The acute effects of eccentric exercise on indicators of hamstring muscle damage
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The prevention and treatment of hamstring strains are important as they represent a large proportion of non-contact sporting injuries. With insufficient rehabilitation, hamstring strains can become recurrent and lead to extended periods away from training and match play. Eccentric hamstring training using the Nordic hamstrings exercise has been shown to decrease the incidence of hamstring strains in several studies, however the overall rate of reported hamstring injuries has not reduced. This may be because the areas of the hamstrings that are stressed during Nordic hamstrings training and the subsequent areas that undergo hypertrophy are different from the areas of the hamstrings that are commonly injured. Therefore, a different form of eccentric hamstring training that more closely replicates the biomechanics of hamstring strain injury and stresses the areas that are commonly injured may offer superior injury protection and rehabilitation. A literature review on the acute effects of eccentric exercise showed that eccentric exercise causes a decrease in peak torque (PT), a shift in the optimal angle of peak torque (APT) to longer muscle lengths, increased ultrasound echo intensity (EI) and muscle thickness (MT) and cross sectional area (CSA), and limb circumference (LC) and increase visual analogue scale (VAS) scores. The literature review exposed the lack of studies specifically on the hamstring muscles and a lack of reliability studies on ultrasound echo intensity and cross sectional area measurements involving the hamstrings. Furthermore, most studies assessed ultrasound measures at one or a few sites only, thereby potentially missing areas of damage. The aims of this thesis were to establish the reliability of measuring ultrasound EI and CSA of the hamstrings and to compare the acute effects of the NH and DL exercises on markers of eccentric muscle damage. An ultrasound echo intensity and cross sectional area scanning protocol was developed and the within session and between session reliability of an experienced sonographer was assessed at seven locations on the hamstrings. Following a familiarisation period, the protocol was found to have high or greater within-session reliability (ICC range = 0.84 to 1.00) for hamstring cross sectional area (CSA) and echo intensity (EI) at 7 different locations. However, there were a number of sites where the between-session reliability of EI and CSA did not reach the level of high reliability (ICC<0.75). A second study was then conducted and the drop lunge was chosen as an alternative to the Nordic hamstrings exercise because it is a unilateral exercise that incorporates eccentric control of hip flexion. To compare the acute effects of these exercises two groups of eight healthy recreationally active females were randomly assigned to perform either the Nordic hamstrings exercise or the drop lunge. Markers of eccentric muscle damage were assessed (PT, APT, EI, CSA, LC and VAS) before exercise, immediately after exercise and for three days post exercise. The key findings were that following both exercises changes in some markers of muscle damage were evident (ES ranges; PT = -0.29 to -0.83, APT = -0.28 to -0.53, EI = 0.38 to 0.92, CSA = 0.20 to 0.85, VAS = 0.8 to 3.0) but there were few clear differences in the magnitude of these changes between the exercises. Therefore the drop lunge appears to provide a similar eccentric stimulus to the Nordic hamstrings exercise and should be considered as an additional eccentric hamstring exercise for hamstring strain injury prevention and rehabilitation. The drop lunge may be especially beneficial for addressing limb asymmetries. However, longer term studies are needed to investigate if the drop lunge can reduce the risk of HS injury and further investigation is needed to ensure adequate between session reliability when determining EI and CSA for some areas of the hamstrings.