Effects of two different injury prevention resistance exercise protocols on the hamstring torque-angle relationship – a randomized controlled trial, by Naclerio, Larumbe-Zabala, Monajati, and Goss- Sampson, in Research in Sports Medicine (2015)
Hamstring strains are a common injury in many popular team sports and they lead to the loss of many hours of training and competition, as well as a very high re-injury rate. Hamstring strains account for 12 – 16% of all injuries in athletes across a range of popular team sports, including rugby, soccer, American Football, and Australian Rules Football. The re-injury rate for hamstring strains ranges from 16 – 34%, depending upon the sport. Running activities account for most hamstring strains, with 57 – 68% of strains occurring during running. The traditional model for hamstring strain injury is that there are various factors that could cause an injury to occur, including: flexibility, strength, fatigue, core stability, muscle architecture, and damage resulting from previous injury. A modern, more sophisticated approach has suggested that while these factors could individually lead to an injury, it is more likely that they interact with each other in order to create multi-factorial scenarios that raise injury risk. Some researchers have suggested that there are at least two different types of hamstring strain injury: those caused by stretching activities and those caused by high-speed running movements. The hamstring strain injury caused by high-speed running is thought to occur most normally in the long head of biceps femoris, typically involves the proximal muscle-tendon junction, displays a greater reduction in strength following injury than those following stretching movements, and leads to a relatively long recovery time to reach pre-injury levels of performance (e.g. around 50 weeks). The biceps femoris (long head) is generally thought to be the most commonly-injured hamstring muscle, although some researchers have suggested that this perception might be incorrect because of inherent errors in common diagnostic approaches. Biomechanically, however, there are good reasons for assuming that the biceps femoris might be most at risk. Firstly, this muscle increases in length by more than the other hamstring muscles during sprinting. Secondly, the moment arm lengths of the biceps femoris in the sagittal plane increase in the late swing position compared to the anatomical position. Previous research has identified that hamstring strains occur most frequently either in the late swing or early stance phases of gait. Late swing involves the greatest strain in the muscle, while early stance involves the largest joint moments. There is good evidence to suggest that hamstring strain injuries can be reduced by eccentric hamstring training but not by flexibility training alone. This has encouraged many strength coaches to incorporate the Nordic hamstring curl into hamstring strain prevention programs. However, there is only limited evidence to suggest that hamstring weakness predicts strain injury risk.
OBJECTIVE: To compare the effects of two different 6-week lower body injury prevention programs on the knee flexion torque–angle relationship (by measuring maximal voluntary isometric contraction (MVIC) knee flexion torque at 35, 45, 60, 80, 90, and 100 degrees) before and after the intervention.
POPULATION: 32 recreationally-trained male soccer players, aged 22.2 ± 2.6 years, randomly allocated to 3 groups: hamstring eccentric (ECC), unstable squat (UNS), and control (CON).
INTERVENTION:The two training groups trained twice per week for 6 weeks using 3 individual hamstring eccentric (ECC) or unstable squat (UNS) exercises, respectively. ECC performed the coach- and band-assisted assisted Nordic hamstring curl, the eccentric single-leg stiff-legged deadlift, and the eccentric two-leg stiff-legged deadlift. UNS performed the single-leg squat, the single-leg squat on a BOSU ball, and forward lunges on a BOSU ball.
Knee flexion torque-angle relationship
At baseline, the researchers reported that all 3 groups displayed peak torque at between 45 and 80 degrees of knee flexion angle and there were no differences between groups.
Changes in knee flexion torque-angle relationship
The researchers reported that MVIC knee flexion torque increased at 35 and 45 degrees in ECC and at 60, 80 and 90 degrees in UNS.
What did the researchers conclude?
The researchers concluded that ECC increased knee flexion torques at the two most open knee flexion angles (35 and 45 degrees) where the hamstrings are lengthened while UNS improved at more closed knee angles (60, 80, and 90 degrees) where the hamstrings are less lengthened. This suggests that exercises that display peak torque at long muscle lengths tend to increase strength most at long muscle lengths while exercises that display peak torque at short muscle lengths tend to increase strength most at short muscle lengths.
The study was limited in that it is unclear whether the unstable surface was necessary for the UNS condition. It is possible that a simple single-leg squat program would have produced similar results without the need for the instability condition.