Prevention of Hamstring Strain Injuries in Sports: a literature review.
Hamstring strain injuries are one of the major problems with a high incidence of re-injury in sports involving sprinting and jumping, such as in soccer, rugby and other sports with high explosive power demands. Recent high-quality studies have postulated various evidence-based causes and risk factors for hamstring injuries. Based on this information, various researches have proposed different training protocols targeting to reduce the risk of hamstring injuries with varying success. However, due to underpowered trials or lack of high quality and large scale randomized controlled trials, it is difficult to draw a concrete conclusion on preventive measures for hamstring strain injuries. This article presents an overview of the current literature available on the best physiotherapy approach to prevent hamstring strain injuries.
Keywords: hamstrings injury, muscle strain, hamstring strain, prevention of muscle injury, prevention of hamstring injury, strength training, muscle fatigue, rehabilitation, and physical therapy.
Introduction:
Hamstring strain injuries (HSIs) are the most common injuries in sports that require maximum sprinting, kicking, acceleration and deceleration or high-speed skilled movements (Askling, Karlsson & Thorstensson, 2003; Brughelli & Crohin, 2008; Sherry, Best, Slider, Thelen & Heiders, 2011). Acute pain in the posterior thigh with disruption of hamstring muscle fibres are the general characteristic features of HSIs (Verrall, Slavotinck, Barnes, Fon & Spriggins, 2001). Hamstring muscle injuries can be a direct injury with laceration and contusion and indirect injury with muscle strains, depending on trauma mechanism (Petersen & Holmich, 2005). In terms of severity of injury, HSIs may range from minor microscopic tearing of few muscle fibres with minimal discomfort and loss of function (Grade 1) to complete rupture of muscle (Grade 3) resulting in a total functional loss of muscle (Blankenbaker & Tuite, 2001; Kujala, Orava & Jarvinen, 1997). Hamstring muscle strains usually occur in the Biceps Femoris muscle with muscle-tendon junction and adjacent muscle fibres being the common site of injury (Askling, Tengvar, Saartok & Thorstensson, 2007; Koulouris & Connell, 2003).
Many studies have reported HSIs as 16-29% of all injuries in various sports like soccer (Arnason, Sigurdsson, Gudmundsson, Home, Engebresten & Bahr, 2004; Croisier, 2004), Australian Rules football (Orchard, & Seward, 2002) and rugby union (Brooks, Fuller, Kemp & Reddin, 2006). An audit of injuries in professional football done by Woods, Hawkins, Malty, Hulse, and Hudson (2004) has reported that hamstring injury accounted for 12% of all injuries in the English Premier League (EPL) over a period of two seasons. Hamstring injuries have been frustrating and devastating to the athletes, coaches, and managers because of its persistent symptoms, poor and slow healing response, and a high percentage (12-31%) of re-injuries (Croisier, 2004; Woods et al., 2004). Elite football players miss the competition for a mean of 14 days sustaining HSIs (Ekstrand, Hagglund & Walden, 2011). In EPL, HSIs accounted for an average of 90 days and 15 matches missed per club per season. Therefore, prevention of initial injury must be emphasized (Woods et al., 2004).
Hamstring strain injuries are well-documented issues in literature for a long time. However, epidemiological data obtained from various sports like soccer, rugby union across a number of years shows no improvement in the incidence rates of HSIs in recent decades (Petersen & Holmich, 2005; Schache, Dorn, Blanch, Brown & Pandy, 2012; Sherry et al., 2011; Witvrouw, Danneels, Asselman, D’Have & Cambier, 2003). This lack of decline in HSI rates may suggest the requirement of a more scientific and evidence-based approach to determine the most effective preventive measures of HSIs.
Therefore, the present study aimed to investigate the current evidence on physical therapy approaches used in the prevention of hamstring strain injuries in sports.
Methods:
A literature review in the databases PubMed, LILACS, SciELO, and the Cochrane Database of Systematic Reviews (Cochrane Library) was made. The following keywords were used: hamstrings injury, muscle strain, hamstring strain, prevention of muscle injury, prevention of hamstring injury, strength training, muscle fatigue, rehabilitation, and physical therapy.
The inclusion criteria for this study were studies with high-quality evidence, such as systematic reviews, meta-analyses, randomized controlled trials, and classical studies relevant to the aim of the study. The exclusion criteria were articles that did not match the proposed theme.
Mechanism of injury:
In order to prevent hamstring injuries, it is important to consider causes and risk factors and understand how they happen. The mechanical strain of a muscle is caused due to an excessive lengthening of an actively working muscle which ultimately leads to injury (Thelen, Chumanov & Hoerth, 2005). During the terminal swing phase of running, there is hip flexion and extension of knee occurring simultaneously (Thelen et al., 2005). During this phase, biarticular hamstring will be lengthened producing peak force and performing much negative work (energy absorption) at the same point. Consequently, putting hamstring at the greatest injury risk (Schache et al., 2012). Hamstring has to actively lengthen when kicking a ball and due to forward trunk lean, hamstring will be in a more vulnerable position of injury when kicking while running or running incline (Thelen et al., 2005). Schache et al., (2012) have shown that biceps femoris is the most vulnerable muscle to injuries as it experiences greater musculotendon strain (12%) during sprinting, compared with semimembranosus (9.8%) and semitendinosus (8.7%).
Risk factors:
Several alterable and unalterable risk factors for HSIs have been proposed in various literature. Unalterable risk factors include but not limited to increasing age (Gabbe, Bennell, Finch, Wajswelner, & Orchard, 2006a; Woods et al., 2004), previous injury (Arnason et al., 2004; Verral et al., 2001), and ethnicity (Verral et al., 2001; Woods et al., 2004). Alterable risk factors include but not limited to strength imbalances (Croisier, Ganteaumne, Binet, Genty, & Ferret, 2008), muscle flexibility (Witvrouw et al., 2003) and muscle fatigue (Mair, Richard, William, & Garrett, 1996).
Flexibility Training:
Flexibility training has traditionally been practiced in sports training as prevention of muscle injury despite a lack of convincing scientific evidence and explanation. To date, there are no scientifically proven repetitive values that can explain the mechanism of use of optimum flexibility in the prevention of HSIs (Slavko, Dirk, Thomas, & Dietmar, 2013).
A recent systemic review (Slavko et al., 2013) done on the effect of static hamstring stretching in the prevention of HSIs in football has concluded the lack of sufficient evidence because of lack of qualitative and quantitative randomized control studies (RCTs) done. Furthermore, the stretching protocols in terms of intervention and follow-up and its effects are significantly diverse. Weldon and Hill, (2003) reviewed four RCTs and postulated that stretching is not effective to prevent muscle strain injuries. However, the studies included in this review did not focus on one muscle group or any particular sport. Arnason, Andersen, Holme, Engebresten, & Bahr, (2008) performed an intervention study on elite soccer players and showed no effect of prescribed partner contract-relax flexibility training during the warm-up. This study also tested the effect of eccentric strength training with warm-up stretching in the prevention of HSIs. They demonstrated significant reduction by 65% in the incidence of HSIs among the intervention teams with an eccentric strength program, compared with the teams that did not use strength program. The incidence of HSIs was also low during the intervention season (2001-2002), compared with the baseline study season (1999-2000) with the same intervention teams.
Hamstring muscles are not completely stretched to its maximal length during sprinting; instead, it performs the most eccentric muscle work (Schache et al., 2013). This mechanism of running may explain the failure of flexibility training alone or combined with warm-up stretching to reduce the incidence of HSIs. Hamstring strain injuries are a complex, interactive and multifactorial problem involving flexibility, strength, warm-up, and fatigue (Hoskins and Pollard, 2005). Dadebo, White, & George (2004) performed a questionnaire-based survey on flexibility training protocols and hamstring strains among 30 English professional football clubs in four divisions during the 1998/1999 season. They postulated the need for further study to determine the most effective standard stretching protocol, stretching technique and stretching holding time. These factors probably act in a complex synergism with other training factors like strength and endurance which reduce hamstring strains. Standard stretching protocol in this survey included static stretching or PNF stretching technique, preceded by a warm-up session and holding time of 10-15 seconds. A prospective cohort study by Witvrouw et al., (2003) examined the flexibility of 146 male professional soccer players and demonstrated the strong association between preseason hamstring tightness and subsequent risk of HSIs. Therefore, the flexibility of muscles may be important to prevent some of the injuries in elite soccer (Witvrouw et al., 2003). However, the standard parameter of flexibility required for preventing subsequent muscle injuries is still a contentious issue in the literature.
So, recent studies suggest that stretching protocols need to be reviewed and may be adjunct in the preventive measures of HSIs when practiced along with other training factors. Further prospective and RCTs are needed to investigate this issue.
Fatigue:
Muscle fatigue has been advocated as a causative factor for muscle strain injuries by many studies (Mair et al., 1996; Worrell et al., 1992). It has been supported by the reports of injury incidence studies showing that HSIs mostly occur at the latter stages of competitions or training (Brooks et al., 2006; Ekstrand, Hagglund, & Walden, 2011; Woods et al., 2004).
Mair et al., (1996) examined the role of fatigue in susceptibility to acute muscle strain injuries in a laboratory setting. Their experiment showed that the pre-fatigued muscles via electrical stimulation absorbed less energy before failure compared to un-fatigued muscles. But still, both fatigued and un-fatigued muscles failed at the same length, suggesting the weakness of fatigued muscle to resist over-lengthening and hence sustaining strain injury. A recent study was done by Small, McNaughton, Greig, & Lovell, (2010) investigated the effects of multidirectional soccer-specific fatigue on HSI risk on 16 semi-professional male soccer players. They concluded that the interventions which can reduce muscle fatigue and its effects during competitions may help in the reduction of HSIs rates. This study has convincingly supported this hypothesis by their substantial findings of a time-dependent decrease in eccentric hamstring strength, functional H: Q ratio and shifting of eccentric hamstring muscle action towards shorter muscle length.
Pinniger, Steele, & Groeller, (2000) have demonstrated an increase in knee extension during the terminal swing phase of running due to fatigued hamstring muscle induced by repeated dynamic efforts. This increase in knee extension during the terminal swing phase of running puts hamstring muscle to a greater strain (Schache et al., 2013). Further, a recent study by Allen, Leung, & Proske, (2010) have shown the proprioceptive imbalance as the result of fatigued hamstring muscle which may lead to increase risk of HSIs. Similarly, the findings from Dadebo et al., (2004); Verral, Slavotinek & Barnes, (2005) have shown that tired muscles are prone to injury and stretching exercises may help prevent injuries in tired muscles. They argue that due to viscoelastic properties of muscle, stretching can make fatigued muscles more resistant to injury.
Strength Training:
Strength imbalance correction
Conventional Hamstring: Quadriceps (H:Qconv) ratio is the comparisons of concentric strength imbalance of hamstring and quadriceps working across knee joint (Orchard, Marsden, Lord, & Garlick, 1997). A lower H:Qconv ratios suggest the failure of hamstring muscles to counteract strong angular momentum at knee joint produced by forceful contraction of quadriceps during the early phase of running (Opar, Williams, & Shield, 2012). Yeung, Suen, & Yeung, (2009) has demonstrated that a conventional concentric H:Q ratio of less than 0.60 at 1800/sec may increase the risk of HSIs by 17 times. However, it was not the same when ratios were tested at 600/sec and 2400/sec and also with different concentric/eccentric contractions. Knapik, Bauman, Jones, McA, & Vaughan, (1991) found H:Qconv ratio of less than 0.75 at 1800/sec as the significant risk factor for lower limb injuries among female collegiate athletes. Comparatively, another prospective study (Orchard et al., 1997) on professional football players found a significant increase in HSIs when H:Q ratio was less than 0.61 at 600/sec but no difference at 1800/sec and 3000/sec among Australian footballers.
Recent studies (Croisier et al., 2008; Yeung et al., 2009) have suggested the use of functional Hamstring:Quadriceps (H:Qfun) ratio rather than H:Qconv as this H:Qfun ratio describes the eccentric contraction of hamstring to break the effect of concentrically contracting quadriceps during the terminal swing phase of running. Croisier et al., (2008) examined 687 players and found that the players with lower H:Qfun ratio was 4.7 times more at risk of HSIs. In contrast, the cut-off values for H:Qfun ratio was 0.98. However, this study further supports its findings by showing the significant decrease in HSIs rates following the correction of strength imbalance verified by isokinetic testing. Similarly, other prospective studies (Konstantinos, Elias, Poulmedis, Spyros, & George, 2011; Sugiura, Saito, Sakuraba, Sakuma, & Suzuki, 2008) have also demonstrated H:Qfun ratio asymmetry between legs as a significant risk factor for HSIs.
These findings of different H:Q ratios at different speed among athletes in different sports suggest that the standard H:Q ratio cut-off point should be determined by further large scale RCTs depending on the particular sports. Also, lack of sport-specific characteristics or lack of compatibility with a speed of running with the use of isokinetic testing (Croisier & Crielaard, 2000) should be considered before using it as the predictive element of HSIs.
Eccentric exercise training and shift of optimum length
In recent decades, many studies (Brockett, Morgan, & Proske, 2001; Brockett, Morgan, & Proske, 2004; Brooks et al., 2006; Proske, Morgan, Brockett, & Percival, 2004) have demonstrated shorter optimum muscle length for tension development as one of the major risk factors of HSIs as most of the tension-producing length would be on the descending portion of the length-tension curve. Eccentric exercise has been advocated as an effective way of shifting optimum length of tension development to longer lengths (Brockett et al., 2001; Prasartwuth, Allen, Butler, Gandevia, & Taylor, 2006; Whitehead, Weerakkody, Gregory, Moran, & Proske, 2001). This shift in optimum length depends on the volume and intensity of eccentric exercise and the length of muscle during eccentric exercise (Brughelli & Crohin, 2008). Eccentric exercise-induced shifts in optimal length are varied. Whitehead et al., (2001) have reported 3.90 shift whereas Prasartwuth, (2006) have reported up to 180 shift in optimum length of tension development following eccentric exercise.
Brockett et al., (2001) demonstrated 7.70 and 8.50 shift of optimum length immediately after and 4 days after the eccentric hamstring training protocol (naming it ‘hamstring lowers’) respectively. In this study the researchers have used 12 sets 6 repetitions per session of exercise protocol and consequently, causing acute muscle damage. Bowers, Morgan, & Proske, (2004) have reported 15.40 shift in optimum length of tension development in the knee extensors following eccentric exercise. They performed eccentric work of quadriceps at elongated muscle length which consisted of 240 repetitions of stepping down from a box. Both of these studies eventually showed acute muscle damage to improve shift in optimum length. However, Clark, Bryant, Culgan, & Harley, (2005) included only 2 or 3 sets of 6-8 repetitions of exercise protocol proposed by Brockett et al., (2001), twice per day, 2-3 times per week and continued for 4 weeks. They have reported 6.50 shift in optimum length without inducing acute muscle damage. Although this study was a pilot study without a control group, the findings were significant due to moderate shift in optimum length without muscle damage and also increase in isokinetic strength (i.e. concentric and eccentric). Thus, the findings of these studies may suggest that the effective eccentric exercise protocol may improve the optimum length of tension development and consequently, reduce the risk of HSIs.
Nordic Hamstring (NH) Exercise
The eccentric hamstring exercise (hamstring lowers) developed by Brockett et al., (2001) and later developed by Mjolsnes, Arnason, Osthagen, Raastad & Bahr (2004) as Nordic Hamstring has been shown to reduce the risk of HSIs significantly among elite players of various sports demands. Mjolsnes et al., (2004) have reported an increase in eccentric hamstring torque and isometric hamstring strength when they used NH exercise for 10 week period with a gradual increase in volume, compared with traditional hamstring curls exercise. Arnason et al., (2008) compared the effect of NH exercise in addition to warm-up protocol with the effect of proprioceptive neuromuscular facilitation associated contract-relax stretching with the same warm-up protocol in elite soccer players. Their results demonstrated a significantly lower incidence of HSIs during the competitive season in the sub-group who have used NH exercise than the group who have used flexibility program. Furthermore, the participants in the NH exercise group suffered fewer HSIs than they had during the baseline study session during the previous 3 years of the intervention study.
Technique:
The Nordic hamstring exercise is performed with the assistance of a partner (Fig. 1). Athletes start in a kneeling position on the ground with knee flexion at 900, hips slightly flexed and erect body posture. The athlete then falls forward as slowly possible from the knees with eccentric contraction of hamstring while the partner fixes the athlete’s ankles securely (Brockett et al., 2001). Hips should be kept in a slightly flexed position throughout the range of motion. Hands should be used to prevent fall on the chest while just letting the chest touch the ground. The athlete then returns to starting position immediately by thrusting back from the hands to minimize concentric work of hamstring muscles (Mjolsnes et al., 2004). Mjolsnes et al., (2004) have reviewed and developed 10 weeks program with a gradual increase in volume (Table 1). When each repetition can withstand for a significantly longer period for 12 repetitions, NH exercise can be progressed by increasing load. This is done by increasing the speed of falling forward at the starting phase and then control the motion which can be further progressed by pushing at the back of the shoulder by the partner. Nordic hamstring exercise should be done after proper warm-up and should be completed in a non-fatigued state to reduce the negative effects of fatigue (Mjolsnes et al., 2004).
Askling et al., (2003) were the first to study the effects of eccentric hamstring exercise in HSIs rates. They performed Yo-Yo hamstring curl exercise (eccentric hamstring curl in prone) by using flywheel among elite soccer players. After 10 weeks of training in a non-fatigue state, the results showed a significant decrease in HSIs rates by 67% in the experimental group than the control group. Gabbe, Branson, & Bennell, (2006b) investigated the effect of NH exercise in the prevention of HSIs in Australian rules football players. Two hundred and twenty players were divided into an experiment group (n=114) who performed NH exercise protocol with 12 sets of 6 reps, 3 times before the season and 2 during season and control group (n=106) who performed regular sports training and stretching protocol. The compliance rate was very low in this study with only 47% of participants completing at least 2 sessions of training and 30% of players failed to complete even one of 5 prescribed sessions. However, only 4.0% of experiment group compared to 13.2% of the control group suffered a hamstring injury when the data of players from both groups who had completed at least two sessions were only analyzed. Both, Askling et al., (2003) and Gabbe et al., (2006b) have reported the delayed onset of muscle soreness (DOMS) after eccentric strengthening training. DOMS has been explained as a response of muscle to damage induced by eccentric exercise and is believed to have a protective effect against further injury (Brockett et al., 2001). However, low compliance reported in Gabbe et al., (2006b) had reduced player’s ability to participate in subsequent training sessions and developed the fear of possible muscle injuries among the players because of muscle soreness. Further, the training protocol of 12 sets of 6 repetitions used by Gabbe et al., (2006b) is not necessary as the shift in optimum length can be obtained with low volume exercises over a sustained period without needed for excessive muscle damage (Clark et al., 2005).
A recent cluster randomized controlled trial performed by Petersen, Thoborg, Nielsen, Jorgensen, & Holmich, (2011) investigated the preventive effect of eccentric training on acute HSIs in male soccer players. A total of 942 male players from 50 Danish male professional and amateur soccer teams were divided into intervention (n=461) and control (n=81) groups. Both the groups continued their usual training program while the intervention groups were subjected to perform 27 sessions of NH exercise in addition in a 10 weeks period as introduced by Mjolsnes et al., (2004), during the mid-season break. The results demonstrated significantly fewer HSIs in the intervention group with 15 injuries registered, compared with 52 injuries registered in the control group. Furthermore, this study also showed a reduction in the new injury rates per 100 player seasons as 3.1 versus 8.1 in the control group. DOMS was only the side effects reported by most players due to eccentric exercise in this RCT. However, there were no injuries and drop-outs during the intervention period due to eccentric loading. The results of this study are in agreement with Arnason et al., (2008) study who reported 65% fewer HSIs incidence in soccer players after a 10 weeks intervention consisting of warm-up stretching, flexibility training, and NH exercise, compared with a group performing warm-up stretching and flexibility training only.
It may be wise to include NH exercise program along with other regular sports training or warm-up stretching programs when the target is to reduce the incidence or recurrence of HSIs. However, the studies discussed above do not include or investigate the effect of an eccentric hamstring exercise to develop the shift of optimum length-tension. This shift in optimum length or changes in the angle of peak torque has been postulated as a protective adaptation for possible muscle strain injuries in sports (Brockett et al., 2001; Proske et al., 2004). Brughelli, Mendiguchia, Nosaka, Idoate, & Cronin, (2010) recently performed randomized controlled training study on effects of eccentric exercise on optimum length of knee flexors and extensors during the preseason in professional soccer players. They divided 24 Spanish soccer league players into the experiment group (n=13) and control group (n=11). They found 40 and 6.50 increase in the optimum length of hamstring and quadriceps respectively in experiment group who performed NH exercise once per week plus 1 or 2 out of 4 different eccentric exercises (eccentric box drop, in-lunge pushes, forward deceleration steps and reverse Nordic hamstring exercise) for a total of 4-5 sets per session. In the control group who performed NH exercise only, they found 2.30 increases in hamstring optimum length which was significantly less than the shift for experiment group. Further, there was no significant difference in peak torque or Q:H ratios in either group before or after the intervention. However, this study could not determine which exercise has contributed the most to increase the optimum length. Also, it could not explain the effects of a shift in optimum length and if the shift will increase, decrease or remain same during and throughout the season as this study was performed for the period of four weeks only.
Recently, Schmitt, Tyler, & McHigh (2012) have proposed eccentric hamstring in lengthened state of muscle for HSIs rehabilitation and prevention of reinjuries. They have proposed the eccentric exercise in Biodex with patient hip in flexion and then passively extending the knee while the patient resists the passive motion. Alternatively, they have proposed the use of elastic thera bands, cable column or manual resistance to passively extend the knee and the patient eccentrically resists the knee extension while pulling knee to the chest. The authors hypothesize that implementing the lengthened state eccentric training may help to reduce the rate of injury as evidence has shown that the athletes with previous HSIs lack muscle strength in the lengthened state (Brockett et al., 2004). Further, large scale RCTs are warranted to determine the effect of lengthened state hamstring eccentric training to prevent possible re-injury as well as a possible decrease in the incidence of new HSIs.
Manual Therapy:
Wayne & Henry, (2010) demonstrated some potential evidence for the use of manual therapy on the prevention of lower limb but could not show a statistically significant result to prevent HSIs. They used a multimodal and multiregional approach consisting of high-velocity low amplitude joint manipulation, mobilization and/or soft tissue therapies. Also, this was a small descriptive study done as an RCT investigation on 29 elite Australian Rules football. So, the findings of this study were not strong enough to draw a significant conclusion.
Recently, Szlezak, Georgilopoulos, Saxton, & Steele (2011) have reported that unilateral zygapophyseal joint mobilization has a significant effect on the restoration of posterior chain neurodynamics which may help to reduce the number of hamstring injuries. Orchard, Best, & Verrall (2005); Sherry, & Best (2004) have advocated the requirement of the restoration of neuromuscular control and normal movement patterns in the lumbopelvic region to enable optimum hamstring function during sports activities. Further studies are warranted to determine the effects of manual therapy on the restoration of neurodynamics of the posterior chain and its relationship with hamstring strain injuries.
Perspectives
To date, not much evidence-based research has been published on the prevention of HSIs. Hamstring strain injuries are most common in high explosive power events which require rapid contract-relax demands of muscles such as in soccer, football, rugby and other sports involving sprinting and jumping. There is a need for further research focusing to correct the most predictive modifiable risk factors found in epidemiological researches for HSIs, preferably in the form of randomized controlled trials.
The failure of warm-up stretching or flexibility training may reflect the lack of strong qualitative studies on its effects particularly on hamstring muscle strain in a specific sport. Also due to a widely accepted mechanism of injury and heterogeneity of HSIs, flexibility may not be considered as the only predictor of HSIs. However, findings from the studies included suggest that the optimum flexibility of hamstring may help to reduce HSIs. There is a need for further studies on this.
Muscle endurance is one of the most important factors for optimal muscle function (Mair et al., 1996; Small et al., 2010). So, muscle fatigue should be considered carefully in future studies, when the target is to prevent the HSIs.
Recent literature has mostly focused and reported significant evidence towards the role of optimum isokinetic strength, especially eccentric strength and optimum muscle length of tension development for the prevention of HSIs. Nordic hamstring exercise has been shown to have the potential effect on the prevention of both new HSIs and reinjuries. However, due to underpowered trials, insufficient data, statistical heterogeneity, poor functional approach, and lower compliance values have hampered to postulate the concrete conclusion on effects and hence the applicability of strengthening protocols. Further, large scale researches are warranted to determine more functional approaches of hamstring strengthening protocols, preferably focusing on eccentric hamstring training.
Although Wayne & Henry (2010) trial of manual therapy did not find the significant result on prevention of HSIs, there was a significant reduction in lower limb and back strains. Also, there was a significant improvement in posterior chain neurodynamics following lumbar spine Z-joint mobilization (Szlezak et al., 2011). These facts point manual therapy as a promising intervention for the prevention of HSIs. Further studies are needed on this issue.
Conclusion:
On the basis of the studies included in this review, it can be suggested that the effective training protocol for the prevention of HSIs should be based on the improvement of flexibility, eccentric strength and endurance of hamstring muscle. In addition, optimum neurodynamics control is necessary. The functional eccentric hamstring training points to be a more promising approach, there is a need for further standard qualitative and quantitative studies on this. It is therefore expected that athletes, trainers, coaches, and even further researchers will be highly motivated to focus on these topics to reduce the incidence of hamstring strain injuries.
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Static stretching before sporting activity may impair performance.
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