Background A history of hamstring strain injury (HSI) has been indicated as a primary risk factor for a future injury. It is unknown whether a prior strain influences the architectural characteristics of the hamstring.

Objective To determine if alterations occur to the architecture of the long head (BFlh) muscle in those with a history of strain injury to this muscle.

Design Reliability and retrospective case-control study design.

Setting Clinical setting with recreationally active males and elite Australian Footballers.

Participants 31 males (Australian Footballers n=14, recreational males n=17). Of the 31 participants, 18 had a prior unilateral HSI, whereas the other 13 formed an uninjured control group.

Risk factor assessment The independent variables were muscle fascicle length (FL), pennation angle (PA) and thickness (MT)).

Main outcome measure The BFlh muscle architecture of both limbs was assessed utilising B-mode ultrasound (General Electric Vivid-i) whilst rested in a prone position. The scan site was determined as 50% of the distance between the ischial tuberosity and the lateral knee joint line. The hypothesis prior to recruitment was that a history of injury would result in a reduction of FL and MT and an increase in PA. Reliability was assessed at a second time point within ten days of the initial visit and was determined utilising intra-class correlations (ICC).

Results The assessment of the architectural characteristics of the BFlh was shown as reliable whilst in a rested state (FL and PA ICC=0.97, MT=0.96). PA's (P=.001) and FL's (P=.013) were significantly less in the previously injured BFlh then the contralateral uninjured limb. Muscle thickness was not significantly different (p=0.466).

Conclusions Ultrasound is a reliable measure of architectural characteristics in the BFlh. A previously injured BFlh displays reductions in FL and increases in PA. These variations may partly explain the high rate of recurrence of HSI's.