To aid in effective injury prevention, mode specificity is a vital premise on which to focus. This involves applying stresses to the body in the situations which commonly cause injury. Rapid acceleration or deceleration during both distance or sprint running (Anderson, 2005) has been found to be a common denominator to hamstring injuries, including sports such as soccer, track and field, football and rugby (Sports Injury Bulletin). Injuries are particularly apparent in movements that require a high output of force, speed and strength (Askling et al, 2003), resulting in high angular velocities (Jonhagen, 2005).
Specific training programs can be used to reduce the incidence of hamstring injuries. The majority of the training programs are based on the premise that stress levels that are higher than the maintenance stress range, though lower than the injury threshold will have a positive effect of tissue adaptation, thus increasing resistance to injury.
Strength training forms an integral part of a hamstring injury prevention program, with hamstring strength found to be the most important protective factor for hamstring injury (Sports Injury Bulletin). With the achievement of a stable strength base through traditional concentric strength training exercises, more sport specific strengthening initiatives can be undertaken to improve functional eccentric strength. Changes in performance are primarily attributable to specific training induced adaptations in the neuromuscular system (Hakkinen and Komi, 1986). Efficient concentric and eccentric strengthening is motivated from a functional point of view and allows effective muscle balance between muscle groups, and between opposite sides of the body. This ensures that undesirable muscle imbalances (as alluded to re: hamstring quadriceps ratios), which may increase the likelihood of injury, are not present. Once again emphasising the importance of eccentric training, a key variable in hamstring strength training is shifting the point of equality towards a more flexed joint position to incur stability throughout a greater range of motion. This is particularly viable in explosive sports, those sports which instigate the highest percentage of hamstring injuries. Eccentric training is vital, as functionally, “…eccentric antagonist co-activation and serial elastic tension resist concentric agonist contraction eccentrically.” (Coombs and Garbutt, 2002). Greater cross education is apparent with eccentric training than with concentric training, with more global strength improvements during differing concentric, eccentric and isometric contractions (Hortobagyi et al, 1997).
Eccentric training involves a greater load torque than the muscle torque, resulting in an active muscle lengthening (Enoka, 2002). Eccentric training results in muscular specialisations that “may generate alterations in involved articular forces and cause static and dynamic changes” (Siqueira et al, 2002). This can be explained by the physical stress theory (Mueller and Maluf, 2002), which posits that tissues accommodate to physical stresses by “…altering their structure and composition to best meet the mechanical demands of routine loading”. As previously discussed, eccentric exercise can lead to micro-tears, which have been hypothesised to lead to gross muscle damage (Middleton and Montero, 2004). During such a contraction, some sarcomeres are stretched beyond myofilament overlap, leading eventually to muscle fibre dennervation. This results in the effective series compliance of the fibre increasing, leading to a shift in the length-tension relation to longer muscle lengths (Camilla et al, 2001). This disruption in sarcomeres combines with a second, more sustained shift in length tension which represents a training effect by an increase of sarcomeres in series. The micro-tears associated with this exercise can be reduced by exercise with an eccentric bias (Camilla et al, 2001). As the majority of hamstring injuries occur at longer muscle lengths (with the knee extended and hip flexed), a shift of this peak torque to longer muscle lengths and an avoidance of the descending limb of the length-tension curve, will allow resistance to quadriceps contraction in the target range. Not only does concentric training alone fail to offer the protective benefits of eccentric training, it actually reduces sarcomeres in series, thus shifting the optimal muscle length to the right on the length-tension curve and causing an increase in injury occurrence. Additionally, a single bout of eccentric exercise has been shown to have a protective effect during future eccentric exercise.
As previously alluded to, two common sites of hamstring injury are the musculo-tendinous junction and the tendon itself. It has been found that eccentric exercise is an effective method of rehabilitating and strengthening muscle tendons (Jonhagen, 2005; Middleton and Montero, 2004), further advocating the use of eccentric exercise due to its effect on this ‘anatomical weak link’. In fact, eccentric exercise is used to treat Achilles and patellar tendinopathy, emphasising its valuable role in tendon strengthening. The outcome of eccentric training results in hypertrophy and a change in the spring characteristics of muscle, and the ability for tendon to tolerate higher tensions; both favourable outcomes (LaStayo et al, 2003).
A primary training method is the Nordic Hamstrings exercise, which involves a subject’s ankle being held in place while kneeling, as the subject lowers themselves forward towards the ground, thus creating an eccentric contraction, and the accompanying benefits just discussed.
An 11% increase in eccentric strength was observed in subjects undertaking this training mechanism, and with this, a functional hamstring quadriceps ratio improvement was also seen. To mirror functional movements, the exercises should be performed at the angular velocities experienced during running, up to 700 degrees per second (Running Research News), as length-tension properties are impaired at high velocities (Coombs and Garbutt, 2002). “The optimal exercise would be an eccentric one in which a very high angular knee velocity is most aggressively controlled at around 30 degrees of knee flexion” (Bahr and Holme, 2000). The eccentric hamstring lower based exercise is advantageous over conventional hamstring exercises, as they have a major effect on loss of co-contraction by unlearning stabilising activation patterns (Coombs and Garbutt, 2002), thus removing them from an injury prevention program.
In terms of injury prevention and rehabilitation, a four step sequence (Bahr and Krosshaug, 2005) must be employed, where the magnitude of the problem is identified, the risk factors and injury mechanisms are established, measures are introduced to reduce the risk of injury, and finally, the effect of these measures must be evaluated.
