Collegiate athletes who participate in sports associated with sprinting have an increased risk of hamstring strain. Investigators have identified flexibility, strength, and fatigue as potential contributing factors that can be addressed through sport-specific training and rehabilitation.
By Kevin M. Cross, PhD, ATC, PT, Susan Saliba, PhD, ATC, PT, and Jay Hertel, PhD, ATC
Hamstring strains are considered a plague for any athlete, especially for soccer players, as time lost from competition averages between two to three weeks and three matches.1,2 Most research on hamstring strains has involved European professional soccer athletes. Practitioners referring to these studies as part of an evidence-based approach to prevention and treatment must assume that professional athletes are similar to their patient population. This may not always be the case, however.
Even among professional soccer players, the risk of hamstring injury varies significantly among different countries.3,4 Given the popularity of high school and collegiate sports in the US, data specific to these populations would provide a more focused approach to hamstring injury management.
Sport, gender, and reinjury
For many years, practitioners have anecdotally identified a high incidence of hamstring strains among sports associated with sprinting. Research has confirmed this in collegiate athletics. Among men’s and women’s sports that were monitored by the National Collegiate Athletic Association, excluding track and field, soccer athletes had the highest incidence of hamstring strain.5 Only football and field hockey had comparable rates among male and female athletes, respectively. All of these sports require repeated bouts of high-intensity running. On professional soccer teams, approximately 25% of the total distance a player runs during a match is at a high intensity.6 In addition, the pace of activity is very quick and fluid. There are minimal rest breaks, and the play occurs with few interruptions. Due to the use of fast-twitch fiber during high-intensity activity, the hamstrings may fatigue more rapidly than in other sports, and thus may be predisposed to strain.
Data also show track and field athletes, especially sprinters and jumpers, have a high occurrence of hamstring strains.7-9 Unlike the previously mentioned sports, which have periods of variable-intensity activity for long durations, most track and field athletes with a high prevalence of hamstring strains perform running and jumping activities at maximal effort for short periods of time.7
The repetitive high-load stress and strain on the muscle tissue that occurs at terminal swing most likely overloads the musculotendinous unit.10 At this point in the running cycle, the hamstring muscle is positioned at peak stretch while simultaneously undergoing a rapid and maximal eccentric contraction. Early stance phase, which is also characterized by repetitive high force through the hamstrings, is another period during which the hamstrings may be susceptible to injury.11
The sport-specific requirements of the hamstring muscles during track and field events appear different from those of field and court sports that are associated with a high incidence of hamstring strains. Unfortunately, hamstring injury data on collegiate track and field athletes have not been reported. Future research should examine the epidemiology and risk factors of this population separately.
Gender is another significant characteristic that influences the incidence of hamstring strains. Among collegiate athletes, men are 64% more likely than women to sustain a hamstring strain.12 Flexibility, strength, and fatigue have been identified as potential intrinsic risk factors of hamstring strain.13
Of these, muscle flexibility and stiffness are the factors that differ most conclusively between genders. Male athletes not only have less active and passive hamstring flexibility,14-18 but also have increased muscle stiffness compared with women.15,18
Theoretically, this stiffness may reduce the “cushioning” of the muscle during high-velocity eccentric contractions,19 such as those occurring during high-intensity running. Using an animal model, Safran et al20 confirmed the influence of increased muscle stiffness on the development of acute muscle injuries.
Conclusive evidence is lacking with regard to hamstring injury risk and other physiological differences between male and female athletes, including muscle fatigue.21 Likewise, gender differences in hamstring strength are variable, and dependent on the method used to normalize the strength measure.22,23
As noted by Worrell,13 determining the interaction of the various intrinsic and extrinsic factors would provide better insight into the predisposition for hamstring strains.
Another common intrinsic factor associated with hamstring strain is a previous history of hamstring strain.24-26 While not absolute,27 hamstring flexibility and strength deficits have been confirmed among athletes with a prior strain.28-32
However, the prior injury may actually be a surrogate for the unique predisposition to injure a specific tissue type.33 For hamstring strains, this factor has not been readily identified. Recurrence rates among collegiate soccer players are similar to those of professional teams,1,2,12,34 and male athletes have a significantly larger proportion of recurrent strains than female athletes.12
Research is sparse regarding the influence of extrinsic factors, such as time of season, on hamstring strains. Among US collegiate soccer players, male athletes have a higher incidence rate of hamstring strains than female athletes in matches, practice, and during the in-season.12
During professional soccer matches, male athletes generally perform a larger volume of high-intensity runs and sprinting activities than female athletes.35,36 This may lead to increased fatigue, decreased eccentric strength, and consequent changes in sprinting and technical performance.37-39 One may speculate that, if the collegiate match is played similarly, then the relatively elevated exertion among male athletes may help explain the increased incidence of hamstring strains associated with male gender during matches, and consequently, during the in-season.
Every coach has a unique philosophy for planning practices, so there is large variability in practice intensity within and between genders. Although male athletes still have a higher incidence of hamstring strains than female athletes during practice, the difference between genders is less dramatic than it is during games.
Relative exertion levels may also predispose soccer players to hamstring strain during matches, regardless of gender. Among professionals, both male and female players perform a larger percentage of high-intensity activities during matches than during practice. The exercise-to-rest ratio is also much higher during matches, allowing for less recovery time.40,41 The result may be a more rapid onset of fatigue and, ultimately, a magnified physiological response to exercise during a match. The same alterations to skill and running performance, as mentioned above,36,42 may increase likelihood of a hamstring strain for either gender during matches compared with practices.
Most interesting is the influence of the time of season on hamstring strain incidence among collegiate soccer players. Regardless of gender, there is a higher incidence of hamstring strains during the preseason than during the in-season12 even though very few matches are played during the preseason. The influence of the preseason on muscle strain incidence has not been demonstrated conclusively within professional soccer.1,43
Among professional Italian leagues, however, the potential mismanagement of the preseason schedule has been identified as a potential contributor to increased injury occurrence. Athletes frequently do not adhere to the recommended off-season training regimens, and coaches respond with intense preseason training.44 The consequence of such contrary responses may be magnified in the collegiate soccer population because of the significantly shorter preseason. With approximately two weeks to prepare for the first regular season match, collegiate coaches may believe that more intense training is necessary for match-readiness. Unfortunately, the result may be fatigue-induced injuries such as muscle strains.
Based on these extrinsic factors, we believe that muscle fatigue is an important component in explaining the incidence of hamstring strains. Animal45 and laboratory research37,38,46 have demonstrated that fatigue creates muscle impairments and functional adaptations that may predispose the muscle to injury.
The primary preventive measure must be appropriate sport-specific conditioning.
For the sports with the highest incidence of hamstring strains, such as soccer, preseason and in-season training that emphasizes high-intensity running and sprinting with large exercise-to-rest ratios is necessary to properly challenge the aerobic and anaerobic energy systems. For professional male soccer players, for example, recent evidence suggests that performing longer sprints (40 m) with an exercise-to-rest ratio of 1:6 best replicates their physiology.47 Such specifics should be considered for each sport with regard to gender and level of play.
Likewise, performance testing is necessary to identify athletes who are not properly conditioned, and thus at risk for injury. There are many good options for assessing the sport-specific fitness of athletes in sports such as soccer. The Yo-Yo intermittent recovery test level 2 (Yo-Yo IR2) is one of the most studied and utilized performance measures. Research has validated it as an accurate assessment of the fitness and performance of athletes who participate in soccer and other sports with intermittent high-intensity activity patterns.48-51
The Yo-Yo IR2 consists of two 20-m runs that must be completed within a given amount of time. The time to complete the runs is progressively increased by audio beeps, previously recorded, that identify the start and finish for the run. Ten seconds of rest is given between each set of runs. The athlete completes the test when he or she fails twice to finish the distance before the second beep.
For soccer athletes, the Yo-Yo-IR2 is reproducible and can differentiate among different times of the season, player positions, and competitive levels.50-52 Moreover, there is a strong correlation between the performance on the Yo-Yo IR2 and the volume of high-intensity activity performed during a match.51 In the preseason this test may provide insight about which athletes may be at risk for fatiguing quickly, and thus at risk for injury. Also, it may be used to verify improved fitness and provide expectations for the athlete’s actual performance during matches.
With the use of an individual’s multiple time points as a baseline, the Yo-Yo IR2 may guide the sports medicine practitioner’s assessment and the progression of an injured athlete to return to play. Given the influence of fatigue on player performance and risk of injury, the Yo-Yo IR2, or comparable field test, should be a factor in determining a soccer player’s readiness to return to participation.
Prevention and rehabilitation of hamstring strains must consider the unique characteristics of the sport and each particular team. The literature is variable in describing the risk factors and preventive programs53 for hamstring strains.54 In US collegiate athletics, soccer has one of the highest incidences of hamstring strains, and sports with a similar playing pace also have high occurrences of hamstring strains. Prospective data within these populations must be collected to accurately portray the characteristics of athletes who sustain a hamstring strain.
Several variables should be assessed to develop individualized programs. Hamstring flexibility and stiffness may be assessed using the active and passive knee extension tests.55 In the literature, hamstring strength is commonly measured using isokinetic strength tests. Testing may be performed at slow (60°/sec) or fast speeds (180°/sec) for concentric measurement, and eccentric strength may be assessed at 60°/sec.56
Field tests may be more practical for the practitioner who does not have access to isokinetic devices or other weight equipment. Functional tests such as the vertical jump57 and the five-jump test58 are significantly correlated to power and strength measures as well as agility performance among soccer players. Dynamic balance and stability may be assessed using the Star Excursion Balance Test (SEBT),59 which can identify balance deficits and predict injury risk in multiple populations. Sports medicine practitioners should provide athletes with defined impairments a patient-specific program for prevention and treatment.
Of greatest importance is the proper assessment of the athletes’ fitness relative to the sport and position. Collegiate sports with a high incidence of hamstring strains arguably involve intermittent energy systems. Conditioning programs that focus solely on the aerobic or anaerobic system will not properly train the muscles to withstand the fatigue and strain of a competition.
Furthermore, when treating an athlete with a hamstring strain, sports medicine clinicians must not focus only on the impairments. We must also assess the athlete’s sport-specific fitness to guide return-to-play decisions and assure appropriate conditioning programs to permit optimal recovery.
Evidence to guide the management of amateur and female soccer players is growing. However, the population of adolescents who are playing high school or club soccer has not been thoroughly addressed. Due to the multiple competitive seasons throughout the calendar year, the potential for overuse and stress-related injuries is increased because of fatigue on all tissues and structures. Muscle strains, specifically hamstring strains, can be a consequence of the same pattern.
As recommended by many highly acclaimed sports medicine physicians, it is beneficial for players to skip at least one soccer season per year and to participate in a different sport that uses different muscles and movement patterns.60 High school soccer athletes who participate in traditional seasons may approach the prevention and treatment of hamstring strains as previously noted, but practitioners and coaches must be cautious to test and train athletes at intensities and volumes appropriate to their age and current level of conditioning.
Kevin M. Cross, PhD, ATC, PT, is a physical therapist and research coordinator at UVA-HealthSouth in Charlottesville, VA. Susan Saliba, PhD, ATC, PT, is an associate professor, and Jay Hertel, PhD, ATC, is the Joe H. Gieck Professor of Sports Medicine in the Department of Kinesiology at the University of Virginia in Charlottesville.
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