Hamstring strain injuries : Understanding risk factors and intervention strategies

Hamstring strain injuries (HSI) is a common lower-limb injury sustained by many athletes of sprint-based sports (e.g. football) throughout the course of their career. In several large-scale studies looking at the prevalence of injuries over a competitive playing season, it was found to be the most prevalent injury sustained by players. The high recurrence rates associated with HSIs further exacerbates its problematic nature. Consequently, HSIs have been found to result in high individual and financial costs to the athlete and the support systems surrounding him or her. Therefore, the purpose of this programme of research was to understand the risk factors associated with HSIs and examine the characteristics of different hamstring strengthening exercises that are commonly prescribed. Another purpose of this study was to understand if an intervention designed to address injury-mechanism related parameters (e.g. weakness in eccentric strength at longer muscle lengths) could effectively reduce the susceptibility to HSIs. In order to meet the purposes set out in this research study, three separate experimental studies were designed and conducted. In the first study, structural and functional characteristics of the hamstrings were compared between athletes who previously sustained an HSI and those who were uninjured. The objective of this study was to determine characterisations of a previously-injured hamstring muscle which might be interpreted as possible risk factors. The results of the study suggested that previously-injured hamstrings had shorter muscle fascicles and were more pennate; and were generally stiffer. Subtle differences in sprinting kinematics relating to hip flexion angle were also observed. Additionally, inter-session reliability measures of architectural and viscoelastic assessments also found ultrasonography and myotonometry to be reliable as structural measures of the hamstring. The objective of the second study was to examine the muscle activation characteristics of a range of hamstring strengthening exercises. Besides looking at commonly prescribed exercises such as the Nordic Hamstring Lowers (NHL), variations of exercises were also examined. These variations were designed based on current understanding of the mechanism of HSIs (e.g. state of muscle during onset of injury). In this study, it was found that despite the high muscle activation levels elicited during the NHL, it was typically performed when the muscle was under minimal elongation stress. On the other hand, Lengthened-state Training (LST) exercise, more specifically, a seated-leg curl performed with the hip flexed at 60° elicited high muscle activation levels accompanied by the greatest amount of elongation stress. This suggested that LST could be a more effective hamstring strengthening exercise than what is currently widely prescribed. By utilising the results of the second study, the objective of the final study was to determine if a training intervention utilising LST would be effective in modifying the muscle architecture and eccentric strength measures identified as possible risk factors from the first study. In this study, 43 participants underwent a 12-week intervention programme consisting of control (CON), NHL or LST training. Muscle architecture and eccentric strength was measured pre and post training. It was found that in participants in both training groups, there was significant hypertrophy of the muscles reflected in an increase in muscle thickness, however, these changes were not accompanied by changes in fascicle length and pennation angle. There was also a larger eccentric strength increase in the LST compared to the NHL, suggesting that LST could be more effective than NHL in reducing injury risk. Thus, this research study suggests that muscle structure and function could be modifiable risk factors of HSI that can be addressed through a strengthening programme that is designed to strengthen the muscle at the state (contraction type and muscle length) at which the injury commonly occurs. This could have implications for approaches to designing training interventions for reducing the risk of a range of muscular injuries.