This article attempts to demonstrate the size and scope of the acute hamstring injury by describing its incidence in various sports and the difficulty in return to those sports without impaired performance and a high risk of reinjury. It will also help the reader understand what happens anatomically and physiologically after an acute hamstring injury. This understanding is the prerequisite to the ultimate purpose, which is to provide practical applications for the sports medicine and performance team that help return athletes to sport with reduced risk for recurrent injury.
Acute hamstring strain injuries are common in sports that involve sprinting, kicking, or high-speed skilled movements (2,4,10,15,21,23,33,34,38,41,52,60-62). A retrospective review of the National Collegiate Athletic Association Injury Surveillance System found that male college athletes were 62% more likely to sustain a hamstring injury than female athletes and more common in field sports than in court sports (19). A National Football League team published injury data, including preseason training camp from 1998 to 2007, and found that hamstring strains were the second most common injury, only surpassed by knee sprains (23). Injury rates varied by position, with it being the highest percentage of total injuries among running backs (22%), defensive backs (14%), and wide receivers (12%) (23). A 4-year study of injury rates within a Division 1 football team showed that hamstring strains were the third most common orthopedic problem, behind knee and ankle injuries (15). A 2-year analysis of professional soccer teams revealed that 12% of all injuries were hamstring strains (61). In addition to high-speed sports, there is an increased risk for hamstring strains in sports involving slow extreme stretching-type maneuvers, such as dancing (3,4).
Hamstring strain injuries often result in significant recovery time and have a lengthy period of increased susceptibility for recurrent injury (36,46,47). Reinjury rates reported in the literature vary depending on the population, the interventions used, and the duration of follow-up. A study that analyzed 858 hamstring strains in Australian footballers showed that the rate of recurrence was 12.6% during the first week of return to sports and 8.1% for the second week. The cumulative risk of reinjury for the entire 22-week season was 30.6% (46). Another study reported the recurrence rate at 1 year to be as low as 7.7% (52), but most often, recurrence rates are near 30% or higher (10,38,52).
RISK FACTORS FOR HAMSTRING INJURY
The high incidence of injury and frustration associated with trying to return to sport without reinjury have led several researchers to search for risk factors that predispose athletes to hamstring injury. If these risk factors were identifiable, they could then potentially be addressed and modified through injury prevention programs. To date, there is some evidence to suggest previous hamstring injury, older age (relative for competitive athletes), decreased quadriceps flexibility, and muscle imbalances of the thigh are risk factors for hamstring injury.
Gabbe et al. (24) showed that decreased quadriceps flexibility, as assessed by the modified Thomas test, was an independent risk factor for hamstring strains in community-level Australian rules football players. However, measurements, such as hamstring flexibility when measured with the sit and reach test, passive straight leg raise, and the active knee extension test, have not been related to a higher incidence of hamstring strain injury (24,26,27). One study found that hamstring-to-quadriceps strength imbalances can be a risk factor for reinjury (18). It is important to note that 31% of the individuals with a recurrent hamstring injury in that study displayed normal hamstring strength, suggesting that strength imbalances alone cannot explain the risk for reinjury after a hamstring strain. Older age, relative for competitive athletes, has also been identified as a risk factor for hamstring injury in several studies (24,26,33). A recent prospective study evaluated 508 soccer players in an attempt to determine if player position, age, previous hamstring injury, subjective rating, or physical performance capabilities could determine risk for hamstring injury (22). The physical performance tests included a Nordic hamstring strength test, 40-m sprint test, and countermovement jump test. Their results suggest that previous acute hamstring injury was the only significant risk factor for a new hamstring injury. Specifically, the previously injured players were more than twice as likely to sustain a new hamstring injury as their noninjured counterparts. Other studies have also found that a previous hamstring injury is a significant risk factor for recurrent injury, suggesting that postinjury changes to the muscle and altered movement patterns may persist that contribute to this increased risk (6,22,24,33,38,47).
MECHANISM OF HAMSTRING INJURY
Most hamstring strain injuries happen while running. It is generally believed that they occur during terminal swing phase of the gait cycle (29,45). This is supported by the objective findings from 2 separate hamstring injury case studies (50). During the second half of the swing, the hamstrings undergo an eccentric contraction and absorb energy from the swing limb before foot contact (16,63). Thus, the hamstrings are stretched while subjected to load (eccentric contraction), with the biceps femoris incurring the greatest amount of length change and performing the greatest amount of negative work during this time (58,59). This may contribute to the tendency of the biceps femoris to be more often injured than the semimembranosus and semitendinosus (5).
ANATOMY AND PHYSIOLOGY OF HAMSTRING INJURY
Most hamstring injuries occur along the proximal musculotendon junction (MTJ) (20), where the muscle fibrils intersect with the tendon (30). Like most acute strain injuries, hamstring strains do not typically involve the muscle tearing away from the tendon. In fact, it is the muscle tissue adjacent to the MTJ that is damaged (31). Immediately after injury, there is an acute inflammatory response that is followed by muscle and collagen regeneration (8). An injury such as this can result in fibrous scar formation. Structural changes within the muscle immediately after an acute hamstring strain injury have been investigated (17,32,39,40). The amount and extent of edema and hemorrhage on magnetic resonance (MR) images can confirm the presence and severity of initial muscle fiber damage and can also provide a reasonable estimate of the rehabilitation period, especially in the moderate and severe cases (17,32,56). MR imaging and clinical assessment with regard to the less severe acute hamstring strains may not necessarily be definitive (51). For example, in 18 of the 58 cases studied, a clinical diagnosis of hamstring injury was made with no positive identification of injury on MR images (51). It is unknown whether MR is not sensitive enough to identify more mild strains or if other injuries may clinically mimic mild hamstring strains.
Animal models of muscle injury have shown that the growth of fibrous tissue prevails over muscle regeneration and eventually leads to the presence of mature acellular scarring at the site of injury (37,44). For example, imaging studies in humans have found evidence of scar tissue as soon as 6 weeks after injury (17). Animal models suggest that scar tissue may persist indefinitely (8,39). These changes may increase the stiffness of the MTJ and thereby alter the relative amount of stretch taken up by the adjacent muscle and tendinous tissue (7,49). The long-term effects of a hamstring strain injury have been shown to persist in some people until at least 23 months after injury (53). In this study, 14 subjects had returned to full sporting activity without self-perceived symptoms or performance deficits, yet residual scar tissue was present along the MTJ adjacent to the site of presumed previous injury for 11 of the 14 subjects.
The significance of these persistent musculotendon morphological changes to reinjury risk is not definitively known at the present time. Proske et al. (48) showed that after hamstring injury, the optimum length for active force generation was reduced. This change subsequently causes the angle of peak torque to occur at a greater knee flexion angle (i.e., shorter optimum musculotendon length for active tension) compared with the noninjured side. Proske et al. and Morgan et al. (9,48) then suggested a correlation with the increase in the risk of injury recurrence with the shorter optimum length for tension, as it would create susceptibility to damage from eccentric contractions of the hamstrings occurring in the late swing phase of running. These findings created a speculation that the replacement of muscle with scar tissue after injury was the cause for this. However, a more recent retrospective study of athletes with a history of unilateral hamstring strain injuries found that a consistent shift in the angle of peak torque was not observed (55). The same study investigated the effect of scar tissue on musculotendon dynamics by assessing running kinematics at 4 speeds ranging from 60 to 100% of maximum sprinting speed (55). It was speculated that peak stretch of the hamstring muscles might be reduced in the previously injured limb compared with the contralateral side as a compensation for the modified tissue. However, no significant asymmetries in overall hamstring musculotendon stretch were observed at any of the speeds tested (55). Other studies have also shown similar findings in a group of athletes tested at submaximal sprinting speeds (12,42). It seems that joint-level mechanics or local neuromuscular control patterns do not appear to be consistently altered.
It is possible that scar tissue may alter local contraction mechanics, thereby influencing reinjury risk. To investigate this possibility, CINE phase-contrast MR imaging has been used to measure muscle tissue velocities adjacent to the previous injury in a group of previously injured athletes (55). This type of imaging technique allowed us to measure tissue velocities within the biceps femoris muscle tissue, adjacent to the proximal MTJ. Measurements were taken while the subjects performed cyclic knee flexion-extension for both elastic and inertial loading conditions. The elastic and inertial loads induced active shortening and lengthening contractions, respectively. Muscle tissue velocities obtained during these tasks were integrated to estimate displacements and subsequently used to calculate tissue strain (54). Both healthy and previously injured subjects exhibited increased muscle strains near the proximal MTJ (54). In addition, subjects with previous injury presented with significantly greater muscle tissue strains when compared with their healthy counterparts (54). It therefore seems likely that residual scar tissue at the site of a previous injury may adversely affect local tissue mechanics in a way that could contribute to reinjury risk.
REHABILITATION AND RECONDITIONING
Rehabilitation programs should address components of these basic science findings in addition to clinical findings. In response to eccentric exercise, an increase in serial sarcomeres has been suggested (43). This would allow the muscle-tendon unit to operate at longer lengths and decrease the magnitude of the stretch absorbed by each sarcomere and likely the corresponding strain. Clinical investigations involving eccentric training have also shown benefits in reducing the incidence of hamstring strain injuries. One study showed a decrease in hamstring injury after a program of concentric and eccentric contractions on a YoYo flywheel ergometer (2), whereas 3 other studies have shown a decrease in hamstring injury after eccentric training using the Nordic curl exercise (1,10,25). Despite the benefit of these programs, they can have significantly low compliance rates (21,25). There are also authors who are critical of the training specificity of the Nordic curl, noting that it is a bilateral movement that only generates movement from the knees (11). Thus, despite its demonstrated benefit, there may be potential for even greater benefit using a unilateral eccentric exercise that incorporated hip and knee motion.
Rehabilitation and reconditioning efforts must also appreciate more regional factors influencing function. Musculoskeletal modeling has recently demonstrated the substantial influence that lumbopelvic muscles can have on the overall stretch of the hamstrings (16). For example, contralateral hip flexor (i.e., iliopsoas) activity during high-speed running has a large influence on ipsilateral hamstring stretch. That is because activity of the iliopsoas can produce an increase in anterior pelvic tilt during early swing phase, the stretch of the contralateral hamstrings is increased. A recent experimental study of normal running mechanics has confirmed the bilateral coupling between hip extension and contralateral hamstring stretch (57). This coupling may, in part, explain why rehabilitation exercises targeting neuromuscular control of muscles in the lumbopelvic region are effective at reducing hamstring reinjury rates (52).
This influence of lumbopelvic muscles on hamstring dynamics was prospectively assessed by comparing reinjury rates in athletes with hamstring strains who were treated with a progressive agility and trunk stabilization (PATS) program and those treated with a hamstring strengthening and stretching (SS) program (52). Both programs were to be completed at least 5 times per week. The PATS group had a reinjury rate of 0 and 7.7% at 2 weeks and 1 year after return to sport, respectively, whereas the SS group had a reinjury rate of 54.5 and 70% at 2 weeks and 1 year after return to sport, respectively (52). Although the morphological and neuromuscular factors were not measured, it does suggest that there may be a role of lumbopelvic neuromuscular control in the prevention of future hamstring injury. In fact, Cameron et al. (13) demonstrated that below-average neuromuscular control can predispose athletes to hamstring injury. They prospectively investigated limb neuromuscular control with a leg swing movement discrimination test in a weight-bearing position in 28 Australian Football League players. The movement discrimination test involved backward swinging of the leg to a contact plate without visual reference. The purpose of the test was to assess lower limb neuromuscular control (13). Of those 28 players, 6 subsequently injured their hamstring that season. All 6 players had movement discrimination scores below the mean. This led to the creation of the “HamSprint program” during which a series of drills are conducted to improve running technique, coordination, and hamstring function (14). Some drills in this program included leg cycling, pawing, ankle pops, high knee marching, quick support running drills, forward falling running drills, and explosive starts (Table 1). After 6 weeks of training using the HamSprint program, athletes significantly improved their movement discrimination scores when compared with a control group that performed regular stretching, running, and football drills (14). Based on the findings from these 2 studies, Cameron et al. (14) theorized that the HamSprint program could be an effective hamstring injury prevention program. These drills are similar to the drills that Gambetta and Benton (28) have advocated for hamstring injury prevention. They theorized that these drill would improve running mechanics and sport-specific training of the hamstrings. A similar hypothesis was used for soccer athletes. A training program consisting of a variety of single-leg balance, takeoff and landing exercises, that were theorized to improve neuromuscular control for soccer, were studied. A positive effect was seen for this proprioceptive balance training program by an observed decrease in noncontact hamstring injuries in female soccer players (41). At the completion of the 3-year prospective program, noncontact hamstring injury rates were reduced from 22.4 to 8.2/1,000 hours (p = 0.021) (41). These studies suggest that proprioceptive and neuromuscular control mechanisms may be affected by injury and just as importantly have an important role in preventing future injury.
The scientific evidence presented creates a sound basis for the following practical applications. Consistent implementation of these practical applications consistently should improve return to sport after injury by expediting return to optimal athletic function and reducing the chance of recurrent injury.
Upon return to sport after injury, athletes should incorporate a dynamic warm-up before practice or competition. The HamSprint program by Cameron et al. (14) demonstrated that dynamic agility drills can improve lower limb motor control and that this has a relationship to hamstring injury. Postinjury research has also shown that the use of progressive agility exercises is an effective way to prevent reinjury (52). Based on these principles, an appropriate dynamic warm-up program should include specific drills shown to improve running technique, lumbopelvic control, and hamstring function. Such drills could include A marching, A skips, B skips, short stride cariocas, side shuffles, leg cycling, leg pawing, ankle pops, quick support running drills, forward falling running drills, and explosive starts (Table 1) (see Video, Supplemental Digital Content 1, https://links.lww.com/SCJ/A5, labeled “Dynamic Warm-Up Drills”).
TRUNK STABILIZATION AND NEUROMUSCULAR CONTROL EXERCISES
Upon return to sport after injury, athletes should perform trunk stabilization and neuromuscular control exercises at least 3-4 times per week. These exercises may vary depending on the sport that the athlete is returning to but generally should involve exercises that incorporate control of trunk rotation, weight bearing, and multiple angles of hip flexion. Such exercises could include low to high wood chops, high to low wood chops, rotating core planks, physioball bridging with alternating leg holds and alternating hip position, or single-leg stand rotating reaches (Table 2) (see Video, Supplemental Digital Content 2, https://links.lww.com/SCJ/A7, labeled “Trunk Stabilization and Core Control Exercises”) (35,52).
The eccentric contraction basis for injury and the positive prophylactic effect of eccentric training strongly suggest that eccentric training should be a component of a reconditioning program upon return to sport. Alternative exercises, such as the eccentric box drops, eccentric loaded lunge drops, eccentric forward pulls, split-stance Zerchers, and single-leg deadlifts, may be good alternatives to the Nordic curls because these exercises create biarticular muscle function in a unilateral asymmetric fashion, similar to that needed for sprinting and most sport activities (Table 3) (see Video, Supplemental Digital Content 3, https://links.lww.com/SCJ/A8, labeled “Eccentric Training Exercises”) (11).
Given the frequency of hamstring injuries and the high rate of injury recurrence, successful recovery and return to sport pose a great challenge to the rehabilitation professional and sports performance professional. Understanding the morphological and functional effects of injury can help optimize rehabilitation and reconditioning strategies. As outlined in this article, determining appropriate readiness for sport, using an appropriate dynamic warm-up program, integrating neuromuscular control and trunk stabilization exercises into sports performance programs, and the use of functional eccentric strengthening have shown potential to prevent a recurrent injury and keep athletes in the game.
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