flexibility, may also be included within a structured warm-up to improve the overall time efficiency of the athlete performance program.

Although there is limited evidence to suggest that reduced levels of flexibility play a significant role in increasing the risk of HSI (^{10,43,76,112,116}), it has been demonstrated that flexibility training can have a positive effect on biceps femoris fascicle lengths (³³). The study by Freitas et al. (³³) described the effects of an 8-week high-volume stretching intervention, which involved stretching the hamstring at maximum range of motion for 450 seconds 5 times per week, on biceps femoris muscle architecture, as measured through ultrasound sonography. They reported significant increases (+12.3 mm, p = 0.04) in biceps femoris fascicle lengths as well as significant improvements in passive knee extension range of motion (+14.2°, p = 0.04) (³³). As short biceps femoris fascicle lengths have been highlighted as a potential risk factor for HSI (¹⁰²), it would appear prudent to include elements of flexibility training within a HSI prevention program. To increase time efficiency, such exercises can be incorporated into a structured warm-up routine.

ECCENTRIC STRENGTH

Strength, and more specifically eccentric strength, has been previously highlighted as a contributing risk factor for HSI, demonstrating the need for the inclusion of eccentric strength exercises within HSI prevention programs. When selecting strength-based exercises, it is important to note which type of HSI the athlete is likely to suffer and, therefore, which muscle group is likely to be the site of damage (type I: biceps femoris and type II: semimembranosus) (^5,6,14). This enables practitioners to program exercises with a focus toward a particular hamstring muscle (¹⁴). The work by Bourne et al. (¹⁴) provides a framework for selecting the most appropriate strength training exercises within HSI prevention programs.

Team sport athletes (Figure 1) are most likely to suffer a type I HSI (sprint related strain) but may also experience type II strains in actions such as kicking (^5,6). Within these populations, the inclusion of the Nordic curl exercise in injury prevention programs has been well reported within the literature (^2,90,107). Arnason et al. (²) implemented a flexibility and hamstring strength training intervention among elite soccer players from Iceland and Norway. They found no effect upon injury reduction amongs those players performing flexibility training alone (p = 0.22) (²). However, when Nordic hamstring curls were included as part of the program, hamstring injury was reduced by 65% compared with the control group (²). These findings are further corroborated by Van der Horst et al. (¹⁰⁷), who found that the inclusion of Nordic hamstring curls within a 13-week training program significantly reduced the incidence of hamstring injuries compared with a control group (intervention group = 0.25 per 1,000 player hours; control group = 0.8 per 1,000 player hours; p = 0.005) within a large population of amateur soccer players. Furthermore, in the year before the intervention, 24 and 20 HSIs were reported in the intervention and control group, respectively (¹⁰⁷). This was reduced to 11 in the intervention group but increased to 25 in the control group during the 52-week surveillance period (which included 13 weeks of the intervention) (¹⁰⁷). A successful Nordic hamstring intervention was also seen within a group of baseball athletes implemented across the entire 2012 season (⁹⁰). It was demonstrated that zero hamstring injuries were reported among the intervention group, compared with the 10 suffered by the control and noncompliant group (performing <3.5 Nordic curls per week) (⁹⁰). Furthermore, upon the implementation of the Nordic curl intervention, the time loss because of HSI was reduced to 136 days, compared with 273 and 309 days in previous seasons (⁹⁰).

The success of the Nordic curl exercises within these studies may be attributed to its positive affect upon biceps femoris long head muscle volume, size, and strength. Seymore et al. (⁹²) studied the effect of the Nordic curl exercise combined with stretching compared with a control group who only performed stretching exercises. The intervention consisted of a 6-week Nordic curl program where frequency (^1–3) and volume (2 × 5 reps, increasing to 3 × 8–12 reps) were progressively increased (⁹²). The group that performed Nordic hamstring exercises in addition to stretching saw significant increases (p < 0.05) in biceps femoris long head physiological cross-sectional area (16.08 ± 6.43 cm² versus 18.05 ± 7.33 cm²) and muscle volume (131.46 ± 43.32 cm³ versus 145.2 ± 46.42 cm³) compared with baseline (⁹²). Furthermore, Bourne et al. (¹³) found that Nordic curl training promoted longer biceps femoris long head fascicle lengths and greater biceps femoris long head, short head, and semitendinosus muscle volume when training sessions were performed twice a week for 10 weeks. However, it should be noted that within the same study, the hip extension exercise promoted greater changes in biceps femoris long head and semimembranosus (where the Nordic curl promoted no significant differences to the control group) muscle volume (¹³).

Figure 2 highlights a HSI prevention program aimed at reducing the incidence and severity of type II strains. Within this population of athletes, the site of injury is most commonly the semimembranosus, and therefore, exercises should be selected accordingly (^6,14). This should include the “Romanian” or “stiff leg” deadlift, which has been reported to show significantly (p < 0.01) higher levels of semimembranosus activation than both the biceps femoris and semitendinosus (⁷¹). The research by Ono et al. (⁷¹) further explained that following the performance of stiff leg deadlifts, a significant increase in both MRI transverse relaxation time (T2) value and cross-sectional area of the semimembranosus were observed.

STRENGTH IMBALANCES

Additional to the development of eccentric hamstring strength, there is also a need to address both intralimb (differences between the quadriceps and the hamstrings in the same leg) and interlimb (differences between hamstrings bilaterally) strength imbalances within HSI prevention programs. Previous research by Ruas et al. (⁸⁴) has demonstrated that eccentric strength training significantly (p ≤ 0.05) increased the functional H:Q ratio after a 6-week intervention (H:Q pre = 0.73 ± 0.092; H:Q post = 0.87 ± 0.098). Furthermore, Holcomb et al. (⁴⁶) reported that a 6-week hamstring emphasized strength program was able to significantly (p < 0.05) increase the functional H:Q ratio from 0.96 ± 0.09 to 1.08 ± 0.11. The inclusion of the Nordic hamstring curl in strength programs aimed at optimizing the functional H:Q ratio is somewhat supported by Delextrat et al. (²⁷). They reported that significant (p < 0.05) increases of 27.8% in the functional H:Q ratio were seen after a 6-week training program (²⁷). However, it should also be noted that in comparison, the eccentric leg curl promoted greater improvements (38.3%) than the Nordic hamstring curl, and that for both exercises, these results were only evident within the nondominant limb (²⁷). This may suggest that additional unilateral strength training exercises should be included within HSI prevention programs aiming to address intralimb strength imbalances.

Alongside the primary “lifts” (e.g., Nordic curl and stiff leg deadlift), supplementary unilateral exercises have been included within both programs, outlined in Figures 1 and 2, including single-leg stiff leg deadlift, single-leg slider curl, and both the Askling diver and glider exercises. The aim of these exercises is to both correct muscular imbalances (intralimb and interlimb) and to promote joint stability. Previous research has suggested that interlimb strength imbalances should be reduced to <5% to significantly reduce the risk of HSI (²⁵). To achieve this, the single leg stiff leg deadlift is included within Figure 1 and has been previously recommended within hamstring training programs (^60,66), despite Tsaklis et al. (¹⁰³) stating that hamstring EMG was relatively low for this exercise. However, it should be noted that this exercise was performed without external load (i.e., body weight only) during this study, which may have influenced the results (¹⁰³). The single-leg slider curl has also been investigated by Tsaklis et al. (¹⁰³) who measured EMG outputs of 10 hamstring-based exercises and found the slider curl to have the highest mean EMG activation of the biceps femoris and semitendinosus muscles. However, their results should be viewed with caution as the 20 participants performed all exercises in the same order, albeit with a 5-minute rest period between each, with no randomization (¹⁰³). Furthermore, their study did not differentiate between contraction types (concentric and eccentric) and only provided results for combined contraction outputs, both of these methodological factors may have affected the results of the study (¹⁰³).

The Askling diver and glider form part of the Askling L-protocol (lengthening exercises), which has been shown to be successful within hamstring rehabilitation programs (⁷). During the study, the L-protocol reported significantly shorter (mean 28 days, range 8–58 days) return to play time, compared with a conventional hamstring training program (mean 51 days, range 12–94 days) (⁷). EMG studies of these 2 exercises have shown the hamstrings to be eccentrically contracted at similar working points to that of the swing phase during high-speed running (⁹¹), further supporting their use within prevention and rehabilitation programs.

The strength training component should be included as part of the wider strength training program (i.e., athletes should also be performing other exercises to develop all round athletic performance). It may be prudent for practitioners to also consider the rear foot elevated split squat (RFESS) within the overall athletic development plan. The work by McCurdy et al. (⁶⁵), who compared EMG measurements of the RFESS and the traditional back squat exercise, at 85% of a subject's 3 repetition maximum for each exercise, supports its inclusion within athletic training programs that have an emphasis on HSI prevention. Their research showed that the RFESS recorded significantly (p < 0.01) greater mean and mean peak hamstring activation, whereas the traditional back squat showed significantly greater recruitment of the mean quadriceps (p < 0.05), mean peak quadriceps, and mean Q:H (p < 0.01) (⁶⁵). As the RFESS seems to provide a greater demand on the hamstrings, compared with the back squat (which places a greater emphasis on the quadriceps), it may be seen as a viable alternative to the traditional back-squat exercise in athletic programs when an emphasis on hamstring conditioning is required (⁶⁵).

FATIGUE AND FITNESS

Fatigue has been previously linked to HSI occurrence due to injuries being reported to occur toward the end of games, possibly because of the effect of fatigue on the reduction of eccentric knee flexor strength. Furthermore, it has been previously demonstrated that those with reduced aerobic fitness (as measured through a 1-km time trial) were at an increased chance of injury (odds ratio = 1.5–2.5) compared to those with superior aerobic fitness (⁶²). With this evidence in mind, appropriately planned conditioning should be included within the injury prevention plan to improve overall fitness levels and reduce the burden of fatigue upon the hamstrings. For team-sport athletes, this can include MAS training (⁸). This can be prescribed at increased percentages of an individual's MAS across stages 1–3 (outlined in Figure 1) (⁸), after which sport-specific conditioning (i.e., small-sided games in soccer) can be implemented during the in-season period. In nonrunning-based sports, fitness can be developed through sport-specific conditioning. For example, it has been recommended that dancers can build cardiorespiratory fitness by using dance movements with appropriate work:rest periods (¹¹⁵).

HIGH-SPEED RUNNING

The monitoring of running loads and, more importantly in the case of HSI prevention, high-speed running loads is common within sports performance (^{28,38,48,61,63,85}). All running-based training, and particularly that covered above 24 km/h, should be carefully monitored to prevent spikes in training load and to ensure that the athlete has been exposed to appropriate training doses that may provide a preventative effect upon HSI occurrence (^{28,38,48,61,63,85}). The inclusion of conditioning-based drills and supplemental maximal velocity training should be informed by the data collected from this monitoring process.

PLYOMETRICS

Plyometrics are often included within athletic training programs; however, their potential role in HSI prevention is often overlooked. Previously, plyometric-based exercises, including unilateral and bilateral sagittal plane hurdle hops, frontal plane hurdle hops, 180° hops, and split-squat jumps, have been shown to recruit the hamstring musculature (⁹⁸). Furthermore, because of the nature of plyometric exercises, they are likely to produce hamstring muscle actions at high velocities throughout the stretch-shortening cycle (¹⁰⁵). Therefore, they have the potential to stimulate muscle actions that are similar to those reported during the mechanism of injury associated with high-speed running (¹⁰⁵).

Tsang and DiPasquale (¹⁰⁴), implemented a 6-week plyometric training program where subjects performed the intervention 3 times per week. Their findings highlighted increases in hamstring strength alongside maintaining quadriceps strength, thus improving the Q:H ratio (¹⁰⁴). In addition, Vissing et al. (¹¹¹) demonstrated significant (p < 0.001) increases in hamstring cross-sectional area (6.7 ± 1.8%) after a 12-week plyometric training intervention. However, their results should be viewed with an element of caution, as the subjects were classified as “untrained,” and therefore, it could be hypothesized that any training stimulus would have promoted a positive effect.

The plyometric exercises included in stage 1, as well as the drop land in stage 2, of both prevention programs are aimed at developing optimal landing mechanics, which should be established before progressing to exercises of greater intensity and complexity (¹⁰⁶). The additional exercises within Figure 1 are programmed with a bias toward horizontal force production, to replicate similar movement vectors to that during high-speed running. The additional exercises within Figure 2 are focused on developing overall plyometric ability but may be adapted to suit each individual sport.

CONCLUSION

Within sports performance, HSIs are highly prevalent and incur high reinjury rates. Consequently, this leads to athletes missing extended periods of the competitive season, which can have a detrimental effect on both the performance and finances of sporting organizations. Although HSIs commonly occur during high-speed running activities, practitioners should be aware that a variety of injury mechanisms exist. Furthermore, a multitude of possible contributing risk factors for HSI have been well documented within the literature, highlighting the need for injury prevention programs to be multifaceted in nature.

These programs should include an appropriate warm-up, where other elements of the injury prevention plan (i.e., flexibility) can be included. Eccentric strength training, both bilateral (Nordic curl and stiff leg deadlift) and unilateral (single leg stiff leg deadlift, single leg slider curl, Askling glider, and diver), should be included to improve hamstring strength and reduce muscular imbalances. Alongside this, the RFESS should be considered within HSI prevention programs due to its reported benefits to hamstring recruitment. Conditioning drills, either in the form of MAS or sport-specific conditioning, should be incorporated to improve overall fitness levels and reduce the burden of fatigue. For running-based athletes, careful monitoring of high-speed running loads should be initiated and used to inform training load to ensure that athletes are exposed to an appropriate training dose. Finally, plyometrics should be included that may have the potential to activate the hamstrings at high velocities. These should begin by focusing on correct landing mechanics, before progressing to higher velocity exercises. In the case of running-based athletes, it may be prudent to focus on exercises that require athletes to produce horizontal force.