Thigh muscle injuries including the hamstring and adductor groups account for a large proportion of missed training and playing time in sports such as soccer, football and sprinting [1, 2]. There is some evidence to support that altered muscle function during single leg loading may be a contributing factor in hamstring muscle strains , and athletic groin pain [4, 5]. however the mechanisms behind this altered muscle function are not clear [6–8].
In addition, retraining of hip and thigh muscle groups as part of prevention and rehabilitation of these thigh muscle injuries is popular, with a huge variety of exercises and exercise parameters being recommended [9–11]. There is growing support for functional retraining as an important component of injury prevention and rehabilitation strategies, however there remains a lack of understanding regarding factors that strongly influence muscle function during single leg loading. We hypothesized that position of the trunk and pelvis during single leg loading strongly influences the activation patterns of the hip and thigh muscles. To date, a number of studies investigating frontal plane pelvis position (pelvic drop or Trendelenberg posture) in single leg loading show pelvic posture does influence activity of the hip abductor muscles [12–14].
Apart from these studies, there is little evidence regarding how changes in trunk and pelvis position influence muscle activation patterns in common fontal and sagittal plane postures in single leg stance. There is some literature to suggest that changing posture in a sagittal plane whilst in double leg stance changes the activation of different muscles. O’Sullivan and co-workers  demonstrated differences in abdominal and back muscle activity levels when comparing active upright standing to posterior trunk sway standing. However only trunk, not hip and thigh muscle activity was recorded in this study. This knowledge has lead to improved understanding of potential pain mechanisms linked to standing posture  and functional rehabilitation strategies for back pain disorders .
Wang and co-workers  showed that with anterior trunk sway, there was an increase in hamstring and erector spinae activation (dorsal muscles), accompanied by a decrease in rectus femoris and rectus abdominus activation (ventral muscles), with the opposite pattern observed in posterior trunk sway. Neither study evaluated single leg loading. Other studies have shown that altering lower limb or hip position during single leg loading influences hip and thigh muscle activation [19–21] however the influence of more proximal body segment posture on muscle activity has not been investigated.
In summary, despite what would appear to have widespread clinical application, the influence that trunk and pelvis posture has on lower limb muscle activation in single leg stance is largely unknown. The aim of this study was to investigate the influence of changes in frontal and sagittal plane positions of the trunk and pelvis on muscle activation around the hip and thigh in single leg stance in a male pain free population.
It was hypothesized that changes in both trunk and pelvic posture during single leg stance would result in predictable changes in muscle activation. Specifically, changing posture in the frontal plane would alter primarily frontal plane muscle activity, and changes of posture in the sagittal plane would alter primarily sagittal plane muscle activity.