The presence of short BFlh fascicles and low levels of eccentric knee flexor strength in elite soccer players increases the risk of future HSI. The greater risk of a future HSI in older players or those with a previous HSI is reduced when they have longer BFlh fascicles and high levels of eccentric strength.

Low levels of eccentric hamstring strength increased the risk of future HSI. Interaction effects suggest that the additional risk of future HSI associated with advancing age or previous injury was mitigated by higher levels of eccentric hamstring strength.

Hamstring strain injuries (HSIs) are common in a number of sports and incidence rates have not declined in recent times. Additionally, the high rate of recurrent injuries suggests that our current understanding of HSI and re-injury risk is incomplete. Whilst the multifactoral nature of HSIs is agreed upon by many, often individual risk factors and/or causes of injury are examined in isolation. This review aims to bring together the causes, risk factors and interventions associated with HSIs to better understand why HSIs are so prevalent. Running is often identified as the primary activity type for HSIs and given the high eccentric forces and moderate muscle strain placed on the hamstrings during running these factors are considered to be part of the aetiology of HSIs. However, the exact causes of HSIs remain unknown and whilst eccentric contraction and muscle strain purportedly play a role, accumulated muscle damage and/or a single injurious event may also contribute. Potentially, all of these factors interact to varying degrees depending on the injurious activity type (i.e. running, kicking). Furthermore, anatomical factors, such as the biarticular organization, the dual innervations of biceps femoris (BF), fibre type distribution, muscle architecture and the degree of anterior pelvic tilt, have all been implicated. Each of these variables impact upon HSI risk via a number of different mechanisms that include increasing hamstring muscle strain and altering the susceptibility of the hamstrings to muscle damage. Reported risk factors for HSIs include age, previous injury, ethnicity, strength imbalances, flexibility and fatigue. Of these, little is known, definitively, about why previous injury increases the risk of future HSIs. Nevertheless, interventions put in place to reduce the incidence of HSIs by addressing modifiable risk factors have focused primarily on increasing eccentric strength, correcting strength imbalances and improving flexibility. The response to these intervention programmes has been mixed with varied levels of success reported. A conceptual framework is presented suggesting that neuromuscular inhibition following HSIs may impede the rehabilitation process and subsequently lead to maladaptation of hamstring muscle structure and function, including preferentially eccentric weakness, atrophy of the previously injured muscles and alterations in the angle of peak knee flexor torque. This remains an area for future research and practitioners need to remain aware of the multifactoral nature of HSIs if injury rates are to decline.

31The architectural and morphological adaptations of the hamstrings in response to training  Hip extension training promotes more hypertrophy in the biceps femoris long head and semimembranosus than the Nordic hamstring exercise, which preferentially develops the semitendinosus and the short head of biceps femoris.4

Hamstring strain injury (HSI) is the most common injury type in a number of sports 4,9,17 and is notorious for its high recurrence rate. A high proportion of HSIs are thought to occur during the terminal swing phase of high-speed running, 5 when the hamstrings are required to perform a forceful eccentric contraction. 20,23 Lower eccentric knee flexor strength has been reported as a risk factor for future HSI,8,19 indicating the importance of eccentric strength for HSI avoidance. 3,18 Further, previously strained hamstrings display reduced levels of eccentric knee flexor strength compared to those in the uninjured contralateral limb, 7,13 which may partially explain why a previous HSI is the primary risk factor for future injury. 2Currently, the gold standard measure for the assessment of eccentric knee flexor strength is isokinetic dynamometry. 1 However, this technique is limited by the high cost of the device and, therefore, its lack of widespread availability. Although handheld dynamometers have become a popular field-based alternative, they require operator skill and strength to collect reliable and valid data. 22The purposes of this investigation were (1) to determine if a novel device, designed to measure eccentric knee flexor strength via the Nordic hamstring exercise (NHE), would display acceptable test-retest reliability; (2) to determine normative values for eccentric knee flexor strength derived from the device in in-T T STUDY DESIGN: Reliability and case-control injury study. T T OBJECTIVES:To determine if a novel device designed to measure eccentric knee flexor strength via the Nordic hamstring exercise displays acceptable test-retest reliability; to determine normative values for eccentric knee flexor strength derived from the device in individuals without a history of hamstring strain injury (HSI); and to determine if the device can detect weakness in elite athletes with a previous history of unilateral HSI. T T BACKGROUND: HSI and reinjury are the most common cause of lost playing time in a number of sports. Eccentric knee flexor weakness is a major modifiable risk factor for future HSI. However, at present, there is a lack of easily accessible equipment to assess eccentric knee flexor strength. T T METHODS:Thirty recreationally active males without a history of HSI completed the Nordic hamstring exercise on the device on 2 separate occasions. Intraclass correlation coefficients, typical error, typical error as a coefficient of variation, and minimal detectable change at a 95% confidence level were calculated. Normative strength data were determined using the most reliable measurement. An additional 20 elite athletes with a unilateral history of HSI within the previous 12 months performed the Nordic hamstring exercise on the device to determine if residual eccentric muscle weakness existed in the previously injured limb. T T RESULTS:The device displayed high to moderate reliability (intraclass correlation coefficient = 0.83-0.90; typical error, 21.7-27.5 N; typical error as a coefficient of varia...