Hamstring injuries that occur during sprinting are a significant concern in football, not just because of the frequency with which they occur, but also because they are notoriously difficult to rehabilitate, which underscores the need for effective prevention through targeted exercise programs.
By Charles D. Kenyon, MS, RSCC, and Marcus C.C.W. Elliott, MD
Originally published in the Lower Extremity Review
Among US team sport athletes, football players are particularly prone to injury as a result of the ballistic demands of their sport. As such it is imperative to identify areas within all levels of football that provide high-yield opportunities for injury prevention. Due to football’s nature as a contact sport, organized injury prevention efforts typically involve changes to protective equipment, coaching proper tackling techniques, and rule modifications (1-4). Although these factors play a large part in reducing the risk of injury in football, modifiable intrinsic risk factors for targeted injuries may be more pertinent to the everyday sports medicine or human movement professional. By identifying specific athletes at risk of certain injuries, focused injury prevention plans can be developed and implemented in coordination with sports medicine, sports science, and skills coaching personnel (5).
Overall injury rates in youth football are reported at 43 injuries per 1000 athlete-exposures (A-E), nearly double that of the second most injury-prone boys’ sport of youth baseball (24 injuries per 1000 A-E) (6). A 2008 epidemiological study of high school sports reported the injury rate of football to be 2.54 injuries per 1000 A-E during practice, and 12.09 injuries per 1000 A-E during games (7). With football excluded, the average injury rate for boys’ high school sports was reported to be 1.49 injuries per 1000 A-E during practice and 3.08 per 1000 A-E during games. That means that the practice injury rate for high school football is 70% higher than for other boys’ high school sports, while the game injury rate is an alarming 293% above the average.
An analysis of 15 National Collegiate Athletic Association (NCAA) sports, including football, revealed an average practice injury rate of 3.98 injuries per 1000 A-E, and a game injury rate of 13.79 injuries per 1000 A-E. Collegiate football itself has an injury rate of 9.6 injuries per 1000 A-E during practices and 35.9 injuries per 1000 A-E during competition (8). Therefore NCAA football has a 141% increased injury rate during practice, and 160% increased injury rate during games compared with the average collegiate sport. This comparison is even more concerning because football injury rates likely raise the average NCAA injury rate significantly. And, without a doubt, the explosive collisions and rapid pace of the National Football League (NFL) result in a high risk of significant injury.
Muscle strain injuries are devastating in competitive sport, not only because of the initial time lost to injury, but also because decreased performance may be observed upon return to competition (10); in addition, there is an increased risk of more severe injury down the road (10). Muscle strain injuries represent 12% to 24% of all injuries in high school football (11-14). Published data from the NCAA Injury Surveillance System demonstrate that across all divisions from 1988 to 2004, muscle strain injuries accounted for 18.9% of spring practice and 22.2% of fall practice injuries (2). Among these, the upper thigh represented nearly half of all strain injuries observed during practice.
Research specifically on hamstring muscle strain injuries at the elite competitive level is prevalent in sports such as soccer (15). rugby (10,16) and Australian Rules football (17-19). However, only recently has the topic been investigated in detail within the NFL (5). Epidemiological research shows that hamstring muscle strains represent 46% of practice injuries and 22% of game injuries during the NFL preseason (20). A recent 10-year review of hamstring injuries in the NFL revealed that 1716 hamstring muscle strain injuries occurred across the 1989-1998 seasons for an overall injury rate of 0.77 per 1000 A-E (5). Although successful hamstring injury prevention programs have not yet been published in the sport of gridiron football, targeted interventions are proven effective at the elite levels of soccer (21-22) rugby (10), and Australian Rules football (23-24).
Hamstring injury mechanisms
Of reported hamstring strain injuries from 1989-1998 in the NFL, an impressive 81.5% of injuries occurred during noncontact action (5). Specifically, sprinting was cited as the mechanism of injury in 74.4% of noncontact injuries. Such numbers may be surprising in light of collegiate data showing overall injury rates are sustained predominantly during player contact (2). But the data on hamstring strain injuries remain consistent with what we know about the biomechanical demands of the hamstring muscle group during sprinting and acceleration. Biomechanical research demonstrates the posterior thigh is at greatest risk for injury in the latter stages of the swing phase of sprinting as the hamstring muscle group reaches its maximal length and rapid eccentric muscle action occurs prior to and during foot strike (25-27). This action allows the lower limb to control ground reaction forces as the limb transitions to a propulsive mechanism during stance phase and push off.
An analysis of NFL hamstring strains by player position revealed that a significant number of these injuries occurred in wide receivers and players in the defensive secondary, who accounted for 20.8% and 23.1% of all hamstring strains, respectively (5). Running backs (12.3%) and linebackers (11.5%) represented a second tier at-risk group, while offensive (5.2%) and defensive (7.4%) linemen had the lowest risk of hamstring muscle strain injury. Among the wide receivers and defensive secondary, 92.0% and 93.5% of hamstring injuries, respectively, resulted from noncontact situations. It is not coincidental that athletes who play positions demanding the highest intensity sprinting are also the athletes most affected by hamstring strain injury.
The literature regarding risk factors for hamstring muscle strain injury is complex, with many studies presenting seemingly contradictory evidence. Proposed risk factors include age, limitations in flexibility, and decreased strength. However, the risk factor cited most consistently in hamstring muscle strain research is previous injury. In elite athletes, previous injury has been identified as a significant risk factor for hamstring strain in track and field (28), soccer (15, 29-30), rugby, (10) and Australian Rules football (17-18,23). The NFL is no different, with 16.5% of all hamstring muscle strains occurring as reinjuries (5). A comprehensive study of hamstring injuries by the Football Association Medical Research Programme covering British soccer leagues reported a similar reinjury rate of 12% (15). For Australian Rules football, Verrall and colleagues revealed that previously injured athletes were 4.9 times more likely to experience a strain than those without a history of muscle strain injury (19). Likewise, Orchard reported that the risk ratio was as high as 6.33 in athletes with injuries incurred within the previous eight weeks (18). In top level rugby, reinjuries account for 24% of hamstring strains that occur during match play and 20% of those experienced during training (10).
During a study of elite track and field athletes with a primary hamstring strain, approximately 14% of the participants suffered reinjury injury within a 24-month follow-up period (28). Interestingly, 24.1% of these track and field athletes diagnosed with a grade II strain were reinjured, while only 7.7% of athletes with grade III injuries had recurring symptoms. No athlete with a grade IV injury experienced a reinjury during the follow-up period. It is possible that athletes with milder strains are rushed back to competition too quickly, and thus placed at elevated risk. However, the consistency of the data across multiple sports suggests that hamstring muscle strain injuries are particularly difficult to rehabilitate efficiently. Therefore practitioners should focus on prevention of the initial strain injury as well as the development of effective rehabilitation protocols.
Preseason conditioning is essential
Feeley and colleagues published epidemiological data for one NFL team’s preseason injuries over a 10-year period (1998-2007) (20). Hamstring strains were implicated as the second most common preseason injury (after knee sprains) with 1.79 injuries per 1000 A-E for practice and 4.07 injuries per 1000 A-E for preseason games. Elliott et al reported a league-wide preseason injury rate of 2.47 injuries per 1000 A-E for practice (0.82 per 1000 A-E during games), compared with the regular-season practice injury rate of 0.18 per 1000 A-E (5). Over the 10-year study period, 53.1% of all hamstring injuries in the NFL occurred during the seven-week preseason period rather than the 16-week regular season. Among practice injuries specifically, 79.8% occurred during the preseason. Furthermore, 56% of these practice-related injuries occurred in the first month of preseason (July) while 23.8% happened during the second month (August).
Based on this data, the authors recommended an increased focus on conditioning prior to the beginning of training camp (5). Because the preseason is a period of greatly increased risk, an improved effort by athletes to report for camp in adequate shape is necessary to reduce the occurrence of hamstring muscle strain injury. This finding is applicable to football players at all levels, particularly high school and collegiate athletes, as they utilize the summer months to prepare for the grueling fall season.
Preseason muscle weakness has been associated with an increased risk of injury in Australian Rules football as well (31). In addition to reducing the risk of injury, hamstring-specific preseason conditioning in Swedish soccer players significantly improved performance variables such as overall strength and maximal running speed (22). Fatigue has also been implicated as a risk factor in muscle strain injury (15,32). A properly designed offseason conditioning program that replicates the intermittent metabolic demands of football will also make the athlete more resistant to fatigue and encourage a healthy competitive season (24).
The role of eccentric exercise
A common element in almost all published hamstring muscle strain injury prevention programs is the implementation of strength training with a focus on eccentric muscle contraction (10,17,21-24,33). During high-speed activities such as explosive sprinting and deceleration the hamstring muscle group undergoes repetitive eccentric contractions to safely distribute ground reaction forces throughout the body. Through an eccentric strengthening program, athletes can adapt the hamstring muscle group to better withstand the demands of sprinting, deceleration, and change of direction.
One proposed mechanism by which eccentric exercise reduces injury risk is through increases in overall muscle strength. Utilizing an enhanced eccentric loading protocol with hamstring curls, Kaminski et al demonstrated that eccentric exercise induces significantly greater gains in strength than a concentric-focused execution of the movement (34). There is also encouraging evidence showing that adaptations to eccentric muscle loading include a shift in the optimal joint angle for torque generation at longer muscle lengths (35). By increasing the hamstring muscle group’s ability to absorb eccentric forces at longer muscle lengths, damage and soreness from repeated sprinting bouts can be minimized.
A primary mode of eccentric exercise utilized in injury prevention studies is the Nordic hamstring lower exercise. The Nordic hamstring lower has been shown to reduce muscle strain injuries (10). Brughelli and colleagues, however, propose the introduction of functional single-leg movements as more sport-specific modes of eccentric exercise for elite athletes in both prevention and rehabilitation of muscle strain injuries (33,36).
These authors argue that the Nordic hamstring curl may allow one leg to dominate the movement, leading to imbalances and compensations across the kinetic chain. During exercise progression, the implementation of multijoint eccentric movements such as single-leg Romanian deadlifts or split stance lunge drops may be a more sport- and task-specific method of developing eccentric muscle adaptations (33). Furthermore, a study found these protocols were effective in the treatment of an Australian Rules football player suffering from recurrent hamstring injury (36). The eccentric-based intervention used in the study was both safe and effective for shifting the angle of peak torque toward longer muscle lengths, as well as restoring the performance and physical comfort of the injured athlete.
A comprehensive training program should utilize both double- and single-leg movements to progressively challenge the athlete. Through both open- and closed-kinetic chain movements the hamstring muscle group can adapt to a variety of forces and help the athlete prepare for the competitive season.
An area for future research in hamstring strain injury prevention is the potential role of neuromuscular activation training in injury risk reduction. Research has demonstrated that muscles are capable of absorbing more force under strain before failure when they are activated than when passively strained (37). Since high-intensity sprinting demands a high level of hamstring neuromuscular coordination (38), improving the neuromuscular system’s ability to initiate and appropriately time forceful contractions should lead to both increased sprinting speeds and reduced injury rates.
Drills that mimic various phases of sprinting have been shown to improve neuromuscular function and positional awareness of the lower limb (39). Research has also shown that plyometric training influences the stiffness of musculotendinous units and improves lower extremity strength and power as well as the function of the stretch-shortening cycle (40). Brughelli et al used eccentric drop jumps as part of their functional rehabilitation protocol (36) but large-scale controlled studies are necessary to further evaluate the value of plyometric and neuromuscular training for prevention of muscle strain injuries. The impact of such drills on sprint technique and performance (40), however, suggests the same methods can be effective in modifying injury risk for the hamstring muscle group.
By becoming more powerful athletes through improvements in neuromuscular efficiency, football players will be able to reach their full potential with a significantly reduced risk of hamstring muscle strain injury throughout all stages of their development and competitive careers.