Dear Spekis
Do you have a supervisor or tutor? if you do, that person is perhaps the best person to put you through...
what is the title of your topic? from your description, it is unlikely that the focus of your dissertation will generate any papers sufficient for a systematic review.. i say this because it doesnt sound like a clinical question is being asked here...
every muscle in the body has a primary and secondary role depending on the activity it is being subjected to...the iliopsoas muscle group will flex the hip as well as stabilize the pelvis/lumbar spine...
maybe what you should be thinking about doing is a literature review...with this you would find several materials, papers, textbooks etc that would mention the role of the iliopsoas muscle group, however you will not be able to systematically review this papers you find because many of them would be asking clinical questions that will not be aimed at explaining the role of the iliopsoas muscle...
On the other hand you may want to review your topic with a clinical question attached to it...for instance you could say you want to review papers that have looked at the effect of low back pain on iliopsaos function? or the effect of hip arthritis on iliopsoas function..? or what is the effective management of iliopsoas tendinopathy? these kind of topics will generate more clinically based studies which you can now review systematically...
If one of these topics is what you choose...your literature review can be now explain the controversies in the assumed role of the iliopsoas muscle in lumbar/pelvic stabilization? how poor stability in the lumbar spine/pelvis can contribute to lowback pain/hip arthritis/iliopsoas tendinopathy? what the general concensus of the function of these muscle is preventing or causing these problems? what groups of people are more susceptible to these problems etc...
the other option is to generally carry out an experimental study...wherein you measure the EMG values of the iliopsoas muscle in low back and non low back pain patients and find out if there is any difference when lets say carrying out a onle leg stance for instance...this can give you an idea of the role the iliopsoas muscle plays in certain musculoskeletal dysfunction, and perhaps at the end of this study...give you an insight to what the therapist needs to do or involve the iliopsoas muscle in when rehabing such patients...
i hope i have been able to give you several options that should make your dissertation more meaningful...
Cheers