'Participation' as defined within the World Health Organisation's International Classification of Functioning, Disability and Health classification tree is concerned with the free-living activity of individuals. The measurement of participation is difficult, except through a questionnaire based approach. To overcome the subjective nature of questionnaires, body worn monitors have been developed that allow the quantification of aspects of posture and stepping activity. Whilst the measurement of ability goes some way towards understanding the positive benefits of hip replacement as an intervention, knowledge of free-living activity would considerably extend the clinician/health professional understanding of the patient's recovery following the operation. Conventional tools for assessing participation include questionnaires such as the SF36, WOMAC, UCLA, Hip dysfunction and Osteoarthritis Outcome Score, Harris Hip Score and the Oxford Hip Score. Objective measurement of physical activity (e.g. using body worn monitors) removes the subjectivity of questionnaires. There is clearly a cost and time implication for the use of monitoring devices, so it is important that they provide additional, valuable information on free-living physical activity in comparison to quick to implement and readily accessible questionnaires.