INTRODUCTION Hamstring strain injury is the primary injury type sustained across a number of sports. Of further importance reinjury rates have been high for many years and a previous insult is commonly identified as the primary risk factor for future injury. These data suggest that maladaptation associated with previous injury increases the risk of reinjury, however scant attention has been paid to the impact of hamstring strains on the architecture of the previously injured hamstring. Therefore, the purpose of this study was to determine the impact of a previous hamstring strain injury on biceps femoris long head muscle thickness, pennation angle and fascicle length. METHODS Thirty-one participants consented to participate in the study (22.4 ± 3.2 years; 1.82 ± 0.08m; 80.5 ± 9.6kg), which consisted of elite male Australian footballers (n=14) and recreational male athletes (n=17). Of the 31 participants, 18 reported a history of prior unilateral hamstring strain injury, whilst the remaining 13 formed an uninjured control group. Details of injury history were ascertained from notes taken during clinical examination. B-mode ultrasound (General Electric Vivid-i) was performed on both limbs at a capture depth of 8cm and a frequency of 12Hz with a 7Mhz linear-array transducer. Scan site was determined as 50% of the distance between the greater trochanter of the femur and the lateral knee joint line, with the probe positioned directly over the biceps femoris long head. All ultrasonography was performed with the muscle in a rested state and was collected and analysed by the same investigator (KNP), between 9am and 12pm, who was blinded to the injury history of each participant. A subset of 26 of the 31 participants were assessed five to ten days following their first assessment to determine the test-retest reliability of the between limb differences of the measures. Analysis was performed using Image J freeware to determine biceps femoris long head muscle thickness, pennation angle and fascicle length (Figure 1). To determine reliability intra-class correlation co-efficeint (ICC) and typical error (TE) was determined. Dependent t-tests were used to compared dependent variables in the injured and uninjured limbs. Mean between limb differences with 95% confidence intervals were reported. Significance was set at p < Biceps femoris fascicle length is shorter in a previously hamstring injured athlete. David Opar 1#, Kirsten Porter 2, Morgan Willaims 2,3, Anthony Shield 1 1 School of Exercise and Nutrition Sciences and Institute of Health and Biomedical Innovation, Queensland University of Technology, Australia 2 School of Exercise Science, Australian Catholic University, Australia 3 Division of Sport and Science, University of Glamorgan, Wales # Corresponding author: RESULTS Data pertaining to the reliability of the measures of architecture variables assessed can be found in Table 1. Based on intra-class correlation co- efficient all measures displayed high levels of reliability. Pennation angles (p = 0.001) and fascicle lengths (p = 0.013) were significantly different in the previously injured biceps femoris long head compared to the contralateral uninjured muscle, however muscle thickness did not differ (p = 0.466) (Figure 2). There were no significant between limb differences in muscle thickness (mean difference = 0.02 cm, 95%CI = 0.00 to 0.03 cm; p = ), pennation angle (mean difference = -0.1°, 95%CI = -0.9 to 0.8°; p = ) or fascicle length (mean difference = cm, 95%CI = to 0.67 cm; p = ) in the uninjured group. CONCLUSION Ultrasound displayed high levels of measurement reliability for biceps femoris long head muscle thickness, pennation angle and fascicle length. A previously injured biceps femoris long head displays differences in pennation angle and fascicle length compared to an uninjured contralateral biceps femoris long head. These architectural variations may partly explain the high tendency for hamstring strain injuries to reoccur. Further work is needed to confirm if differences in architecture confer an increased risk of hamstring strain injury or re-injury. ‘REAL WORLD’ IMPLICATIONS Hamstring muscle architecture data obtained via ultrasound may present a methodology which allows more objective assessment of rehabilitation success. Clinicians should consider the assessment of hamstring muscle architecture as a screening tool for hamstring strain injury and reinjury. Test 1 ( Mean  SD)Test 2 ( Mean  SD) ICC (95% CI)TE (95% CI) Muscle Thickness (cm)  (0.93 to 0.99)0.02 (0.02 to 0.03) Pennation Angle (°) -0.3   (0.93 to 0.99)0.4 (0.3 to 0.5) Fascicle Length (cm)0.05   (0.92 to 0.98)0.29 (0.23 to 0.40) Table 1 Test-retest reliability for between limb mean differences (left – right) of biceps femoris long head architecture variables assessed from ultrasound (n=26). SD, standard deviation; 95%CI, 95% confidence intervals; ICC, intra-class correlation co-efficient ; TE, typical error. Figure 1 Example ultrasound image of the biceps femoris long head. Muscle thickness (MT), pennation angle (PA) and fascicle length (FL) was determined using Image J. Superficial aponuerosis Deep aponuerosis MT FL PA * * Figure 2 Comparisons of architectural variables of previously injured and uninjured biceps femoris long head muscle. *p < 0.05 compared to injured limb.