Abstract
The Nordic hamstring exercise (NHE) has been shown to reduce hamstring injury risk when employed in training programs. This study investigates a novel device to modify the NHE torque-length relationship of the knee flexors, as targeting the hamstrings at a more extended length may have benefits for hamstring strain injury prevention and rehabilitation. Eighteen recreational male participants completed three bilateral NHE repetitions at a conventional 0° flat position, a 10° incline, and a 10° decline slope on a novel device (HALHAM°). Measures of peak torque and break-torque angle explored the effect of inclination on the knee flexors' length-tension relationship. Relative thigh-to-trunk angle and angular velocity of the knee joint were used to assess influence of inclination on technique and exercise quality. Break-torque angle increased when performed at an incline (134.1 ± 8.6°) compared to both the decline (112.1 ± 8.3°, p <0.0001, g = 2.599) and standard flat NHE positions (126.0 ± 9.8°, p = 0.0002, g = 0.885). Despite this, altering inclination did not affect eccentric knee flexor peak torque (decline = 132.0 ± 63.1 Nm, flat = 149.7 ± 70.1 Nm, incline = 148.9 ± 64.9 Nm, F = 0.952, p = 0.389), angular velocity of the knee joint at break-torque angle (decline = 23.8 ± 14.4°, flat = 29.2 ± 22.6°, incline = 24.5 ± 22.6°, F = 0.880, p = 0.418) or relative thigh-to-trunk angle at break-torque angle (decline = 20.4 ± 10.4°, flat = 16.7 ± 10.8°, incline = 20.2 ± 11.2°, F = 1.597, p = 0.207). The report recommends the use of arbitrary metrics such as break-torque angle that can be replicated practically in the field by practitioners to assess proxy muscle length changes i.e., the angular range over which the torque can be produced. Inclination of the Nordic hamstring exercise leads to hamstring muscle failure at longer muscle lengths without reductions in the maximal force exuded by the muscle. Therefore, the NHE performed on an incline may be a more effective training intervention, specific to the proposed mechanism of hamstring strain injury during sprinting that occurs whilst the muscle is rapidly lengthening. Using a graded training intervention through the inclinations could aid gradual return-to-play rehabilitation.
Highlights
Hamstring strain injuries (HSIs) are common lower limb injuries that occur in intensive, acyclic sports (Ekstrand et al, 2011) presenting an 18% re-injury rate, (Hägglund et al, 2006) worsened by poor quality formation of scar tissue in the myotendinous junction post-injury (Hägglund et al, 2006)
Changing inclination significantly affected break-torque angle” (BTA) (F = 63.85, p < 0.01), which increased when the Nordic hamstring exercise (NHE) was performed at INC (134.1 ± 8.6◦) compared to both the decline angle (DEC) (112.1 ± 8.3◦) (p < 0.01, g = 2.599) and conventional flat angle position (FLAT) NHE (126.0 ± 9.8◦) (p < 0.01, g = 0.885)
Altering inclination had no significant effect on the angular velocity of the knee joint (AVK) (F = 0.880, p = 0.418) large variability was reported for Angular velocity of the knee joint (AVK) at BTA in all inclinations, with FLAT reporting the highest mean difference (29.2 ± 22.6 deg·s−1)
Summary
Hamstring strain injuries (HSIs) are common lower limb injuries that occur in intensive, acyclic sports (Ekstrand et al, 2011) presenting an 18% re-injury rate, (Hägglund et al, 2006) worsened by poor quality formation of scar tissue in the myotendinous junction post-injury (Hägglund et al, 2006). HSIs present a major challenge in elite football, with a missed mean playing time of 14 competition days per injury resulting in substantial financial strain: and potentially having a detrimental impact on a team’s performance (Ekstrand et al, 2011). In the late swing phase, the hamstrings rapidly change from acting eccentrically (decelerating the extended knee) to concentrically (supporting hip extension). This places them in a more susceptible extended position under high mechanical stress, as the hamstrings develop maximal tension to stabilize the knee joint (Guex et al, 2012). By targeting the hamstrings in a lengthened position during eccentric hamstring training, the muscle orientation at injury can be more closely reproduced
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