Hamstring strains often are difficult to treat, slow to rehabilitate, and are compounded by a high recurrence rate (23,58,60). Normal recurrence rates are between 12% and 31% within 1 year of returning to sport (60) but may be as high as 54.5% (68). Factors responsible for the recurrence in hamstring injury include reduced tensile strength of the scar tissue at the site of injury, reduced strength of surrounding musculature as the result of disuse atrophy, reflex inhibition, reduced flexibility of the muscle tendon unit (MTU), and possible adaptive changes in the biomechanics of sporting movements after the original injury (23,58,60).
Strains to the MTU are among the most prevalent forms of injury for athletes (17,44,48,80) with muscles that span 2 joints, such as the hamstring, being particularly susceptible to strain (22). Hamstrings are the most commonly injured muscles in the lower limb, with strains prevalent in sports such as soccer and track and field athletics (14). Hamstring strains represent 12-16% of injuries among soccer (28,60) and Australian Rules Football players (60), 11% in cricket players (71), and 24% among collegiate sprinters and jumpers (79).
The high recurrence rates reported for hamstring injuries (23,58,60,68,71,79) may be reduced by a progressive reintroduction of activities that will prepare the athlete fully for the demands of the sport. This complete rehabilitation should incorporate and allow sufficient time for several stages of healing and treatment. These stages include mobilization and stretching, to avoid loss of extensibility, improve range of motion (ROM), and help reduce formation of scar tissue (39,40,50,52); avoidance of atrophy and regaining of strength (60,68); and advanced strength and conditioning that is sport specific to appropriately prepare the athlete for their return to sport (15,19,30,32,36,46,54).
CAUSES OF INJURY
The literature suggests that there are 2 types of hamstring strains, one resulting from explosive high-speed running (75,79) and the other during stretching movements carried out at extreme ROM (5). Reported causes of hamstring strains include poor lumbar posture (33), previous injury (3,21,27,73), lack of flexibility (12,14,22,27,29,33,45,48,74), inadequate warm up (76,77), fatigue (76,77), strength imbalance and inadequate quadriceps to hamstring ratio (16,21,27,48,79), and poor coordination (14,16,27). Hamstring strains also have been associated with eccentric loading (14,16,50), such as during rapid deceleration.
Hamstring strains most commonly occur in the long head of the biceps femoris (7,51,72,75) with most located near the muscle-tendon junction (51,60). Hamstring strains are commonly reported in sprinters when speed is maximal or close to maximal (6) and during powerful eccentric muscle actions (14).
The strain is most likely to occur during 2 phases of the running cycle; late forward swing and toe off (69), because during this phase the hamstrings decelerate hip flexion and knee extension (39,40), resulting in large eccentric loads. It has also been found that although sprinters sustain their injuries during high-speed running, dancers sustain injuries while performing slow stretching-type exercises (6). In activities such as dancing, most hamstring injuries occur during stretching (hip flexion with knee extension) (6,8), resulting in an eccentric load, with the proximal end of the semimembranosus as the site of injury (8).
Consensus of how to effectively treat hamstring injury is limited, although a multidisciplinary approach can be recommended (20). Treatment and rehabilitation should be tailored to the severity of injury, and the constraints of the healing process (44,50). Rehabilitation time varies depending on the severity of the injury, with an average duration of 16 weeks, but a range of 6-50 weeks (7). The use of magnetic resonance imaging has identified that hamstring injuries in dancers take an average of 50 weeks to return to preinjury status (8). Cryotherapy (34,41), nonsteroidal antiinflammatory drugs (18,34,35), electrotherapy modalities (34,65), and pain-free strengthening and stretching exercises (60,68) are all incorporated into the rehabilitation of soft tissue injuries; however, to further reduce the recurrence of injury, these strengthening exercises need to be specific to the demand of the individual's chosen sport/activity (15,19,30,32,36,46,54).
The healing of soft tissue is characterized by the formation of connective fibrous tissue that is both shorter and less elastic than the original structure, leading to a loss of flexibility and impaired function (24). In muscle, the development of scar tissue leads to restricted contractions and increased risk of rupture (23), which can lead to a decrease in the elasticity of the stretch shortening cycle because lengthening is impaired (43), which further increases the risk of recurrence. To regain the lost flexibility and prevent further injury and inflammation (18,24,76,77), the performance of concurrent pain-free stretching and strengthening exercises, beginning with isometrics and progressing to dynamic exercises, is essential (60). During the remodeling phase of rehabilitation, stretching of the muscle determines the stress lines along which collagen will be oriented. If this procedure fails to take place, tensile strength is not regained properly, leading to prolonged pain, limited function, and increased susceptibility to tissue injury (52).
Stretching the hamstrings has been shown to increase ROM at the hip joint with the most successful technique being static stretching performed 3-5 times per week, for a duration of 30-45 seconds, repeated up to 4 times (9-11,52,55,63,64). However, when restoring range of motion in an injured athlete, research has demonstrated that regular stretching (4 × 30 seconds; 3-4 times per day; from 48 hours after injury) reduces the time to restore normal ROM compared with stretching once per day (5.7 versus 7.3 days) (10,52). It is also worth noting that injured muscles with changed viscoelasticity may require longer stretches (>30 seconds) or more repetitions to obtain the same benefits as healthy muscles (52).
Stretching also has been shown to be a powerful stimulant of muscle protein synthesis and muscle growth that can be associated with an adaptation to increased functional length by adding or removing sarcomeres in series (31) and therefore should be continued in conjunction with strengthening exercises. Sarcomere length is adjusted back to the optimum for force generation, velocity, and power output (31) when stretching is performed after exercise or after injury.
Stretching combined with other treatment protocols such as strengthening increases success, resulting in a decrease in recurrence (65,68,77). Sherry and Best (68) demonstrated only a 7% recurrence of injury when using progressive agility and trunk stabilization exercises compared with a 70% recurrence (within 1 year) in the stretching and strengthening group. The average time to return to sport was 22 days and 37 days, respectively. The progressive agility and trunk stabilization group were subject to isometric, slow and fast concentric and eccentric training, which is more representative of the demands of sport than the stretching and strengthening protocol. Hamstring-specific exercises, such as the leg curl and stiff-leg dead lift (Figure 1), should be incorporated into the strengthening program because they result in greater hamstring activity compared with the back squat (1,78). It is also worth noting that research suggests the back squat, regardless of technique variation, produces comparatively low activation of the hamstring compared to the quadriceps (1,25,26,42,53,57,59,67). The squat may not therefore be an appropriate exercise to strengthen the hamstrings against hamstring strain, although it will form an essential exercise to appropriately condition athletes prior to the performance of plyometric activities. Strength training may help prevent recurrence because it increases MTU stiffness and strength (49). The incorporation of running and agility drills also has demonstrated a much quicker (10-14 days) return to sport in a series of case studies, when combined with stretching, cryotherapy, and electrotherapy (34), with no recurrence of injury for the rest of the season.
The inclusion of eccentric (deceleration) training also appears to have a beneficial effect in preventing and rehabilitating hamstring strains and therefore may reduce recurrence of injury (13,14,19,46,61,62,79). This may be due to the fact that hamstring strains are associated with rapid eccentric loading (14,50). Eccentric training such as performing Nordic hamstring lowers (Figures 2 and 3) has been shown to decrease the risk of hamstring injuries (2,4,15,19,30,54) by creating greater strength gains than concentric training (46,47,54) and improving the hamstring:quadriceps ratio, especially at greater velocities (38,54). Eccentric training alters the angle of peak torque closer to full extension (13,19,47,54), which may also aid prevention of injury.
The incorporation of plyometrics and agility drills into an athlete's training program has also been shown to increase hamstring peak torque and improve hamstring quadriceps ratios (36). The incorporation of plyometric training may decrease the risk of reinjury (32), which can be attributed to the rapid eccentric loading. Progression of sport-specific and plyometric activities should develop from unidirectional (e.g., squat jumps) to bidirectional (e.g., bounding), and then to multidirectional movements (e.g., zigzag bounding) (32). Before the athlete commences high-intensity plyometrics, it is recommended that he or she can squat >150% body mass for >1 repetition (37,56). However, if plyometrics are performed in water, creating an unloading effect through buoyancy, they can be introduced earlier and have been shown to be highly effective (66,70).
APPLICATION: THE CONDITIONING CONTINUUM
From a review of selected literature, it would appear that, post hamstring strain, restoration of ROM, is inadequate as a measure of an athlete's ability to return to sport. Moreover, because of a high recurrence of injury, stretching alone does not appear to be sufficient to fully prepare an athlete for a return to sport (68). However, stretching, combined with strengthening and sport-specific training, may increase success (32,64,68,77). Training should, therefore, be specific to the demands of the individual athlete's sport and consider a range of factors that include the forces exerted, types of muscle action involved, movement patterns and movement velocity, the mechanism of injury, which is commonly eccentric loading (6-8,39,40,69). These principles can be incorporated into a progressive rehabilitation framework that should fully prepare the athlete for their return to sport (Table 1).
To reduce the risk of an injury recurring, it is essential to understand the mechanism of injury and address these issues by implementing appropriate, progressive exercises in any subsequent training. In the case of hamstring strain injuries, the mechanism of injury appears to be eccentric loading (6,8) at a high velocity (39,40,69); therefore, training needs to address eccentric loading (6-8,39,40,69) and progressively increase velocity of movement using plyometric activities (32,36).
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